--- pgp/doc/pgpdoc2.txt 2018/04/24 16:40:34 1.1.1.2 +++ pgp/doc/pgpdoc2.txt 2018/04/24 16:41:29 1.1.1.3 @@ -1,2218 +1,2443 @@ - Phil's Pretty Good Software - Presents - - === - PGP - === - - Pretty Good Privacy - Public Key Encryption for the Masses - - - ------------------------- - PGP User's Guide - Volume II: Special Topics - ------------------------- - by Philip Zimmermann - Revised 14 Jun 93 - - - PGP Version 2.3a - 1 Jul 93 - Software by - Philip Zimmermann - with - Branko Lankester, Hal Finney, and Peter Gutmann - - - - -Synopsis: PGP uses public-key encryption to protect E-mail and data -files. Communicate securely with people you've never met, with no -secure channels needed for prior exchange of keys. PGP is well -featured and fast, with sophisticated key management, digital -signatures, data compression, and good ergonomic design. - - -Software and documentation (c) Copyright 1990-1993 Philip Zimmermann. -For information on PGP licensing, distribution, copyrights, patents, -trademarks, liability limitations, and export controls, see the -"Legal Issues" section. - - -Contents -======== - -Quick Overview -Special Topics - Selecting Keys via Key ID - Separating Signatures from Messages - Decrypting the Message and Leaving the Signature on it - Sending ASCII Text Files Across Different Machine Environments - Leaving No Traces of Plaintext on the Disk - Displaying Decrypted Plaintext on Your Screen - Making a Message For Her Eyes Only - Preserving the Original Plaintext Filename - Editing Your User ID or Pass Phrase - Editing the Trust Parameters for a Public Key - Checking If Everything is OK on Your Public Key Ring - Verifying a Public Key Over the Phone - Using PGP as a Unix-style Filter - Suppressing Unneccessary Questions: BATCHMODE - Force "Yes" Answer to Confirmation Questions: FORCE - PGP Returns Exit Status to the Shell - Environmental Variable for Pass Phrase - Setting Configuration Parameters: CONFIG.TXT - TMP - Directory Pathname for Temporary Files - LANGUAGE - Foreign Language Selector - MYNAME - Default User ID for Making Signatures - TEXTMODE - Assuming Plaintext is a Text File - CHARSET - Specifies Local Character Set for Text Files - ARMOR - Enable ASCII Armor Output - ARMORLINES - Size of ASCII Armor Multipart Files - KEEPBINARY - Keep Binary Ciphertext Files After Decrypting - COMPRESS - Enable Compression - COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed - MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed - CERT_DEPTH - How Deep May Introducers Be Nested - BAKRING - Filename for Backup Secret Keyring - PAGER - Selects Shell Command to Display Plaintext Output - SHOWPASS - Echo Pass Phrase to User - TZFIX - Timezone Adjustment - CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text - VERBOSE - Quiet, Normal, or Verbose Messages - INTERACTIVE - Ask for Confirmation for Key Adds - Protecting Against Bogus Timestamps - A Peek Under the Hood - Random Numbers - PGP's Conventional Encryption Algorithm - Data Compression - Message Digests and Digital Signatures - Compatibility with Previous Versions of PGP -Vulnerabilities - Compromised Pass Phrase and Secret Key - Public Key Tampering - "Not Quite Deleted" Files - Viruses and Trojan Horses - Physical Security Breach - Tempest Attacks - Exposure on Multi-user Systems - Traffic Analysis - Cryptanalysis -Legal Issues - Trademarks, Copyrights, and Warranties - Patent Rights on the Algorithms - Licensing and Distribution - Export Controls -Computer-Related Political Groups -Recommended Readings -To Contact the Author -Appendix A: Where to Get PGP - - -Quick Overview -============= - -Pretty Good(tm) Privacy (PGP), from Phil's Pretty Good Software, is a -high security cryptographic software application for MSDOS, Unix, -VAX/VMS, and other computers. PGP combines the convenience of the -Rivest-Shamir-Adleman (RSA) public key cryptosystem with the speed of -conventional cryptography, message digests for digital signatures, -data compression before encryption, good ergonomic design, and -sophisticated key management. - -This volume II of the PGP User's Guide covers advanced topics about -PGP that were not covered in the "PGP User's Guide, Volume I: -Essential Topics". You should first read the Essential Topics -volume, or this manual won't make much sense to you. Reading this -Special Topics volume is optional. - - - -Special Topics -=============== - - -Selecting Keys via Key ID -------------------------- - -In all commands that let the user type a user ID or fragment of a -user ID to select a key, the hexadecimal key ID may be used instead. -Just use the key ID, with a prefix of "0x", in place of the user ID. -For example: - - pgp -kv 0x67F7 - -This would display all keys that had 67F7 as part of their key IDs. - -This feature is particularly useful if you have two different keys -from the same person, with the same user ID. You can unambiguously -pick which key you want by specifying the key ID. - - -Separating Signatures from Messages ------------------------------------ - -Normally, signature certificates are physically attached to the text -they sign. This makes it convenient in simple cases to check -signatures. It is desirable in some circumstances to have signature -certificates stored separately from the messages they sign. It is -possible to generate signature certificates that are detached from -the text they sign. To do this, combine the 'b' (break) option with -the 's' (sign) option. For example: - - pgp -sb letter.txt - -This example produces an isolated signature certificate in a file -called "letter.sig". The contents of letter.txt are not appended to -the signature certificate. - -After creating the signature certificate file (letter.sig in the -above example), send it along with the original text file to the -recipient. The recipient must have both files to check the signature -integrity. When the recipient attempts to process the signature -file, PGP notices that there is no text in the same file with the -signature and prompts the user for the filename of the text. Only -then can PGP properly check the signature integrity. If the -recipient knows in advance that the signature is detached from the -text file, she can specify both filenames on the command line: - - pgp letter.sig letter.txt -or: pgp letter letter.txt - -PGP will not have to prompt for the text file name in this case. - -A detached signature certificate is useful if you want to keep the -signature certificate in a separate certificate log. A detached -signature of an executable program is also useful for detecting a -subsequent virus infection. It is also useful if more than one party -must sign a document such as a legal contract, without nesting -signatures. Each person's signature is independent. - -If you receive a ciphertext file that has the signature certificate -glued to the message, you can still pry the signature certificate -away from the message during the decryption. You can do this with -the -b option during decrypt, like so: - - pgp -b letter - -This decrypts the letter.pgp file and if there is a signature in it, -PGP checks the signature and detaches it from the rest of the -message, storing it in the file letter.sig. - - -Decrypting the Message and Leaving the Signature on it ------------------------------------------------------- - -Usually, you want PGP to completely unravel a ciphertext file, -decrypting it and checking the nested signature if there is one, -peeling away the layers until you are left with only the original -plaintext file. - -But sometimes you want to decrypt an encrypted file, and leave the -inner signature still attached, so that you are left with a decrypted -signed message. This may be useful if you want to send a copy of a -signed document to a third party, perhaps re-enciphering it. For -example, suppose you get a message signed by Charlie, encrypted to -you. You want to decrypt it, and, leaving Charlie's signature on it, -you want to send it to Alice, perhaps re-enciphering it with Alice's -public key. No problem. PGP can handle that. - -To simply decrypt a message and leave the signature on it intact, -type: - - pgp -d letter - -This decrypts letter.pgp, and if there is an inner signature, it is -left intact with the decrypted plaintext in the output file. - -Now you can archive it, or maybe re-encrypt it and send it to someone -else. - - - -Sending ASCII Text Files Across Different Machine Environments --------------------------------------------------------------- - -You may use PGP to encrypt any kind of plaintext file, binary 8-bit -data or ASCII text. Probably the most common usage of PGP will be for -E-mail, when the plaintext is ASCII text. - -ASCII text is sometimes represented differently on different -machines. For example, on an MSDOS system, all lines of ASCII text -are terminated with a carriage return followed by a linefeed. On a -Unix system, all lines end with just a linefeed. On a Macintosh, all -lines end with just a carriage return. This is a sad fact of life. - -Normal unencrypted ASCII text messages are often automatically -translated to some common "canonical" form when they are transmitted -from one machine to another. Canonical text has a carriage return -and a linefeed at the end of each line of text. For example, the -popular KERMIT communication protocol can convert text to canonical -form when transmitting it to another system. This gets converted -back to local text line terminators by the receiving KERMIT. This -makes it easy to share text files across different systems. - -But encrypted text cannot be automatically converted by a -communication protocol, because the plaintext is hidden by -encipherment. To remedy this inconvenience, PGP lets you specify -that the plaintext should be treated as ASCII text (not binary data) -and should be converted to canonical text form before it gets -encrypted. At the receiving end, the decrypted plaintext is -automatically converted back to whatever text form is appropriate for -the local environment. - -To make PGP assume the plaintext is text that should be converted to -canonical text before encryption, just add the "t" option when -encrypting or signing a message, like so: - - pgp -et message.txt her_userid - -This mode is automatically turned off if PGP detects that the -plaintext file contains what it thinks is non-text binary data. - -For PGP users that use non-English 8-bit character sets, when PGP -converts text to canonical form, it may convert data from the local -character set into the LATIN1 (ISO 8859-1 Latin Alphabet 1) character -set, depending on the setting of the CHARSET parameter in the PGP -configuration file. LATIN1 is a superset of ASCII, with extra -characters added for many European languages. - - - -Leaving No Traces of Plaintext on the Disk ------------------------------------------- - -After PGP makes a ciphertext file for you, you can have PGP -automatically overwrite the plaintext file and delete it, leaving no -trace of plaintext on the disk so that no one can recover it later -using a disk block scanning utility. This is useful if the plaintext -file contains sensitive information that you don't want to keep -around. - -To wipe out the plaintext file after producing the ciphertext file, -just add the "w" (wipe) option when encrypting or signing a message, -like so: - - pgp -esw message.txt her_userid - -This example creates the ciphertext file "message.pgp", and the -plaintext file "message.txt" is destroyed beyond recovery. - -Obviously, you should be careful with this option. Also note that -this will not wipe out any fragments of plaintext that your word -processor might have created on the disk while you were editing the -message before running PGP. Most word processors create backup -files, scratch files, or both. Also, it overwrites the file only -once, which is enough to thwart conventional disk recovery efforts, -but not enough to withstand a determined and sophisticated effort to -recover the faint magnetic traces of the data using special disk -recovery hardware. - - - -Displaying Decrypted Plaintext on Your Screen ---------------------------------------------- - -To view the decrypted plaintext output on your screen (like the -Unix-style "more" command), without writing it to a file, use the -m -(more) option while decrypting: - - pgp -m ciphertextfile - -This displays the decrypted plaintext display on your screen one -screenful at a time. - - - -Making a Message For Her Eyes Only ----------------------------------- - -To specify that the recipient's decrypted plaintext will be shown -ONLY on her screen and cannot be saved to disk, add the -m option: - - pgp -sem message.txt her_userid - -Later, when the recipient decrypts the ciphertext with her secret key -and pass phrase, the plaintext will be displayed on her screen but -will not be saved to disk. The text will be displayed as it would if -she used the Unix "more" command, one screenful at a time. If she -wants to read the message again, she will have to decrypt the -ciphertext again. - -This feature is the safest way for you to prevent your sensitive -message from being inadvertently left on the recipient's disk. This -feature was added at the request of a user who wanted to send -intimate messages to his lover, but was afraid she might accidentally -leave the decrypted messages on her husband's computer. - - - -Preserving the Original Plaintext Filename ------------------------------------------- - -Normally, PGP names the decrypted plaintext output file with a name -similar to the input ciphertext filename, but dropping the -extension. Or, you can override that convention by specifying an -output plaintext filename on the command line with the -o option. -For most E-mail, this is a reasonable way to name the plaintext file, -because you get to decide its name when you decipher it, and your -typical E-mail messages often come from useless original plaintext -filenames like "to_phil.txt". - -But when PGP encrypts a plaintext file, it always saves the original -filename and attaches it to the plaintext before it compresses and -encrypts the plaintext. Normally, this hidden original filename is -discarded by PGP when it decrypts, but you can tell PGP you want to -preserve the original plaintext filename and use it as the name of -the decrypted plaintext output file. This is useful if PGP is used -on files whose names are important to preserve. - -To recover the original plaintext filename while decrypting, add -the -p option, like so: - - pgp -p ciphertextfile - -I usually don't use this option, because if I did, about half of my -incoming E-mail would decrypt to the same plaintext filenames of -"to_phil.txt" or "prz.txt". - - - -Editing Your User ID or Pass Phrase ------------------------------------ - -Sometimes you may need to change your pass phrase, perhaps because -someone looked over your shoulder while you typed it in. - -Or you may need to change your user ID, because you got married and -changed your name, or maybe you changed your E-mail address. Or -maybe you want to add a second or third user ID to your key, because -you may be known by more than one name or E-mail address or job -title. PGP lets you attach more than one user ID to your key, any -one of which may be used to look up your key on the key ring. - -To edit your own userid or pass phrase for your secret key: - - pgp -ke userid [keyring] - -PGP prompts you for a new user ID or a new pass phrase. - -The optional [keyring] parameter, if specified, must be a public -keyring, not a secret keyring. The userid field must be your own -userid, which PGP knows is yours because it appears on both your -public keyring and your secret keyring. Both keyrings will be -updated, even though you only specified the public keyring. - - - -Editing the Trust Parameters for a Public Key ---------------------------------------------- - -Sometimes you need to alter the trust parameters for a public key on -your public key ring. For a discussion on what these trust -parameters mean, see the section "How Does PGP Keep Track of Which -Keys are Valid?" in the Essential Topics volume of the PGP User's -Guide. - -To edit the trust parameters for a public key: - - pgp -ke userid [keyring] - -The optional [keyring] parameter, if specified, must be a public -keyring, not a secret keyring. - - - -Checking If Everything is OK on Your Public Key Ring ----------------------------------------------------- - -Normally, PGP automatically checks any new keys or signatures on your -public key ring and updates all the trust parameters and validity -scores. In theory, it keeps all the key validity status information -up to date as material is added to or deleted from your public key -ring. But perhaps you may want to explicitly force PGP to perform a -comprehensive analysis of your public key ring, checking all the -certifying signatures, checking the trust parameters, updating all -the validity scores, and checking your own ultimately-trusted key -against a backup copy on a write-protected floppy disk. It may be a -good idea to do this hygienic maintenance periodically to make sure -nothing is wrong with your public key ring. To force PGP to perform -a full analysis of your public key ring, use the -kc (key ring check) -command: - - pgp -kc - -You can also make PGP check all the signatures for just a single -selected public key by: - - pgp -kc userid [keyring] - -For further information on how the backup copy of your own key is -checked, see the description of the BAKRING parameter in the -configuration file section of this manual. - - - -Verifying a Public Key Over the Phone -------------------------------------- - -If you get a public key from someone that is not certified by anyone -you trust, how can you tell if it's really their key? The best way -to verify an uncertified key is to verify it over some independent -channel other than the one you received the key through. One -convenient way to tell, if you know this person and would recognize -them on the phone, is to call them and verify their key over the -telephone. Rather than reading their whole tiresome (ASCII-armored) -key to them over the phone, you can just read their key's -"fingerprint" to them. To see this fingerprint, use the -kvc -command: - - pgp -kvc userid [keyring] - -This will display the key with the 16-byte digest of the public key -components. Read this 16-byte fingerprint to the key's owner on the -phone, while she checks it against her own, using the same -kvc -command at her end. - -You can both verify each other's keys this way, and then you can sign -each other's keys with confidence. This is a safe and convenient way -to get the key trust network started for your circle of friends. - - - -Using PGP as a Unix-style Filter --------------------------------- - -Unix fans are accustomed to using Unix "pipes" to make two -applications work together. The output of one application can be -directly fed through a pipe to be read as input to another -application. For this to work, the applications must be capable of -reading the raw material from "standard input" and writing the -finished output to "standard output". PGP can operate in this mode. -If you don't understand what this means, then you probably don't need -this feature. - -To use a Unix-style filter mode, reading from standard input and -writing to standard output, add the -f option, like so: - - pgp -feast her_userid outputfile - -This feature makes it easier to make PGP work with electronic mail -applications. - -When using PGP in filter mode to decrypt a ciphertext file, you may -find it useful to use the PGPPASS environmental variable to hold the -pass phrase, so that you won't be prompted for it. The PGPPASS -feature is explained below. - - - -Suppressing Unneccessary Questions: BATCHMODE ----------------------------------------------- - -With the BATCHMODE flag enabled on the command line, PGP will not ask -any unneccessary questions or prompt for alternate filenames. Here -is an example of how to set this flag: - - pgp +batchmode cipherfile - -This is useful for running PGP non-interactively from Unix shell -scripts or MSDOS batch files. Some key management commands still -need user interaction even when BATCHMODE is on, so shell scripts may -need to avoid them. - -BATCHMODE may also be enabled to check the validity of a signature on -a file. If there was no signature on the file, the exit code is 1. -If it had a signature that was good, the exit code is 0. - - -Force "Yes" Answer to Confirmation Questions: FORCE ----------------------------------------------------- - -This command-line flag makes PGP assume "yes" for the user response -to the confirmation request to overwrite an existing file, or when -removing a key from the keyring via the -kr command. Here is an -example of how to set this flag: - - pgp +force cipherfile -or: - pgp -kr +force Smith - -This feature is useful for running PGP non-interactively from a Unix -shell script or MSDOS batch file. - - - -PGP Returns Exit Status to the Shell ------------------------------------- - -To facilitate running PGP in "batch" mode, such as from an MSDOS -".bat" file or from a Unix shell script, PGP returns an error exit -status to the shell. An exit status code of zero means normal exit, -while a nonzero exit status indicates some kind of error occurred. -Different error exit conditions return different exit status codes to -the shell. - - - -Environmental Variable for Pass Phrase --------------------------------------- - -Normally, PGP prompts the user to type a pass phrase whenever PGP -needs a pass phrase to unlock a secret key. But it is possible to -store the pass phrase in an environmental variable from your -operating system's command shell. The environmental variable PGPPASS -can be used to hold the pass phrase that PGP will attempt to use -first. If the pass phrase stored in PGPPASS is incorrect, PGP -recovers by prompting the user for the correct pass phrase. - -For example, on MSDOS, the shell command: - - SET PGPPASS=zaphod beeblebrox for president - -would eliminate the prompt for the pass phrase if the pass phrase -were indeed "zaphod beeblebrox for president". - -This dangerous feature makes your life more convenient if you have to -regularly deal with a large number of incoming messages addressed to -your secret key, by eliminating the need for you to repeatedly type -in your pass phrase every time you run PGP. - -I added this feature because of popular demand. However, this is a -somewhat dangerous feature, because it keeps your precious pass -phrase stored somewhere other than just in your brain. Even worse, -if you are particularly reckless, it may even be stored on a disk on -the same computer as your secret key. It would be particularly -dangerous and stupid if you were to install this command in a batch -or script file, such as the MSDOS AUTOEXEC.BAT file. Someone could -come along on your lunch hour and steal both your secret key ring and -the file containing your pass phrase. - -I can't emphasize the importance of this risk enough. If you are -contemplating using this feature, be sure to read the sections -"Exposure on Multi-user Systems" and "How to Protect Secret Keys from -Disclosure" in this volume and in the Essential Topics volume of the -PGP User's Guide. - -If you must use this feature, the safest way to do it would be to -just manually type in the shell command to set PGPPASS every time you -boot your machine to start using PGP, and then erase it or turn off -your machine when you are done. And you should definitely never do -it in an environment where someone else may have access to your -machine. Someone could come along and simply ask your computer to -display the contents of PGPPASS. - - - -Setting Configuration Parameters: CONFIG.TXT -============================================ - -PGP has a number of user-settable parameters that can be defined in a -special configuration text file called "config.txt", in the directory -pointed to by the shell environmental variable PGPPATH. Having a -configuration file enables the user to define various flags and -parameters for PGP without the burden of having to always define -these parameters in the PGP command line. - -Configuration parameters may be assigned integer values, character -string values, or on/off values, depending on what kind of -configuration parameter it is. A sample configuration file is -provided with PGP, so you can see some examples. - -In the configuration file, blank lines are ignored, as is anything -following the '#' comment character. Keywords are not -case-sensitive. - -Here is a short sample fragment of a typical configuration file: - - # TMP is the directory for PGP scratch files, such as a RAM disk. - TMP = "e:\" # Can be overridden by environment variable TMP. - Armor = on # Use -a flag for ASCII armor whenever applicable. - # CERT_DEPTH is how deeply introducers may introduce introducers. - cert_depth = 3 - -If some configuration parameters are not defined in the configuration -file, or if there is no configuration file, or if PGP can't find the -configuration file, the values for the configuration parameters -default to some reasonable value. - -Note that it is also possible to set these same configuration -parameters directly from the PGP command line, by preceding the -parameter setting with a "+" character. For example, the following -two PGP commands produce the same effect: - - pgp -e +armor=on message.txt smith -or: pgp -ea message.txt smith - - -The following is a summary of the various parameters than may be -defined in the configuration file. - - -TMP - Directory Pathname for Temporary Files --------------------------------------------- - -Default setting: TMP = "" - -The configuration parameter TMP specifies what directory to use for -PGP's temporary scratch files. The best place to put them is on a -RAM disk, if you have one. That speeds things up quite a bit, and -increases security somewhat. If TMP is undefined, the temporary -files go in the current directory. If the shell environmental -variable TMP is defined, PGP instead uses that to specify where the -temporary files should go. - - -LANGUAGE - Foreign Language Selector ------------------------------------- - -Default setting: LANGUAGE = "en" - -PGP displays various prompts, warning messages, and advisories to the -user on the screen. For example, messages such as "File not found.", -or "Please enter your pass phrase:". These messages are normally in -English. But it is possible to get PGP to display its messages to -the user in other languages, without having to modify the PGP -executable program. - -A number of people in various countries have translated all of PGP's -display messages, warnings, and prompts into their native languages. -These hundreds of translated message strings have been placed in a -special text file called "language.txt", distributed with the PGP -release. The messages are stored in this file in English, Spanish, -Dutch, German, French, Italian, Russian, Latvian, and Lithuanian. -Other languages may be added later. - -The configuration parameter LANGUAGE specifies what language to -display these messages in. LANGUAGE may be set to "en" for English, -"es" for Spanish, "de" for German, "nl" for Dutch, "fr" for French, -"it" for Italian, "ru" for Russian, "lt3" for Lithuanian, "lv" for -Latvian, "esp" for Esperanto. For example, if this line appeared in -the configuration file: - - LANGUAGE = "fr" - -PGP would select French as the language for its display messages. -The default setting is English. - -When PGP needs to display a message to the user, it looks in the -"language.txt" file for the equivalent message string in the selected -foreign language and displays that translated message to the user. -If PGP can't find the language string file, or if the selected -language is not in the file, or if that one phrase is not translated -into the selected language in the file, or if that phrase is missing -entirely from the file, PGP displays the message in English. - -To conserve disk space, most foreign translations are not included -in the standard PGP release package, but are available separately. - - -MYNAME - Default User ID for Making Signatures ----------------------------------------------- - -Default setting: MYNAME = "" - -The configuration parameter MYNAME specifies the default user ID to -use to select the secret key for making signatures. If MYNAME is not -defined, the most recent secret key you installed on your secret key -ring will be used. The user may also override this setting by -specifying a user ID on the PGP command line with the -u option. - - -TEXTMODE - Assuming Plaintext is a Text File --------------------------------------------- - -Default setting: TEXTMODE = off - -The configuration parameter TEXTMODE is equivalent to the -t command -line option. If enabled, it causes PGP to assume the plaintext is a -text file, not a binary file, and converts it to "canonical text" -before encrypting it. Canonical text has a carriage return and a -linefeed at the end of each line of text. - -This mode will be automatically turned off if PGP detects that the -plaintext file contains what it thinks is non-text binary data. - -For VAX/VMS systems, the current version of PGP defaults TEXTMODE=ON. - -For further details, see the section "Sending ASCII Text Files Across -Different Machine Environments". - - -CHARSET - Specifies Local Character Set for Text Files ------------------------------------------------------- - -Default setting: CHARSET = NOCONV - -Because PGP must process messages in many non-English languages with -non-ASCII character sets, you may have a need to tell PGP what local -character set your machine uses. This determines what character -conversions are performed when converting plaintext files to and from -canonical text format. This is only a concern if you are in a -non-English non-ASCII environment. - -The configuration parameter CHARSET selects the local character set. -The choices are NOCONV (no conversion), LATIN1 (ISO 8859-1 Latin -Alphabet 1), KOI8 (used by most Russian Unix systems), ALT_CODES -(used by Russian MSDOS systems), ASCII, and CP850 (used by most -western European languages on standard MSDOS PCs). - -LATIN1 is the internal representation used by PGP for canonical text, -so if you select LATIN1, no conversion is done. Note also that PGP -treats KOI8 as LATIN1, even though it is a completely different -character set (Russian), because trying to convert KOI8 to either -LATIN1 or CP850 would be futile anyway. This means that setting -CHARSET to NOCONV, LATIN1, or KOI8 are all equivalent to PGP. - -If you use MSDOS and expect to send or receive traffic in western -European languages, set CHARSET = "CP850". This will make PGP -convert incoming canonical text messages from LATIN1 to CP850 after -decryption. If you use the -t (textmode) option to convert to -canonical text, PGP will convert your CP850 text to LATIN1 before -encrypting it. - -For further details, see the section "Sending ASCII Text Files Across -Different Machine Environments". - - -ARMOR - Enable ASCII Armor Output ---------------------------------- - -Default setting: ARMOR = off - -The configuration parameter ARMOR is equivalent to the -a command -line option. If enabled, it causes PGP to emit ciphertext or keys in -ASCII Radix-64 format suitable for transporting through E-mail -channels. Output files are named with the ".asc" extension. - -If you tend to use PGP mostly for E-mail, it may be a good idea to -enable this parameter. - -For further details, see the section "Sending Ciphertext Through -E-mail Channels: Radix-64 Format" in the Essential Topics volume. - - -ARMORLINES - Size of ASCII Armor Multipart Files ------------------------------------------------- - -Default setting: ARMORLINES = 720 - -When PGP creates a very large ".asc" radix-64 file for sending -ciphertext or keys through the E-mail, it breaks the file up into -separate chunks small enough to send through Internet mail -utilities. Normally, Internet mailers prohibit files larger than -about 50000 bytes, which means that if we restrict the number of -lines to about 720, we'll be well within the limit. The file chunks -are named with suffixes ".as1", ".as2", ".as3", ... - -The configuration parameter ARMORLINES specifies the maximum number -of lines to make each chunk in a multipart ".asc" file sequence. If -you set it to zero, PGP will not break up the file into chunks. - -Fidonet email files usually have an upper limit of about 32K bytes, -so 450 lines would be appropriate for Fidonet environments. - -For further details, see the section "Sending Ciphertext Through -E-mail Channels: Radix-64 Format" in the Essential Topics volume. - - -KEEPBINARY - Keep Binary Ciphertext Files After Decrypting ----------------------------------------------------------- - -Default setting: KEEPBINARY = off - -When PGP reads a ".asc" file, it recognizes that the file is in -radix-64 format and will convert it back to binary before processing -as it normally does, producing as a by-product a ".pgp" ciphertext -file in binary form. After further processing to decrypt the ".pgp" -file, the final output file will be in normal plaintext form. - -You may want to delete the binary ".pgp" intermediate file, or you -may want PGP to delete it for you automatically. You can still rerun -PGP on the original ".asc" file. - -The configuration parameter KEEPBINARY enables or disables keeping -the intermediate ".pgp" file during decryption. - -For further details, see the section "Sending Ciphertext Through -E-mail Channels: Radix-64 Format" in the Essential Topics volume. - - -COMPRESS - Enable Compression ------------------------------ - -Default setting: COMPRESS = on - -The configuration parameter COMPRESS enables or disables data -compression before encryption. It is used mainly for debugging PGP. -Normally, PGP attempts to compress the plaintext before it encrypts -it. Generally, you should leave this alone and let PGP attempt to -compress the plaintext. - - -COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed ------------------------------------------------------------------- - -Default setting: COMPLETES_NEEDED = 1 - -The configuration parameter COMPLETES_NEEDED specifies the minimum -number of completely trusted introducers required to fully certify a -public key on your public key ring. This gives you a way of tuning -PGP's skepticism. - -For further details, see the section "How Does PGP Keep Track of -Which Keys are Valid?" in the Essential Topics volume. - - -MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed ------------------------------------------------------------------- - -Default setting: MARGINALS_NEEDED = 2 - -The configuration parameter MARGINALS_NEEDED specifies the minimum -number of marginally trusted introducers required to fully certify a -public key on your public key ring. This gives you a way of tuning -PGP's skepticism. - -For further details, see the section "How Does PGP Keep Track of -Which Keys are Valid?" in the Essential Topics volume. - - -CERT_DEPTH - How Deep May Introducers Be Nested ------------------------------------------------ - -Default setting: CERT_DEPTH = 4 - -The configuration parameter CERT_DEPTH specifies how many levels deep -you may nest introducers to certify other introducers to certify -public keys on your public key ring. For example, If CERT_DEPTH is -set to 1, there may only be one layer of introducers below your own -ultimately-trusted key. If that were the case, you would be required -to directly certify the public keys of all trusted introducers on -your key ring. If you set CERT_DEPTH to 0, you could have no -introducers at all, and you would have to directly certify each and -every key on your public key ring in order to use it. The minimum -CERT_DEPTH is 0, the maximum is 8. - -For further details, see the section "How Does PGP Keep Track of -Which Keys are Valid?" in the Essential Topics volume. - - -BAKRING - Filename for Backup Secret Keyring --------------------------------------------- - -Default setting: BAKRING = "" - -All of the key certification that PGP does on your public key ring -ultimately depends on your own ultimately-trusted public key (or -keys). To detect any tampering of your public key ring, PGP must -check that your own key has not been tampered with. To do this, PGP -must compare your public key against a backup copy of your secret key -on some tamper-resistant media, such as a write-protected floppy -disk. A secret key contains all the information that your public key -has, plus some secret components. This means PGP can check your -public key against a backup copy of your secret key. - -The configuration parameter BAKRING specifies what pathname to use -for PGP's trusted backup copy of your secret key ring. On MSDOS, you -could set it to "a:\secring.pgp" to point it at a write-protected -backup copy of your secret key ring on your floppy drive. This check -is performed only when you execute the PGP -kc option to check your -whole public key ring. - -If BAKRING is not defined, PGP will not check your own key against -any backup copy. - -For further details, see the sections "How to Protect Public Keys -from Tampering" and "How Does PGP Keep Track of Which Keys are -Valid?" in the Essential Topics volume. - - -PAGER - Selects Shell Command to Display Plaintext Output ---------------------------------------------------------- - -Default setting: PAGER = "" - -PGP lets you view the decrypted plaintext output on your screen (like -the Unix-style "more" command), without writing it to a file, if you -use the -m (more) option while decrypting. This displays the -decrypted plaintext display on your screen one screenful at a time. - -If you prefer to use a fancier page display utility, rather than -PGP's built-in one, you can specify the name of a shell command that -PGP will invoke to display your plaintext output file. The -configuration parameter PAGER specifies the shell command to invoke -to display the file. For example, on MSDOS systems, you might want -to use the popular shareware program "list.com" to display your -plaintext message. Assuming you have a copy of "list.com", you may -set PAGER accordingly: - - PAGER = "list" - -However, if the sender specified that this file is for your eyes -only, and may not be written to disk, PGP always uses its own -built-in display function. - -For further details, see the section "Displaying Decrypted Plaintext -on Your Screen". - - -SHOWPASS - Echo Pass Phrase to User ------------------------------------ - -Default setting: SHOWPASS = off - -Normally, PGP does not let you see your pass phrase as you type it -in. This makes it harder for someone to look over your shoulder -while you type and learn your pass phrase. But some typing-impaired -people have problems typing their pass phrase without seeing what -they are typing, and they may be typing in the privacy of their own -homes. So they asked if PGP can be configured to let them see what -they type when they type in their pass phrase. - -The configuration parameter SHOWPASS enables PGP to echo your typing -during pass phrase entry. - - -TZFIX - Timezone Adjustment ---------------------------- - -Default setting: TZFIX = 0 - -PGP provides timestamps for keys and signature certificates in -Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC), which -means the same thing for our purposes. When PGP asks the system for -the time of day, the system is supposed to provide it in GMT. - -But sometimes, because of improperly configured MSDOS systems, the -system time is returned in US Pacific Standard Time time plus 8 -hours. Sounds weird, doesn't it? Perhaps because of some sort of US -west-coast jingoism, MSDOS presumes local time is US Pacific time, -and pre-corrects Pacific time to GMT. This adversely affects the -behavior of the internal MSDOS GMT time function that PGP calls. -However, if your MSDOS environmental variable TZ is already properly -defined for your timezone, this corrects the misconception MSDOS has -that the whole world lives on the US west coast. - -The configuration parameter TZFIX specifies the number of hours to -add to the system time function to get GMT, for GMT timestamps on -keys and signatures. If the MSDOS environmental variable TZ is -defined properly, you can leave TZFIX=0. Unix systems usually -shouldn't need to worry about setting TZFIX at all. But if you are -using some other obscure operating system that doesn't know about -GMT, you may have to use TZFIX to adjust the system time to GMT. - -On MSDOS systems that do not have TZ defined in the environment, you -should make TZFIX=0 for California, -1 for Colorado, -2 for Chicago, --3 for New York, -8 for London, -9 for Amsterdam. In the summer, -TZFIX should be manually decremented from these values. What a mess. - -It would be much cleaner to set your MSDOS environmental variable TZ -in your AUTOEXEC.BAT file, and not use the TZFIX correction. Then -MSDOS gives you good GMT timestamps, and will handle daylight savings -time adjustments for you. Here are some sample lines to insert into -AUTOEXEC.BAT, depending on your time zone: - -For Los Angeles: SET TZ=PST8PDT -For Denver: SET TZ=MST7MDT -For Arizona: SET TZ=MST7 - (Arizona never uses daylight savings time) -For Chicago: SET TZ=CST6CDT -For New York: SET TZ=EST5EDT -For London: SET TZ=GMT0BST -For Amsterdam: SET TZ=MET-1DST -For Moscow: SET TZ=MSK-3MSD -For Aukland: SET TZ=NZT-13 - - -CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text ------------------------------------------------------------------- - -Default setting: CLEARSIG = off - -Normally, unencrypted PGP signed messages have a signature -certificate prepended in binary form. To send this through a 7-bit -E-mail channel, radix-64 ASCII armor is applied (see the ARMOR -parameter), rendering the message unreadable to casual human eyes, -even though the message is not actually encrypted. The recipient -must use PGP to strip the armor off before reading the message. - -If the original plaintext message is in text (not binary) form, there -is a way to send it through an E-mail channel in such a way that the -ASCII armor is applied only to the binary signature certificate, but -not to the plaintext message. This makes it possible to read the -signed message with human eyes, without the aid of PGP. Of course, -you still need PGP to actually check the signature. - -To enable this feature, set CLEARSIG=ON, and set ARMOR=ON (or use -the -a option), and set TEXTMODE=ON (or use the -t option). For -example, you can set CLEARSIG directly from the command line: - - pgp -sta +clearsig=on message.txt - -This message representation is analogous to the MIC-CLEAR message type -used in Internet Privacy Enhanced Mail (PEM). It is important to -note that since this method only applies ASCII armor to the binary -signature certificate, and not to the message text itself, there is -some risk that the unarmored message may suffer some accidental -molestation while en route. This can happen if it passes through -some E-mail gateway that performs character set conversions, or in -some cases extra spaces may be added to or stripped from the ends of -lines. If this occurs, the signature will fail to verify, which may -give a false indication of intentional tampering. But since PEM -lives under a similar vulnerability, it seems worth having this -feature despite the risks. - -Beginning with PGP version 2.2, trailing blanks are ignored on each -line in calculating the signature for text in CLEARSIG mode. - - -VERBOSE - Quiet, Normal, or Verbose Messages --------------------------------------------- - -Default setting: VERBOSE = 1 - -VERBOSE may be set to 0, 1, or 2, depending on how much detail you -want to see from PGP diagnostic messages. The settings are: - -0 - Display messages only if there is a problem. Unix fans wanted -this "quiet mode" setting. - -1 - Normal default setting. Displays a reasonable amount of detail -in diagnostic or advisory messages. - -2 - Displays maximum information, usually to help diagnose problems -in PGP. Not recommended for normal use. Besides, PGP doesn't have -any problems, right? - - -INTERACTIVE - Ask for Confirmation for Key Adds ------------------------------------------------ - -Default Setting: INTERACTIVE = off - -Enabling this mode will mean that if you add a key file containing -multiple keys to your key ring, PGP will ask for confirmation for -each key before adding it to your key ring. - - - -Protecting Against Bogus Timestamps -=================================== - -A somewhat obscure vulnerability of PGP involves dishonest users -creating bogus timestamps on their own public key certificates and -signatures. You can skip over this section if you are a casual user -and aren't deeply into obscure public key protocols. - -There's nothing to stop a dishonest user from altering the date and -time setting of his own system's clock, and generating his own public -key certificates and signatures that appear to have been created at a -different time. He can make it appear that he signed something -earlier or later than he actually did, or that his public/secret key -pair was created earlier or later. This may have some legal or -financial benefit to him, for example by creating some kind of -loophole that might allow him to repudiate a signature. - -A remedy for this could involve some trustworthy Certifying Authority -or notary that would create notarized signatures with a trustworthy -timestamp. This might not necessarily require a centralized -authority. Perhaps any trusted introducer or disinterested party -could serve this function, the same way real notary publics do now. -A public key certificate could be signed by the notary, and the -trusted timestamp in the notary's signature would have some legal -significance. The notary could enter the signed certificate into a -special certificate log controlled by the notary. Anyone can read -this log. - -The notary could also sign other people's signatures, creating a -signature certificate of a signature certificate. This would serve -as a witness to the signature the same way real notaries do now with -paper. Again, the notary could enter the detached signature -certificate (without the actual whole document that was signed) into -a log controlled by the notary. The notary's signature would have a -trusted timestamp, which might have greater credibility than the -timestamp in the original signature. A signature becomes "legal" if -it is signed and logged by the notary. - -This problem of certifying signatures with notaries and trusted -timestamps warrants further discussion. This can of worms will not -be fully covered here now. There is a good treatment of this topic -in Denning's 1983 article in IEEE Computer (see references). There -is much more detail to be worked out in these various certifying -schemes. This will develop further as PGP usage increases and other -public key products develop their own certifying schemes. - - -A Peek Under the Hood -===================== - -Let's take a look at a few internal features of PGP. - - -Random Numbers --------------- - -PGP uses a cryptographically strong pseudorandom number generator for -creating temporary conventional session keys. The seed file for this -is called "randseed.bin". It too can be kept in whatever directory -is indicated by the PGPPATH environmental variable. If this random -seed file does not exist, it is automatically created and seeded with -truly random numbers derived from timing your keystroke latencies. - -This generator reseeds the disk file each time it is used by mixing -in new key material partially derived with the time of day and other -truly random sources. It uses the conventional encryption algorithm -as an engine for the random number generator. The seed file contains -both random seed material and random key material to key the -conventional encryption engine for the random generator. - -This random seed file should be at least slightly protected from -disclosure, to reduce the risk of an attacker deriving your next or -previous session keys. The attacker would have a very hard time -getting anything useful from capturing this random seed file, because -the file is cryptographically laundered before and after each use. -Nonetheless, it seems prudent to at least try to keep it from falling -into the wrong hands. - -If you feel uneasy about trusting any algorithmically derived random -number source however strong, keep in mind that you already trust the -strength of the same conventional cipher to protect your messages. -If it's strong enough for that, then it should be strong enough to -use as a source of random numbers for temporary session keys. Note -that PGP still uses truly random numbers from physical sources -(mainly keyboard timings) to generate long-term public/secret key -pairs. - - - -PGP's Conventional Encryption Algorithm ---------------------------------------- - -As described earlier, PGP "bootstraps" into a conventional single-key -encryption algorithm by using a public key algorithm to encipher the -conventional session key and then switching to fast conventional -cryptography. So let's talk about this conventional encryption -algorithm. It isn't the DES. - -The Federal Data Encryption Standard (DES) is a good algorithm for -most commercial applications. However, the Government does not trust -the DES to protect its own classified data. Perhaps this is because -the DES key length is 56 bits, short enough for a brute force attack -with a special purpose machine built from massive numbers of DES -chips. Also, Biham and Shamir have had some success recently on -attacking the full 16-round DES. - -PGP does not use the DES as its conventional single-key algorithm to -encrypt messages. Instead, PGP uses a different conventional -single-key block encryption algorithm, called IDEA(tm). A future -version of PGP may support the DES as an option, if enough users -ask for it. But I suspect IDEA is better than DES. - -For the cryptographically curious, the IDEA cipher has a 64-bit block -size for the plaintext and the ciphertext. It uses a key size of 128 -bits. It is based on the design concept of "mixing operations from -different algebraic groups". It runs much faster in software than -the DES. Like the DES, it can be used in cipher feedback (CFB) and -cipher block chaining (CBC) modes. PGP uses it in 64-bit CFB mode. - -The IPES/IDEA block cipher was developed at ETH in Zurich by James L. -Massey and Xuejia Lai, and published in 1990. This is not a -"home-grown" algorithm. Its designers have a distinguished -reputation in the cryptologic community. Early published papers on -the algorithm called it IPES (Improved Proposed Encryption Standard), -but they later changed the name to IDEA (International Data -Encryption Algorithm). So far, IDEA has resisted attack much better -than other ciphers such as FEAL, REDOC-II, LOKI, Snefru and Khafre. -And preliminary evidence suggests that IDEA may be more resistant -than the DES to Biham & Shamir's highly successful differential -cryptanalysis attack. Biham and Shamir have been examining the IDEA -cipher for weaknesses, without success. Academic cryptanalyst groups -in Belgium, England, and Germany are also attempting to attack it, as -well as the military services from several European countries. As -this new cipher continues to attract attack efforts from the most -formidable quarters of the cryptanalytic world, confidence in IDEA is -growing with the passage of time. - -A famous hockey player once said, "I try to skate to where I think -the puck will be." A lot of people are starting to feel that the -days are numbered for the DES. I'm skating toward IDEA. - -It is not ergonomically practical to use pure RSA with large keys to -encrypt and decrypt long messages. Absolutely no one does it that way -in the real world. But perhaps you are concerned that the whole -package is weakened if we use a hybrid public-key and conventional -scheme just to speed things up. After all, a chain is only as strong -as its weakest link. Many people less experienced in cryptography -mistakenly believe that RSA is intrinsically stronger than any -conventional cipher. It's not. RSA can be made weak by using weak -keys, and conventional ciphers can be made strong by choosing good -algorithms. It's usually difficult to tell exactly how strong a good -conventional cipher is, without actually cracking it. A really good -conventional cipher might possibly be harder to crack than even a -"military grade" RSA key. The attraction of public key cryptography -is not because it is intrinsically stronger than a conventional -cipher-- its appeal is because it helps you manage keys more -conveniently. - - - -Data Compression ----------------- - -PGP normally compresses the plaintext before encrypting it. It's too -late to compress it after it has been encrypted; encrypted data is -incompressible. Data compression saves modem transmission time and -disk space and more importantly strengthens cryptographic security. -Most cryptanalysis techniques exploit redundancies found in the -plaintext to crack the cipher. Data compression reduces this -redundancy in the plaintext, thereby greatly enhancing resistance to -cryptanalysis. It takes extra time to compress the plaintext, but -from a security point of view it seems worth it, at least in my -cautious opinion. - -Files that are too short to compress or just don't compress well are -not compressed by PGP. - -If you prefer, you can use PKZIP to compress the plaintext before -encrypting it. PKZIP is a widely-available and effective MSDOS -shareware compression utility from PKWare, Inc. Or you can use ZIP, -a PKZIP-compatible freeware compression utility on Unix and other -systems, available from Jean-Loup Gailly. There is some advantage in -using PKZIP or ZIP in certain cases, because unlike PGP's built-in -compression algorithm, PKZIP and ZIP have the nice feature of -compressing multiple files into a single compressed file, which is -reconstituted again into separate files when decompressed. PGP will -not try to compress a plaintext file that has already been -compressed. After decrypting, the recipient can decompress the -plaintext with PKUNZIP. If the decrypted plaintext is a PKZIP -compressed file, PGP automatically recognizes this and advises the -recipient that the decrypted plaintext appears to be a PKZIP file. - -For the technically curious readers, the current version of PGP uses -the freeware ZIP compression routines written by Jean-loup Gailly, -Mark Adler, and Richard B. Wales. This ZIP software uses -functionally-equivalent compression algorithms as those used by -PKWare's new PKZIP 2.0. This ZIP compression software was selected -for PGP mainly because of its free portable C source code -availability, and because it has a really good compression ratio, and -because it's fast. - -Peter Gutmann has also written a nice compression utility called -HPACK, available for free from many Internet FTP sites. It encrypts -the compressed archives, using PGP data formats and key rings. He -wanted me to mention that here. - - - -Message Digests and Digital Signatures --------------------------------------- - -To create a digital signature, PGP encrypts with your secret key. -But PGP doesn't actually encrypt your entire message with your secret -key-- that would take too long. Instead, PGP encrypts a "message -digest". - -The message digest is a compact (128 bit) "distillate" of your -message, similar in concept to a checksum. You can also think of it -as a "fingerprint" of the message. The message digest "represents" -your message, such that if the message were altered in any way, a -different message digest would be computed from it. This makes it -possible to detect any changes made to the message by a forger. A -message digest is computed using a cryptographically strong one-way -hash function of the message. It would be computationally infeasible -for an attacker to devise a substitute message that would produce an -identical message digest. In that respect, a message digest is much -better than a checksum, because it is easy to devise a different -message that would produce the same checksum. But like a checksum, -you can't derive the original message from its message digest. - -A message digest alone is not enough to authenticate a message. The -message digest algorithm is publicly known, and does not require -knowledge of any secret keys to calculate. If all we did was attach -a message digest to a message, then a forger could alter a message -and simply attach a new message digest calculated from the new -altered message. To provide real authentication, the sender has to -encrypt (sign) the message digest with his secret key. - -A message digest is calculated from the message by the sender. The -sender's secret key is used to encrypt the message digest and an -electronic timestamp, forming a digital signature, or signature -certificate. The sender sends the digital signature along with the -message. The receiver receives the message and the digital -signature, and recovers the original message digest from the digital -signature by decrypting it with the sender's public key. The -receiver computes a new message digest from the message, and checks -to see if it matches the one recovered from the digital signature. If -it matches, then that proves the message was not altered, and it came -from the sender who owns the public key used to check the signature. - -A potential forger would have to either produce an altered message -that produces an identical message digest (which is infeasible), or -he would have to create a new digital signature from a different -message digest (also infeasible, without knowing the true sender's -secret key). - -Digital signatures prove who sent the message, and that the message -was not altered either by error or design. It also provides -non-repudiation, which means the sender cannot easily disavow his -signature on the message. - -Using message digests to form digital signatures has other advantages -besides being faster than directly signing the entire actual message -with the secret key. Using message digests allows signatures to be -of a standard small fixed size, regardless of the size of the actual -message. It also allows the software to check the message integrity -automatically, in a manner similar to using checksums. And it allows -signatures to be stored separately from messages, perhaps even in a -public archive, without revealing sensitive information about the -actual messages, because no one can derive any message content from a -message digest. - -The message digest algorithm used here is the MD5 Message Digest -Algorithm, placed in the public domain by RSA Data Security, Inc. -MD5's designer, Ronald Rivest, writes this about MD5: - -"It is conjectured that the difficulty of coming up with two messages -having the same message digest is on the order of 2^64 operations, -and that the difficulty of coming up with any message having a given -message digest is on the order of 2^128 operations. The MD5 -algorithm has been carefully scrutinized for weaknesses. It is, -however, a relatively new algorithm and further security analysis is -of course justified, as is the case with any new proposal of this -sort. The level of security provided by MD5 should be sufficient for -implementing very high security hybrid digital signature schemes -based on MD5 and the RSA public-key cryptosystem." - - - -Compatibility with Previous Versions of PGP -=========================================== - -I'm sorry, PGP version 2.0 is not compatible with PGP version 1.0. -If you have keys generated with version 1.0, you will have to -generate new keys to use with this version. This version of PGP uses -all new algorithms for conventional cryptography, compression, and -message digests, as well as using a much better approach to key -management. There were just too many changes to make it compatible -with the old format messages, signatures, and keys. Perhaps we could -have provided a special conversion utility to convert old keys into -new keys, but we were all tired and wanted to get the new release out -the door. Besides, converting the old keys into new keys would -probably create more problems than it would solve, because we have -changed to a new recommended uniform style for the user ID that -includes the full name and E-mail address in a particular syntax. - -There is compatibility from version 2.0 to higher versions. Because -new features are added, older versions may not always be able to -handle some files created with newer versions. - -We made some effort to design the internal data structures of this -version of PGP to be adaptable to future changes, so that hopefully -you will not be required to discard and regenerate your keys in future -versions. - - -Vulnerabilities -=============== - -No data security system is impenetrable. PGP can be circumvented in -a variety of ways. In any data security system, you have to ask -yourself if the information you are trying to protect is more -valuable to your attacker than the cost of the attack. This should -lead you to protecting yourself from the cheapest attacks, while not -worrying about the more expensive attacks. - -Some of the discussion that follows may seem unduly paranoid, but -such an attitude is appropriate for a reasonable discussion of -vulnerability issues. - - -Compromised Pass Phrase and Secret Key --------------------------------------- - -Probably the simplest attack is if you leave your pass phrase for -your secret key written down somewhere. If someone gets it and also -gets your secret key file, they can read your messages and make -signatures in your name. - -Don't use obvious passwords that can be easily guessed, such as the -names of your kids or spouse. If you make your pass phrase a single -word, it can be easily guessed by having a computer try all the words -in the dictionary until it finds your password. That's why a pass -phrase is so much better than a password. A more sophisticated -attacker may have his computer scan a book of famous quotations to -find your pass phrase. An easy to remember but hard to guess pass -phrase can be easily constructed by some creatively nonsensical -sayings or very obscure literary quotes. - -For further details, see the section "How to Protect Secret Keys from -Disclosure" in the Essential Topics volume of the PGP User's Guide. - - -Public Key Tampering --------------------- - -A major vulnerability exists if public keys are tampered with. This -may be the most crucially important vulnerability of a public key -cryptosystem, in part because most novices don't immediately -recognize it. The importance of this vulnerability, and appropriate -hygienic countermeasures, are detailed in the section "How to Protect -Public Keys from Tampering" in the Essential Topics volume. - -To summarize: When you use someone's public key, make certain it has -not been tampered with. A new public key from someone else should be -trusted only if you got it directly from its owner, or if it has been -signed by someone you trust. Make sure no one else can tamper with -your own public key ring. Maintain physical control of both your -public key ring and your secret key ring, preferably on your own -personal computer rather than on a remote timesharing system. Keep a -backup copy of both key rings. - - -"Not Quite Deleted" Files -------------------------- - -Another potential security problem is caused by how most operating -systems delete files. When you encrypt a file and then delete the -original plaintext file, the operating system doesn't actually -physically erase the data. It merely marks those disk blocks as -deleted, allowing the space to be reused later. It's sort of like -discarding sensitive paper documents in the paper recycling bin -instead of the paper shredder. The disk blocks still contain the -original sensitive data you wanted to erase, and will probably -eventually be overwritten by new data at some point in the future. -If an attacker reads these deleted disk blocks soon after they have -been deallocated, he could recover your plaintext. - -In fact this could even happen accidentally, if for some reason -something went wrong with the disk and some files were accidentally -deleted or corrupted. A disk recovery program may be run to recover -the damaged files, but this often means some previously deleted files -are resurrected along with everything else. Your confidential files -that you thought were gone forever could then reappear and be -inspected by whomever is attempting to recover your damaged disk. -Even while you are creating the original message with a word -processor or text editor, the editor may be creating multiple -temporary copies of your text on the disk, just because of its -internal workings. These temporary copies of your text are deleted -by the word processor when it's done, but these sensitive fragments -are still on your disk somewhere. - -Let me tell you a true horror story. I had a friend, married with -young children, who once had a brief and not very serious affair. -She wrote a letter to her lover on her word processor, and deleted -the letter after she sent it. Later, after the affair was over, the -floppy disk got damaged somehow and she had to recover it because it -contained other important documents. She asked her husband to -salvage the disk, which seemed perfectly safe because she knew she -had deleted the incriminating letter. Her husband ran a commercial -disk recovery software package to salvage the files. It recovered -the files alright, including the deleted letter. He read it, which -set off a tragic chain of events. - -The only way to prevent the plaintext from reappearing is to somehow -cause the deleted plaintext files to be overwritten. Unless you know -for sure that all the deleted disk blocks will soon be reused, you -must take positive steps to overwrite the plaintext file, and also -any fragments of it on the disk left by your word processor. You can -overwrite the original plaintext file after encryption by using the -PGP -w (wipe) option. You can take care of any fragments of the -plaintext left on the disk by using any of the disk utilities -available that can overwrite all of the unused blocks on a disk. For -example, the Norton Utilities for MSDOS can do this. - -Even if you overwrite the plaintext data on the disk, it may still be -possible for a resourceful and determined attacker to recover the -data. Faint magnetic traces of the original data remain on the disk -after it has been overwritten. Special sophisticated disk recovery -hardware can sometimes be used to recover the data. - - -Viruses and Trojan Horses -------------------------- - -Another attack could involve a specially-tailored hostile computer -virus or worm that might infect PGP or your operating system. This -hypothetical virus could be designed to capture your pass phrase or -secret key or deciphered messages, and covertly write the captured -information to a file or send it through a network to the virus's -owner. Or it might alter PGP's behavior so that signatures are not -properly checked. This attack is cheaper than cryptanalytic attacks. - -Defending against this falls under the category of defending against -viral infection generally. There are some moderately capable -anti-viral products commercially available, and there are hygienic -procedures to follow that can greatly reduce the chances of viral -infection. A complete treatment of anti-viral and anti-worm -countermeasures is beyond the scope of this document. PGP has no -defenses against viruses, and assumes your own personal computer is a -trustworthy execution environment. If such a virus or worm actually -appeared, hopefully word would soon get around warning everyone. - -Another similar attack involves someone creating a clever imitation -of PGP that behaves like PGP in most respects, but doesn't work the -way it's supposed to. For example, it might be deliberately crippled -to not check signatures properly, allowing bogus key certificates to -be accepted. This "Trojan horse" version of PGP is not hard for an -attacker to create, because PGP source code is widely available, so -anyone could modify the source code and produce a lobotomized zombie -imitation PGP that looks real but does the bidding of its diabolical -master. This Trojan horse version of PGP could then be widely -circulated, claiming to be from me. How insidious. - -You should make an effort to get your copy of PGP from a reliable -source, whatever that means. Or perhaps from more than one -independent source, and compare them with a file comparison utility. - -There are other ways to check PGP for tampering, using digital -signatures. If someone you trust signs the executable version of -PGP, vouching for the fact that it has not been infected or tampered -with, you can be reasonably sure that you have a good copy. You -could use an earlier trusted version of PGP to check the signature on -a later suspect version of PGP. But this will not help at all if -your operating system is infected, nor will it detect if your -original copy of PGP.EXE has been maliciously altered in such a way -as to compromise its own ability to check signatures. This test also -assumes that you have a good trusted copy of the public key that you -use to check the signature on the PGP executable. - - -Physical Security Breach ------------------------- - -A physical security breach may allow someone to physically acquire -your plaintext files or printed messages. A determined opponent -might accomplish this through burglary, trash-picking, unreasonable -search and seizure, or bribery, blackmail or infiltration of your -staff. Some of these attacks may be especially feasible against -grassroots political organizations that depend on a largely volunteer -staff. It has been widely reported in the press that the FBI's -COINTELPRO program used burglary, infiltration, and illegal bugging -against antiwar and civil rights groups. And look what happened at -the Watergate Hotel. - -Don't be lulled into a false sense of security just because you have -a cryptographic tool. Cryptographic techniques protect data only -while it's encrypted-- direct physical security violations can still -compromise plaintext data or written or spoken information. - -This kind of attack is cheaper than cryptanalytic attacks on PGP. - - -Tempest Attacks ---------------- - -Another kind of attack that has been used by well-equipped opponents -involves the remote detection of the electromagnetic signals from -your computer. This expensive and somewhat labor-intensive attack is -probably still cheaper than direct cryptanalytic attacks. An -appropriately instrumented van can park near your office and remotely -pick up all of your keystrokes and messages displayed on your -computer video screen. This would compromise all of your passwords, -messages, etc. This attack can be thwarted by properly shielding all -of your computer equipment and network cabling so that it does not -emit these signals. This shielding technology is known as "Tempest", -and is used by some Government agencies and defense contractors. -There are hardware vendors who supply Tempest shielding commercially, -although it may be subject to some kind of Government licensing. Now -why do you suppose the Government would restrict access to Tempest -shielding? - - -Exposure on Multi-user Systems ------------------------------- - -PGP was originally designed for a single-user MSDOS machine under -your direct physical control. I run PGP at home on my own PC, and -unless someone breaks into my house or monitors my electromagnetic -emissions, they probably can't see my plaintext files or secret keys. - -But now PGP also runs on multi-user systems such as Unix and VAX/VMS. -On multi-user systems, there are much greater risks of your plaintext -or keys or passwords being exposed. The Unix system administrator or -a clever intruder can read your plaintext files, or perhaps even use -special software to covertly monitor your keystrokes or read what's -on your screen. On a Unix system, any other user can read your -environment information remotely by simply using the Unix "ps" -command. Similar problems exist for MSDOS machines connected on a -local area network. The actual security risk is dependent on your -particular situation. Some multi-user systems may be safe because -all the users are trusted, or because they have system security -measures that are safe enough to withstand the attacks available to -the intruders, or because there just aren't any sufficiently -interested intruders. Some Unix systems are safe because they are -only used by one user-- there are even some notebook computers -running Unix. It would be unreasonable to simply exclude PGP from -running on all Unix systems. - -PGP is not designed to protect your data while it is in plaintext -form on a compromised system. Nor can it prevent an intruder from -using sophisticated measures to read your secret key while it is -being used. You will just have to recognize these risks on -multi-user systems, and adjust your expectations and behavior -accordingly. Perhaps your situation is such that you should consider -only running PGP on an isolated single-user system under your direct -physical control. That's what I do, and that's what I recommend. - - -Traffic Analysis ----------------- - -Even if the attacker cannot read the contents of your encrypted -messages, he may be able to infer at least some useful information by -observing where the messages come from and where they are going, the -size of the messages, and the time of day the messages are sent. -This is analogous to the attacker looking at your long distance phone -bill to see who you called and when and for how long, even though the -actual content of your calls is unknown to the attacker. This is -called traffic analysis. PGP alone does not protect against traffic -analysis. Solving this problem would require specialized -communication protocols designed to reduce exposure to traffic -analysis in your communication environment, possibly with some -cryptographic assistance. - - -Cryptanalysis -------------- - -An expensive and formidable cryptanalytic attack could possibly be -mounted by someone with vast supercomputer resources, such as a -Government intelligence agency. They might crack your RSA key by -using some new secret factoring breakthrough. Perhaps so, but it is -noteworthy that the US Government trusts the RSA algorithm enough in -some cases to use it to protect its own nuclear weapons, according to -Ron Rivest. And civilian academia has been intensively attacking it -without success since 1978. - -Perhaps the Government has some classified methods of cracking the -IDEA(tm) conventional encryption algorithm used in PGP. This is -every cryptographer's worst nightmare. There can be no absolute -security guarantees in practical cryptographic implementations. - -Still, some optimism seems justified. The IDEA algorithm's designers -are among the best cryptographers in Europe. It has had extensive -security analysis and peer review from some of the best cryptanalysts -in the unclassified world. It appears to have some design advantages -over the DES in withstanding differential cryptanalysis, which has -been used to crack the DES. - -Besides, even if this algorithm has some subtle unknown weaknesses, -PGP compresses the plaintext before encryption, which should greatly -reduce those weaknesses. The computational workload to crack it is -likely to be much more expensive than the value of the message. - -If your situation justifies worrying about very formidable attacks of -this caliber, then perhaps you should contact a data security -consultant for some customized data security approaches tailored to -your special needs. Boulder Software Engineering, whose address and -phone are given at the end of this document, can provide such -services. - - -In summary, without good cryptographic protection of your data -communications, it may have been practically effortless and perhaps -even routine for an opponent to intercept your messages, especially -those sent through a modem or E-mail system. If you use PGP and -follow reasonable precautions, the attacker will have to expend far -more effort and expense to violate your privacy. - -If you protect yourself against the simplest attacks, and you feel -confident that your privacy is not going to be violated by a -determined and highly resourceful attacker, then you'll probably be -safe using PGP. PGP gives you Pretty Good Privacy. - - -Legal Issues -============ - - -Trademarks, Copyrights, and Warranties --------------------------------------- - -"Pretty Good Privacy", "Phil's Pretty Good Software", and the "Pretty -Good" label for computer software and hardware products are all -trademarks of Philip Zimmermann and Phil's Pretty Good Software. PGP -is (c) Copyright Philip R. Zimmermann, 1990-1993. Philip Zimmermann -also holds the copyright for the PGP User's Manual, as well as any -foreign language translations of the manual or the software. - -The author assumes no liability for damages resulting from the use of -this software, even if the damage results from defects in this -software, and makes no representations concerning the merchantability -of this software or its suitability for any specific purpose. It is -provided "as is" without express or implied warranty of any kind. - - -Patent Rights on the Algorithms -------------------------------- - -The RSA public key cryptosystem was developed at MIT, which holds a -patent on it (U.S. patent #4,405,829, issued 20 Sep 1983). A company -in California called Public Key Partners (PKP) holds the exclusive -commercial license to sell and sub-license the RSA public key -cryptosystem. - -PKP has not granted a patent license for anyone to use their RSA -algorithm in PGP. The author of this software implementation of the -RSA algorithm is providing this implementation for educational use -only. Licensing this algorithm from PKP is the responsibility of -you, the user, not the author. The author assumes no liability for -any patent infringement that may result from executing the RSA -algorithm on the user's computer without a license from the RSA -patent holder. - -Non-US users should note that the RSA patent does not apply outside -the US, and there is no RSA patent in any other country. Federal -agencies may use it because the Government paid for the development -of RSA with grants from the National Science Foundation and the -Navy. And companies that have already licensed the patent from PKP -may be able to use PGP, depending on the terms of their license. - -I wrote my PGP software from scratch, with my own independently -developed implementation of the RSA algorithm. Before publishing -PGP, I got a formal written legal opinion from a patent attorney with -extensive experience in software patents. I'm convinced that -publishing PGP the way I did does not violate patent law. - -Not only did PKP acquire the exclusive patent rights for the RSA -cryptosystem, but they also acquired the exclusive rights to three -other patents covering other public key schemes invented by others at -Stanford University, also developed with Federal funding. This -essentially gives one company a legal lock in the USA on nearly all -practical public key cryptosystems. They even appear to be claiming -patent rights on the very concept of public key cryptography, -regardless of what clever new original algorithms are independently -invented by others. I find such a comprehensive monopoly troubling, -because I think public key cryptography is destined to become a -crucial technology in the protection of our civil liberties and -privacy in our increasingly connected society. At the very least, -it places these vital tools at risk by affording to the Government -a single pressure point of influence. - -There are negotiations under way with RSA Data Security Inc (RSADSI), -a sister company to PKP, to legalize PGP in the US. This would be -accomplished by integrating RSAREF into PGP. RSAREF is a subroutine -package from RSADSI that implements the RSA algorithm. The RSAREF -subroutines would have to be used instead of PGP's original -subroutines to implement the RSA functions in PGP. There are some -technical obstacles to getting this done, but a solution may be found -if the negotiations with RSADSI proceed favorably. If RSAREF is -integrated into PGP, it will be licensed by RSADSI for noncommercial -usage in the US. Foreign versions of PGP will not use RSAREF, but -will continue to use PGP's original faster subroutine library to do -the RSA calculations. RSADSI may require PGP's name to change in -order to make all this possible. Stay tuned. - -It appears certain that there will be future releases of PGP, -regardless of the outcome of licensing problems with Public Key -Partners. If PKP does not license PGP, then future releases of PGP -will likely not come from me. There are countless fans of PGP outside -the US, and many of them are software engineers who want to improve -PGP and promote it, regardless of what I do. The second release of -PGP was a joint effort of an international team of software -engineers, implementing enhancements to the original PGP with design -guidance from me. It was released by Branko Lankester in The -Netherlands and Peter Gutmann in New Zealand, out of reach of US -patent law. Although released only in Europe and New Zealand, it -spontaneously spread to the USA without help from me or the PGP -development team. - -The IDEA(tm) conventional block cipher used by PGP is covered by a -patent in Europe, held by ETH and a Swiss company called Ascom-Tech -AG. The patent number is PCT/CH91/00117. International patents are -pending. IDEA(tm) is a trademark of Ascom-Tech AG. There is no -license fee required for noncommercial use of IDEA. Ascom Tech AG -has granted permission for PGP to use the IDEA cipher, and places no -restrictions on using PGP for any purpose, including commercial use. -You may not extract the IDEA cipher from PGP and put it in another -commercial product without a license. Commercial users of IDEA may -obtain licensing details from Dieter Profos, Ascom Tech AG, Solothurn -Lab, Postfach 151, 4502 Solothurn, Switzerland, Tel +41 65 242885, -Fax +41 65 235761. - -The ZIP compression routines in PGP come from freeware source code, -with the author's permission. I'm not aware of any patents on the -compression algorithms used in the ZIP routines, but you're welcome to -check into that question yourself. - - -Licensing and Distribution --------------------------- - -In the USA PKP controls, through US patent law, the licensing of the -RSA algorithm. But I have no objection to anyone freely using or -distributing my PGP software, without payment of fees to me. You must -keep the copyright notices on PGP and keep this documentation with -it. However, if you live in the USA, this may not satisfy any legal -obligations you may have to PKP for using the RSA algorithm as -mentioned above. - -PGP is not shareware, it's freeware. Forbidden freeware. Published -as a community service. Giving PGP away for free will encourage far -more people to use it, which hopefully will have a greater social -impact. This could lead to widespread awareness and use of the RSA -public key cryptosystem. - -All the source code for PGP is available for free under the "Copyleft" -General Public License from the Free Software Foundation (FSF). A -copy of the FSF General Public License is included in the source -release package of PGP. - -Regardless of and perhaps contrary to some provisions of the FSF -General Public License, the following terms apply: - -1) Written discussions of PGP in magazines or books may include - fragments of PGP source code and documentation, without - restrictions. - -2) Although the FSF General Public License allows non-proprietary - derivative products, it prohibits proprietary derivative products. - Despite this, I may grant you a special license if you want to derive - a proprietary commercial product from some of PGP's parts. There may - or may not be a fee, depending on the circumstances. Retaining my - copyright notice and attribution might suffice in some cases. Give - me a call and we'll discuss it. I'm real easy to please. Any such - license would not free you of any patent licensing requirements. - -Feel free to disseminate the complete PGP release package as widely -as possible. Give it to all your friends. If you have access to any -electronic Bulletin Boards Systems, please upload the complete PGP -executable object release package to as many BBS's as possible. You -may disseminate the PGP source release package too, if you've got -it. The PGP version 2.3 executable object release package for MSDOS -contains the PGP executable software, documentation, sample key rings -including my own public key, and signatures for the software and this -manual, all in one PKZIP compressed file called pgp22.zip. The PGP -source release package for MSDOS contains all the C source files in -one PKZIP compressed file called pgp22src.zip. The filename for the -release package is derived from the version number of the release. - -You may obtain free copies or updates to PGP from thousands of BBS's -worldwide or from other public sources such as Internet FTP sites. -Don't ask me for a copy directly from me, since I'd rather avoid -further legal problems with PKP at this time. I might be able to -tell you where you can get it, however. - -After all this work I have to admit I wouldn't mind getting some fan -mail for PGP, to gauge its popularity. Let me know what you think -about it and how many of your friends use it. Bug reports and -suggestions for enhancing PGP are welcome, too. Perhaps a future PGP -release will reflect your suggestions. - -This project has not been funded and the project has nearly eaten me -alive. This means you can't count on a reply to your mail, unless -you only need a short written reply and you include a stamped -self-addressed envelope. But I do reply to E-mail. Please keep it in -English, as my foreign language skills are weak. If you call and I'm -not in, it's best to just try again later. I usually don't return -long distance phone calls, unless you leave a message that I can call -you collect. If you need any significant amount of my time, I am -available on a paid consulting basis, and I do return those calls. - -The most inconvenient mail I get is for some well-intentioned person -to send me a few dollars asking me for a copy of PGP. I don't send -it to them because I'd rather avoid any legal problems with PKP. Or -worse, sometimes these requests are from foreign countries, and I -would be risking a violation of US cryptographic export control -laws. Even if there were no legal hassles involved in sending PGP to -them, they usually don't send enough money to make it worth my time. -I'm just not set up as a low cost low volume mail order business. I -can't just ignore the request and keep the money, because they -probably regard the money as a fee for me to fulfill their request. -If I return the money, I might have to get in my car and drive down -to the post office and buy some postage stamps, because these -requests rarely include a stamped self-addressed envelope. And I -have to take the time to write a polite reply that I can't do it. If -I postpone the reply and set the letter down on my desk, it might be -buried within minutes and won't see the light of day again for -months. Multiply these minor inconveniences by the number of -requests I get, and you can see the problem. Isn't it enough that -the software is free? It would be nicer if people could try to get -PGP from any of the myriad other sources. If you don't have a modem, -ask a friend to get it for you. If you can't find it yourself, I -don't mind answering a quick phone call. - -If anyone wants to volunteer to improve PGP, please let me know. It -could certainly use some more work. Some features were deferred to -get it out the door. A number of PGP users have since donated their -time to port PGP to Unix on Sun SPARCstations, to Ultrix, to VAX/VMS, -to OS/2, to the Amiga, and to the Atari ST. Perhaps you can help -port it to some new environments. But please let me know if you plan -to port or add enhancements to PGP, to avoid duplication of effort, -and to avoid starting with an obsolete version of the source code. - -Because so many foreign language translations of PGP have been -produced, most of them are not distributed with the regular PGP -release package because it would require too much disk space. -Separate language translation "kits" are available from a number of -independent sources, and are sometimes available separately from the -same distribution centers that carry the regular PGP release -software. These kits include translated versions of the file -LANGUAGE.TXT, PGP.HLP, and the PGP User's Guide. If you want to -produce a translation for your own native language, contact me first -to get the latest information and standard guidelines, and to find -out if it's been translated to your language already. Colin Plumb -(colin@nyx.cs.du.edu) maintains a comprehensive collection of foreign -language kits from other translators. - -Future versions of PGP may have to change the data formats for -messages, signatures, keys and key rings, in order to provide -important new features. This may cause backward compatibility -problems with this version of PGP. Future releases may provide -conversion utilities to convert old keys, but you may have to dispose -of old messages created with the old PGP. - -If you have access to the Internet, watch for announcements of new -releases of PGP on the Internet newsgroups "sci.crypt" and PGP's own -newsgroup, "alt.security.pgp". There is also an interest group -mailing list called info-pgp, which is intended for users without -access to the "alt.security.pgp" newsgroup. Info-pgp is moderated by -Hugh Miller, and you may subscribe to it by writing him a letter at -info-pgp-request@lucpul.it.luc.edu. Include your name and Internet -address. If you want to know where to get PGP, Hugh can send you a -list of Internet FTP sites and BBS phone numbers. Hugh may also be -reached at hmiller@lucpul.it.luc.edu. - - - -Export Controls ---------------- - -The Government has made it illegal in many cases to export good -cryptographic technology, and that may include PGP. They regard this -kind of software as munitions. This is determined by volatile State -Department policies, not fixed laws. I will not export this software -out of the US or Canada in cases when it is illegal to do so under US -State Department policies, and I assume no responsibility for other -people exporting it on their own. - -If you live outside the US or Canada, I advise you not to violate US -State Department policies by getting PGP from a US source. Since -thousands of domestic users got it after its initial publication, it -somehow leaked out of the US and spread itself widely abroad, like -dandelion seeds blowing in the wind. If PGP has already found its -way into your country, then I don't think you're violating US export -law if you pick it up from a source outside of the US. - -It seems to some legal observers I've talked with, that the framers of -the US export controls never envisioned that they would ever apply to -cryptographic freeware that has been published and scattered to the -winds. It's hard to imagine a US attorney trying to build a real -case against someone for the "export" of software published freely in -the US. As far as anyone I've talked to knows, it's never been done, -despite numerous examples of export violations. I'm not a lawyer and -I'm not giving you legal advice-- I'm just trying to point out what -seems like common sense. - -Starting with PGP version 2.0, the release point of the software has -been outside the US, on publicly-accessible computers in Europe. -Each release is electronically sent back into the US and posted on -publicly-accessible computers in the US by PGP privacy activists in -foreign countries. There are some restrictions in the US regarding -the import of munitions, but I'm not aware of any cases where this -was ever enforced for importing cryptographic software into the US. -I imagine that a legal action of that type would be quite a spectacle -of controversy. - -Some foreign governments impose serious penalties on anyone inside -their country for merely using encrypted communications. In some -countries they might even shoot you for that. But if you live in -that kind of country, perhaps you need PGP even more. - - - -Computer-Related Political Groups -================================= - -PGP is a very political piece of software. It seems appropriate to -mention here some computer-related activist groups. Full details on -these groups, and how to join them, is provided in a separate -document file in the PGP release package. - -The Electronic Frontier Foundation (EFF) was founded in July, 1990, -to assure freedom of expression in digital media, with a particular -emphasis on applying the principles embodied in the US Constitution -and the Bill of Rights to computer-based communication. They can be -reached in Washington DC, at (202) 347-5400. Internet E-mail address: -eff@eff.org. - -Computer Professionals For Social Responsibility (CPSR) empowers -computer professionals and computer users to advocate for the -responsible use of information technology and empowers all who use -computer technology to participate in public policy debates on the -impacts of computers on society. They can be reached at: -415-322-3778 in Palo Alto, E-mail address cpsr@csli.stanford.edu. - -The League for Programming Freedom (LPF) is a grass-roots organization -of professors, students, businessmen, programmers and users dedicated -to bringing back the freedom to write programs. They regard patents -on computer algorithms as harmful to the US software industry. They -can be reached at (617) 433-7071. E-mail address: lpf@uunet.uu.net. - -For more details on these groups, see the accompanying document in -the PGP release package. - - -Recommended Introductory Readings -================================= - -1) Dorothy Denning, "Cryptography and Data Security", Addison-Wesley, - Reading, MA 1982 -2) Dorothy Denning, "Protecting Public Keys and Signature Keys", - IEEE Computer, Feb 1983 -3) Martin E. Hellman, "The Mathematics of Public-Key Cryptography," - Scientific American, Aug 1979 -4) Steven Levy, "Crypto Rebels", WIRED, May/Jun 1993, page 54. - (This is a "must-read" article on PGP and other related topics.) - -Other Readings -============== - -5) Ronald Rivest, "The MD5 Message Digest Algorithm", MIT Laboratory - for Computer Science, 1991 -6) Xuejia Lai, "On the Design and Security of Block Ciphers", - Institute for Signal and Information Processing, ETH-Zentrum, - Zurich, Switzerland, 1992 -7) Xuejia Lai, James L. Massey, Sean Murphy, "Markov Ciphers and - Differential Cryptanalysis", Advances in Cryptology- EUROCRYPT'91 -8) Philip Zimmermann, "A Proposed Standard Format for RSA - Cryptosystems", Advances in Computer Security, Vol III, edited by - Rein Turn, Artech House, 1988 -9) Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and - Source Code in C", John Wiley & Sons, 1993 (coming in November) -10) Paul Wallich, "Electronic Envelopes", Scientific American, Feb - 1993, page 30. (This is an article on PGP) - - -To Contact the Author -===================== - -Philip Zimmermann may be reached at: - -Boulder Software Engineering -3021 Eleventh Street -Boulder, Colorado 80304 USA -Phone 303-541-0140 (voice or FAX) (10:00am - 7:00pm Mountain Time) -Internet: prz@acm.org - - - -Appendix A: Where to Get PGP -============================= - -The following describes how to get the freeware public key -cryptographic software PGP (Pretty Good Privacy) from an anonymous -FTP site on Internet, or from other sources. - -PGP has sophisticated key management, an RSA/conventional hybrid -encryption scheme, message digests for digital signatures, data -compression before encryption, and good ergonomic design. PGP is -well featured and fast, and has excellent user documentation. Source -code is free. - -PGP uses the RSA cryptosystem which is claimed by a US patent held by -a company called Public Key Partners. PGP users outside the US take -note that there is no RSA patent outside the US. Also, bear in mind -that there are US and Canadian export laws prohibiting anyone inside -the US and Canada from exporting cryptographic software like this. -If you live outside the US, you're probably not violating US export -law if you pick it up from a source outside of the US. Note that due -to negotiations with the RSA patent holders, the name of PGP may -change in a future release. - -What follows is a small sample of places that allegedly have PGP, as -of June 1993. This information is not guaranteed to be correct. -Some US sites have occasionally withdrawn PGP because of fear of -legal intimidation from the RSA patent holders. - -There are two compressed archive files in the standard release, with -the file name derived from the release version number. For PGP -version 2.3, you must get pgp23.zip which contains the MSDOS binary -executable and the PGP User's Guide, and you can optionally get -pgp23src.zip which contains all the source code. These files can be -decompressed with the MSDOS shareware archive decompression utility -PKUNZIP.EXE, version 1.10 or later. For Unix users who lack an -implementation of UNZIP, the source code can also be found in the -compressed tar file pgp23src.tar.Z. - -A reminder: Set mode to binary or image when doing an FTP transfer. -And when doing a kermit download to your PC, specify 8-bit binary -mode at both ends. Here are some Internet sites that have PGP via -anonymous FTP: - -Finland: nic.funet.fi (128.214.6.100) - Directory: /pub/unix/security/crypt/ - -Italy: ghost.dsi.unimi.it (149.132.2.1) - Directory: /pub/security/ - -UK: src.doc.ic.ac.uk - Directory: /computing/security/software/PGP - -For those lacking FTP connectivity to the net, nic.funet.fi also -offers the files via email. To get version 2.3, send the following -mail message to mailserv@nic.funet.fi: - - ENCODER uuencode - SEND pub/unix/security/crypt/pgp23src.zip - SEND pub/unix/security/crypt/pgp23.zip - -This will deposit the two zipfiles, as (about) 15 batched messages in -your mailbox within about 24 hours. Save and uudecode. - -In the US, PGP may be found on God knows how many BBS systems, far -too many to list here. Still, if you don't have any local BBS phone -numbers handy, here are some BBS's you might try. - -The GRAPEVINE BBS in Little Rock Arkansas has set up a special -account for people to download PGP for free. The SYSOP is Jim Wenzel, -at jim.wenzel@grapevine.lrk.ar.us. The following phone numbers are -applicable and should be dialed in the order presented (i.e., the -first one is the highest speed line): (501) 753-6859, (501) -753-8121, (501) 791-0124. When asked to login use the following -information: - - name: PGP USER ('PGP' is 1st name, 'USER' is 2nd name) - password: PGP - -PGP is also widely available on Fidonet, a large informal network of -PC-based bulletin board systems interconnected via modems. Check -your local bulletin board systems. It is available on many foreign -and domestic Fidonet BBS sites. - -In New Zealand, try this (supposedly free) dial-up BBS system: - Kappa Crucis: +64 9 817-3714, -3725, -3324, -8424, -3094, -3393 - -Source and binary distributions of PGP are available from the Canadian -Broadcasting Corporation library, which is open to the public. It has -branches in Toronto, Montreal, and Vancouver. Contact Max Allen, at -+1 416 205-6017 if you have questions. - -For information on PGP implementations on the Apple Macintosh, -Commodore Amiga, or Atari ST, or any other questions about where to -get PGP for any other platform, contact Hugh Miller at -hmiller@lucpul.it.luc.edu. - -Here are a few people and their email addresses or phone numbers you -can contact in some countries to get information on local PGP -availability: - -Peter Gutmann Hugh Kennedy -pgut1@cs.aukuni.ac.nz 70042.710@compuserve.com -New Zealand Germany - -Branko Lankester Miguel Angel Gallardo -lankeste@fwi.uva.nl gallardo@batman.fi.upm.es -+31 2159 42242 (341) 474 38 09 -The Netherlands Spain - -Hugh Miller Colin Plumb -hmiller@lucpul.it.luc.edu colin@nyx.cs.du.edu -(312) 508-2727 Toronto, Ontario, Canada -USA - -Jean-loup Gailly -jloup@chorus.fr -France - + + Phil's Pretty Good Software + Presents + + ======= + PGP(tm) + ======= + + Pretty Good(tm) Privacy + Public Key Encryption for the Masses + + + ------------------------- + PGP(tm) User's Guide + Volume II: Special Topics + ------------------------- + by Philip Zimmermann + Revised 7 May 94 + + + PGP Version 2.5 - 7 May 94 + Software by + Philip Zimmermann, and many others. + + + + +Synopsis: PGP(tm) uses public-key encryption to protect E-mail and +data files. Communicate securely with people you've never met, with +no secure channels needed for prior exchange of keys. PGP is well +featured and fast, with sophisticated key management, digital +signatures, data compression, and good ergonomic design. + + +Software and documentation (c) Copyright 1990-1994 Philip Zimmermann. +All rights reserved. For information on PGP licensing, distribution, +copyrights, patents, trademarks, liability limitations, and export +controls, see the "Legal Issues" section. Distributed by the +Massachusetts Institute of Technology. + + +Contents +======== + +Quick Overview +Special Topics + Selecting Keys via Key ID + Separating Signatures from Messages + Decrypting the Message and Leaving the Signature on it + Sending ASCII Text Files Across Different Machine Environments + Leaving No Traces of Plaintext on the Disk + Displaying Decrypted Plaintext on Your Screen + Making a Message For Her Eyes Only + Preserving the Original Plaintext Filename + Editing Your User ID or Pass Phrase + Editing the Trust Parameters for a Public Key + Checking If Everything is OK on Your Public Key Ring + Verifying a Public Key Over the Phone + Handling Large Public Keyrings + Using PGP as a Unix-style Filter + Suppressing Unneccessary Questions: BATCHMODE + Force "Yes" Answer to Confirmation Questions: FORCE + PGP Returns Exit Status to the Shell + Environmental Variable for Pass Phrase + Setting Configuration Parameters: CONFIG.TXT + TMP - Directory Pathname for Temporary Files + LANGUAGE - Foreign Language Selector + MYNAME - Default User ID for Making Signatures + TEXTMODE - Assuming Plaintext is a Text File + CHARSET - Specifies Local Character Set for Text Files + ARMOR - Enable ASCII Armor Output + ARMORLINES - Size of ASCII Armor Multipart Files + KEEPBINARY - Keep Binary Ciphertext Files After Decrypting + COMPRESS - Enable Compression + COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed + MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed + CERT_DEPTH - How Deep May Introducers Be Nested + BAKRING - Filename for Backup Secret Keyring + PUBRING - Filename for Your Public Keyring + SECRING - Filename for Your Secret Keyring + RANDSEED - Filename for Random Number Seed + PAGER - Selects Shell Command to Display Plaintext Output + SHOWPASS - Echo Pass Phrase to User + TZFIX - Timezone Adjustment + CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text + VERBOSE - Quiet, Normal, or Verbose Messages + INTERACTIVE - Ask for Confirmation for Key Adds + Protecting Against Bogus Timestamps + A Peek Under the Hood + Random Numbers + PGP's Conventional Encryption Algorithm + Data Compression + Message Digests and Digital Signatures + Compatibility with Previous Versions of PGP +Vulnerabilities + Compromised Pass Phrase and Secret Key + Public Key Tampering + "Not Quite Deleted" Files + Viruses and Trojan Horses + Physical Security Breach + Tempest Attacks + Exposure on Multi-user Systems + Traffic Analysis + Cryptanalysis +Legal Issues + Trademarks, Copyrights, and Warranties + Patent Rights on the Algorithms + Licensing and Distribution + Export Controls + Philip Zimmermann's Legal Situation + Where to Get a Commercial Version of PGP + Reporting PGP Bugs +Computer-Related Political Groups +Recommended Readings +To Contact the Author +Appendix A: Where to Get PGP + + +Quick Overview +============== + +Pretty Good(tm) Privacy (PGP), from Phil's Pretty Good Software, is a +high security cryptographic software application for MSDOS, Unix, +VAX/VMS, and other computers. PGP combines the convenience of the +Rivest-Shamir-Adleman (RSA) public key cryptosystem with the speed of +conventional cryptography, message digests for digital signatures, +data compression before encryption, good ergonomic design, and +sophisticated key management. + +This volume II of the PGP User's Guide covers advanced topics about +PGP that were not covered in the "PGP User's Guide, Volume I: +Essential Topics". You should first read the Essential Topics +volume, or this manual won't make much sense to you. Reading this +Special Topics volume is optional, except for the legal issues +section, which everyone should read. + + + +Special Topics +=============== + + +Selecting Keys via Key ID +------------------------- + +In all commands that let the user type a user ID or fragment of a +user ID to select a key, the hexadecimal key ID may be used instead. +Just use the key ID, with a prefix of "0x", in place of the user ID. +For example: + + pgp -kv 0x67F7 + +This would display all keys that had 67F7 as part of their key IDs. + +This feature is particularly useful if you have two different keys +from the same person, with the same user ID. You can unambiguously +pick which key you want by specifying the key ID. + + +Separating Signatures from Messages +----------------------------------- + +Normally, signature certificates are physically attached to the text +they sign. This makes it convenient in simple cases to check +signatures. It is desirable in some circumstances to have signature +certificates stored separately from the messages they sign. It is +possible to generate signature certificates that are detached from +the text they sign. To do this, combine the 'b' (break) option with +the 's' (sign) option. For example: + + pgp -sb letter.txt + +This example produces an isolated signature certificate in a file +called "letter.sig". The contents of letter.txt are not appended to +the signature certificate. + +After creating the signature certificate file (letter.sig in the +above example), send it along with the original text file to the +recipient. The recipient must have both files to check the signature +integrity. When the recipient attempts to process the signature +file, PGP notices that there is no text in the same file with the +signature and prompts the user for the filename of the text. Only +then can PGP properly check the signature integrity. If the +recipient knows in advance that the signature is detached from the +text file, she can specify both filenames on the command line: + + pgp letter.sig letter.txt +or: pgp letter letter.txt + +PGP will not have to prompt for the text file name in this case. + +A detached signature certificate is useful if you want to keep the +signature certificate in a separate certificate log. A detached +signature of an executable program is also useful for detecting a +subsequent virus infection. It is also useful if more than one party +must sign a document such as a legal contract, without nesting +signatures. Each person's signature is independent. + +If you receive a ciphertext file that has the signature certificate +glued to the message, you can still pry the signature certificate +away from the message during the decryption. You can do this with +the -b option during decrypt, like so: + + pgp -b letter + +This decrypts the letter.pgp file and if there is a signature in it, +PGP checks the signature and detaches it from the rest of the +message, storing it in the file letter.sig. + + +Decrypting the Message and Leaving the Signature on it +------------------------------------------------------ + +Usually, you want PGP to completely unravel a ciphertext file, +decrypting it and checking the nested signature if there is one, +peeling away the layers until you are left with only the original +plaintext file. + +But sometimes you want to decrypt an encrypted file, and leave the +inner signature still attached, so that you are left with a decrypted +signed message. This may be useful if you want to send a copy of a +signed document to a third party, perhaps re-enciphering it. For +example, suppose you get a message signed by Charlie, encrypted to +you. You want to decrypt it, and, leaving Charlie's signature on it, +you want to send it to Alice, perhaps re-enciphering it with Alice's +public key. No problem. PGP can handle that. + +To simply decrypt a message and leave the signature on it intact, +type: + + pgp -d letter + +This decrypts letter.pgp, and if there is an inner signature, it is +left intact with the decrypted plaintext in the output file. + +Now you can archive it, or maybe re-encrypt it and send it to someone +else. + + + +Sending ASCII Text Files Across Different Machine Environments +-------------------------------------------------------------- + +You may use PGP to encrypt any kind of plaintext file, binary 8-bit +data or ASCII text. Probably the most common usage of PGP will be for +E-mail, when the plaintext is ASCII text. + +ASCII text is sometimes represented differently on different +machines. For example, on an MSDOS system, all lines of ASCII text +are terminated with a carriage return followed by a linefeed. On a +Unix system, all lines end with just a linefeed. On a Macintosh, all +lines end with just a carriage return. This is a sad fact of life. + +Normal unencrypted ASCII text messages are often automatically +translated to some common "canonical" form when they are transmitted +from one machine to another. Canonical text has a carriage return +and a linefeed at the end of each line of text. For example, the +popular KERMIT communication protocol can convert text to canonical +form when transmitting it to another system. This gets converted +back to local text line terminators by the receiving KERMIT. This +makes it easy to share text files across different systems. + +But encrypted text cannot be automatically converted by a +communication protocol, because the plaintext is hidden by +encipherment. To remedy this inconvenience, PGP lets you specify +that the plaintext should be treated as ASCII text (not binary data) +and should be converted to canonical text form before it gets +encrypted. At the receiving end, the decrypted plaintext is +automatically converted back to whatever text form is appropriate for +the local environment. + +To make PGP assume the plaintext is text that should be converted to +canonical text before encryption, just add the "t" option when +encrypting or signing a message, like so: + + pgp -et message.txt her_userid + +This mode is automatically turned off if PGP detects that the +plaintext file contains what it thinks is non-text binary data. + +For PGP users that use non-English 8-bit character sets, when PGP +converts text to canonical form, it may convert data from the local +character set into the LATIN1 (ISO 8859-1 Latin Alphabet 1) character +set, depending on the setting of the CHARSET parameter in the PGP +configuration file. LATIN1 is a superset of ASCII, with extra +characters added for many European languages. + + + +Leaving No Traces of Plaintext on the Disk +------------------------------------------ + +After PGP makes a ciphertext file for you, you can have PGP +automatically overwrite the plaintext file and delete it, leaving no +trace of plaintext on the disk so that no one can recover it later +using a disk block scanning utility. This is useful if the plaintext +file contains sensitive information that you don't want to keep +around. + +To wipe out the plaintext file after producing the ciphertext file, +just add the "w" (wipe) option when encrypting or signing a message, +like so: + + pgp -esw message.txt her_userid + +This example creates the ciphertext file "message.pgp", and the +plaintext file "message.txt" is destroyed beyond recovery. + +Obviously, you should be careful with this option. Also note that +this will not wipe out any fragments of plaintext that your word +processor might have created on the disk while you were editing the +message before running PGP. Most word processors create backup +files, scratch files, or both. Also, it overwrites the file only +once, which is enough to thwart conventional disk recovery efforts, +but not enough to withstand a determined and sophisticated effort to +recover the faint magnetic traces of the data using special disk +recovery hardware. + + + +Displaying Decrypted Plaintext on Your Screen +--------------------------------------------- + +To view the decrypted plaintext output on your screen (like the +Unix-style "more" command), without writing it to a file, use the -m +(more) option while decrypting: + + pgp -m ciphertextfile + +This displays the decrypted plaintext display on your screen one +screenful at a time. + + + +Making a Message For Her Eyes Only +---------------------------------- + +To specify that the recipient's decrypted plaintext will be shown +ONLY on her screen and will not be saved to disk, add the -m option: + + pgp -sem message.txt her_userid + +Later, when the recipient decrypts the ciphertext with her secret key +and pass phrase, the plaintext will be displayed on her screen but +will not be saved to disk. The text will be displayed as it would if +she used the Unix "more" command, one screenful at a time. If she +wants to read the message again, she will have to decrypt the +ciphertext again. + +This feature is the safest way for you to prevent your sensitive +message from being inadvertently left on the recipient's disk. This +feature was added at the request of a user who wanted to send +intimate messages to his lover, but was afraid she might accidentally +leave the decrypted messages on her husband's computer. + +Note that this feature will not prevent a clever and determined +person from finding a way to save the decrypted plaintext to disk-- +it's to help prevent a casual user from doing it inadvertently. + + + +Preserving the Original Plaintext Filename +------------------------------------------ + +Normally, PGP names the decrypted plaintext output file with a name +similar to the input ciphertext filename, but dropping the +extension. Or, you can override that convention by specifying an +output plaintext filename on the command line with the -o option. +For most E-mail, this is a reasonable way to name the plaintext file, +because you get to decide its name when you decipher it, and your +typical E-mail messages often come from useless original plaintext +filenames like "to_phil.txt". + +But when PGP encrypts a plaintext file, it always saves the original +filename and attaches it to the plaintext before it compresses and +encrypts the plaintext. Normally, this hidden original filename is +discarded by PGP when it decrypts, but you can tell PGP you want to +preserve the original plaintext filename and use it as the name of +the decrypted plaintext output file. This is useful if PGP is used +on files whose names are important to preserve. + +To recover the original plaintext filename while decrypting, add +the -p option, like so: + + pgp -p ciphertextfile + +I usually don't use this option, because if I did, about half of my +incoming E-mail would decrypt to the same plaintext filenames of +"to_phil.txt" or "prz.txt". + + + +Editing Your User ID or Pass Phrase +----------------------------------- + +Sometimes you may need to change your pass phrase, perhaps because +someone looked over your shoulder while you typed it in. + +Or you may need to change your user ID, because you got married and +changed your name, or maybe you changed your E-mail address. Or +maybe you want to add a second or third user ID to your key, because +you may be known by more than one name or E-mail address or job +title. PGP lets you attach more than one user ID to your key, any +one of which may be used to look up your key on the key ring. + +To edit your own userid or pass phrase for your secret key: + + pgp -ke userid [keyring] + +PGP prompts you for a new user ID or a new pass phrase. + +The optional [keyring] parameter, if specified, must be a public +keyring, not a secret keyring. The userid field must be your own +userid, which PGP knows is yours because it appears on both your +public keyring and your secret keyring. Both keyrings will be +updated, even though you only specified the public keyring. + + + +Editing the Trust Parameters for a Public Key +--------------------------------------------- + +Sometimes you need to alter the trust parameters for a public key on +your public key ring. For a discussion on what these trust +parameters mean, see the section "How Does PGP Keep Track of Which +Keys are Valid?" in the Essential Topics volume of the PGP User's +Guide. + +To edit the trust parameters for a public key: + + pgp -ke userid [keyring] + +The optional [keyring] parameter, if specified, must be a public +keyring, not a secret keyring. + + + +Checking If Everything is OK on Your Public Key Ring +---------------------------------------------------- + +Normally, PGP automatically checks any new keys or signatures on your +public key ring and updates all the trust parameters and validity +scores. In theory, it keeps all the key validity status information +up to date as material is added to or deleted from your public key +ring. But perhaps you may want to explicitly force PGP to perform a +comprehensive analysis of your public key ring, checking all the +certifying signatures, checking the trust parameters, updating all +the validity scores, and checking your own ultimately-trusted key +against a backup copy on a write-protected floppy disk. It may be a +good idea to do this hygienic maintenance periodically to make sure +nothing is wrong with your public key ring. To force PGP to perform +a full analysis of your public key ring, use the -kc (key ring check) +command: + + pgp -kc + +You can also make PGP check all the signatures for just a single +selected public key by: + + pgp -kc userid [keyring] + +For further information on how the backup copy of your own key is +checked, see the description of the BAKRING parameter in the +configuration file section of this manual. + + + +Verifying a Public Key Over the Phone +------------------------------------- + +If you get a public key from someone that is not certified by anyone +you trust, how can you tell if it's really their key? The best way +to verify an uncertified key is to verify it over some independent +channel other than the one you received the key through. One +convenient way to tell, if you know this person and would recognize +them on the phone, is to call them and verify their key over the +telephone. Rather than reading their whole tiresome (ASCII-armored) +key to them over the phone, you can just read their key's +"fingerprint" to them. To see this fingerprint, use the -kvc +command: + + pgp -kvc userid [keyring] + +This will display the key with the 16-byte digest of the public key +components. Read this 16-byte fingerprint to the key's owner on the +phone, while she checks it against her own, using the same -kvc +command at her end. + +You can both verify each other's keys this way, and then you can sign +each other's keys with confidence. This is a safe and convenient way +to get the key trust network started for your circle of friends. + +Note that sending a key fingerprint via E-mail is not the best way to +verify the key, because E-mail can be intercepted and modified. It's +best to use a different channel than the one that was used to send +the key itself. A good combination is to send the key via E-mail, +and the key fingerprint via a voice telephone conversation. Some +people distribute their key fingerprint on their business cards, +which looks really cool. + +If you don't know me, please don't call me to verify my key over the +phone-- I get too many calls like that. Since every PGP user has a +copy of my public key, no one could tamper with all the copies that +are out there. The discrepancy would soon be noticed by someone who +checked it from more than one source, and word would soon get out on +the Internet. + + + +Handling Large Public Keyrings +------------------------------ + +PGP was originally designed for handling small personal keyrings for +keeping all your friends on, like a personal rolodex. A couple +hundred keys is a reasonable size for such a keyring. But as PGP has +become more popular, people are now trying to add other large +keyrings to their own keyring. Sometimes this involves adding +thousands of keys to your keyring. PGP, in its present form, cannot +perform this operation in a reasonable period of time, while you wait +at your keyboard. Not for huge keyrings. + +You may want to add a huge "imported" keyring to your own keyring, +because you are only interested in a few dozen keys on the bigger +keyring you are bringing in. If that's all you want from the other +keyring, it would be more efficient if you extract the few keys you +need from the big foreign keyring, and then add just these few keys +to your own keyring. Use the -kx command to extract them from the +foreign keyring, specifying the keyring name on the command line. +Then add these extracted keys to your own keyring. + +The real solution is to improve PGP to use advanced database +techniques to manage large keyrings efficiently. Until this happens, +you will just have to use smaller keyrings, or be patient. + + + +Using PGP as a Unix-style Filter +-------------------------------- + +Unix fans are accustomed to using Unix "pipes" to make two +applications work together. The output of one application can be +directly fed through a pipe to be read as input to another +application. For this to work, the applications must be capable of +reading the raw material from "standard input" and writing the +finished output to "standard output". PGP can operate in this mode. +If you don't understand what this means, then you probably don't need +this feature. + +To use a Unix-style filter mode, reading from standard input and +writing to standard output, add the -f option, like so: + + pgp -feast her_userid outputfile + +This feature makes it easier to make PGP work with electronic mail +applications. + +When using PGP in filter mode to decrypt a ciphertext file, you may +find it useful to use the PGPPASS environmental variable to hold the +pass phrase, so that you won't be prompted for it. The PGPPASS +feature is explained below. + + + +Suppressing Unneccessary Questions: BATCHMODE +---------------------------------------------- + +With the BATCHMODE flag enabled on the command line, PGP will not ask +any unneccessary questions or prompt for alternate filenames. Here +is an example of how to set this flag: + + pgp +batchmode cipherfile + +This is useful for running PGP non-interactively from Unix shell +scripts or MSDOS batch files. Some key management commands still +need user interaction even when BATCHMODE is on, so shell scripts may +need to avoid them. + +BATCHMODE may also be enabled to check the validity of a signature on +a file. If there was no signature on the file, the exit code is 1. +If it had a signature that was good, the exit code is 0. + + +Force "Yes" Answer to Confirmation Questions: FORCE +---------------------------------------------------- + +This command-line flag makes PGP assume "yes" for the user response +to the confirmation request to overwrite an existing file, or when +removing a key from the keyring via the -kr command. Here is an +example of how to set this flag: + + pgp +force cipherfile +or: + pgp -kr +force Smith + +This feature is useful for running PGP non-interactively from a Unix +shell script or MSDOS batch file. + + + +PGP Returns Exit Status to the Shell +------------------------------------ + +To facilitate running PGP in "batch" mode, such as from an MSDOS +".bat" file or from a Unix shell script, PGP returns an error exit +status to the shell. An exit status code of zero means normal exit, +while a nonzero exit status indicates some kind of error occurred. +Different error exit conditions return different exit status codes to +the shell. + + + +Environmental Variable for Pass Phrase +-------------------------------------- + +Normally, PGP prompts the user to type a pass phrase whenever PGP +needs a pass phrase to unlock a secret key. But it is possible to +store the pass phrase in an environmental variable from your +operating system's command shell. The environmental variable PGPPASS +can be used to hold the pass phrase that PGP will attempt to use +first. If the pass phrase stored in PGPPASS is incorrect, PGP +recovers by prompting the user for the correct pass phrase. + +For example, on MSDOS, the shell command: + + SET PGPPASS=zaphod beeblebrox for president + +would eliminate the prompt for the pass phrase if the pass phrase +were indeed "zaphod beeblebrox for president". + +This dangerous feature makes your life more convenient if you have to +regularly deal with a large number of incoming messages addressed to +your secret key, by eliminating the need for you to repeatedly type +in your pass phrase every time you run PGP. + +I added this feature because of popular demand. However, this is a +somewhat dangerous feature, because it keeps your precious pass +phrase stored somewhere other than just in your brain. Even worse, +if you are particularly reckless, it may even be stored on a disk on +the same computer as your secret key. It would be particularly +dangerous and stupid if you were to install this command in a batch +or script file, such as the MSDOS AUTOEXEC.BAT file. Someone could +come along on your lunch hour and steal both your secret key ring and +the file containing your pass phrase. + +I can't emphasize the importance of this risk enough. If you are +contemplating using this feature, be sure to read the sections +"Exposure on Multi-user Systems" and "How to Protect Secret Keys from +Disclosure" in this volume and in the Essential Topics volume of the +PGP User's Guide. + +If you must use this feature, the safest way to do it would be to +just manually type in the shell command to set PGPPASS every time you +boot your machine to start using PGP, and then erase it or turn off +your machine when you are done. And you should definitely never do +it in an environment where someone else may have access to your +machine. Someone could come along and simply ask your computer to +display the contents of PGPPASS. + + + +Setting Configuration Parameters: CONFIG.TXT +============================================ + +PGP has a number of user-settable parameters that can be defined in a +special configuration text file called "config.txt", in the directory +pointed to by the shell environmental variable PGPPATH. Having a +configuration file enables the user to define various flags and +parameters for PGP without the burden of having to always define +these parameters in the PGP command line. + +Configuration parameters may be assigned integer values, character +string values, or on/off values, depending on what kind of +configuration parameter it is. A sample configuration file is +provided with PGP, so you can see some examples. + +In the configuration file, blank lines are ignored, as is anything +following the '#' comment character. Keywords are not +case-sensitive. + +Here is a short sample fragment of a typical configuration file: + + # TMP is the directory for PGP scratch files, such as a RAM disk. + TMP = "e:\" # Can be overridden by environment variable TMP. + Armor = on # Use -a flag for ASCII armor whenever applicable. + # CERT_DEPTH is how deeply introducers may introduce introducers. + cert_depth = 3 + +If some configuration parameters are not defined in the configuration +file, or if there is no configuration file, or if PGP can't find the +configuration file, the values for the configuration parameters +default to some reasonable value. + +Note that it is also possible to set these same configuration +parameters directly from the PGP command line, by preceding the +parameter setting with a "+" character. For example, the following +two PGP commands produce the same effect: + + pgp -e +armor=on message.txt smith +or: pgp -ea message.txt smith + + +The following is a summary of the various parameters than may be +defined in the configuration file. + + +TMP - Directory Pathname for Temporary Files +-------------------------------------------- + +Default setting: TMP = "" + +The configuration parameter TMP specifies what directory to use for +PGP's temporary scratch files. The best place to put them is on a +RAM disk, if you have one. That speeds things up quite a bit, and +increases security somewhat. If TMP is undefined, the temporary +files go in the current directory. If the shell environmental +variable TMP is defined, PGP instead uses that to specify where the +temporary files should go. + + +LANGUAGE - Foreign Language Selector +------------------------------------ + +Default setting: LANGUAGE = "en" + +PGP displays various prompts, warning messages, and advisories to the +user on the screen. For example, messages such as "File not found.", +or "Please enter your pass phrase:". These messages are normally in +English. But it is possible to get PGP to display its messages to +the user in other languages, without having to modify the PGP +executable program. + +A number of people in various countries have translated all of PGP's +display messages, warnings, and prompts into their native languages. +These hundreds of translated message strings have been placed in a +special text file called "language.txt", distributed with the PGP +release. The messages are stored in this file in English, Spanish, +Dutch, German, French, Italian, Russian, Latvian, and Lithuanian. +Other languages may be added later. + +The configuration parameter LANGUAGE specifies what language to +display these messages in. LANGUAGE may be set to "en" for English, +"es" for Spanish, "de" for German, "nl" for Dutch, "fr" for French, +"it" for Italian, "ru" for Russian, "lt3" for Lithuanian, "lv" for +Latvian, "esp" for Esperanto. For example, if this line appeared in +the configuration file: + + LANGUAGE = "fr" + +PGP would select French as the language for its display messages. +The default setting is English. + +When PGP needs to display a message to the user, it looks in the +"language.txt" file for the equivalent message string in the selected +foreign language and displays that translated message to the user. +If PGP can't find the language string file, or if the selected +language is not in the file, or if that one phrase is not translated +into the selected language in the file, or if that phrase is missing +entirely from the file, PGP displays the message in English. + +To conserve disk space, most foreign translations are not included +in the standard PGP release package, but are available separately. + + +MYNAME - Default User ID for Making Signatures +---------------------------------------------- + +Default setting: MYNAME = "" + +The configuration parameter MYNAME specifies the default user ID to +use to select the secret key for making signatures. If MYNAME is not +defined, the most recent secret key you installed on your secret key +ring will be used. The user may also override this setting by +specifying a user ID on the PGP command line with the -u option. + + +TEXTMODE - Assuming Plaintext is a Text File +-------------------------------------------- + +Default setting: TEXTMODE = off + +The configuration parameter TEXTMODE is equivalent to the -t command +line option. If enabled, it causes PGP to assume the plaintext is a +text file, not a binary file, and converts it to "canonical text" +before encrypting it. Canonical text has a carriage return and a +linefeed at the end of each line of text. + +This mode will be automatically turned off if PGP detects that the +plaintext file contains what it thinks is non-text binary data. If +you intend to use PGP primarily for E-mail purposes, you should turn +TEXTMODE=ON. + +For VAX/VMS systems, the current version of PGP defaults TEXTMODE=ON. + +For further details, see the section "Sending ASCII Text Files Across +Different Machine Environments". + + +CHARSET - Specifies Local Character Set for Text Files +------------------------------------------------------ + +Default setting: CHARSET = NOCONV + +Because PGP must process messages in many non-English languages with +non-ASCII character sets, you may have a need to tell PGP what local +character set your machine uses. This determines what character +conversions are performed when converting plaintext files to and from +canonical text format. This is only a concern if you are in a +non-English non-ASCII environment. + +The configuration parameter CHARSET selects the local character set. +The choices are NOCONV (no conversion), LATIN1 (ISO 8859-1 Latin +Alphabet 1), KOI8 (used by most Russian Unix systems), ALT_CODES +(used by Russian MSDOS systems), ASCII, and CP850 (used by most +western European languages on standard MSDOS PCs). + +LATIN1 is the internal representation used by PGP for canonical text, +so if you select LATIN1, no conversion is done. Note also that PGP +treats KOI8 as LATIN1, even though it is a completely different +character set (Russian), because trying to convert KOI8 to either +LATIN1 or CP850 would be futile anyway. This means that setting +CHARSET to NOCONV, LATIN1, or KOI8 are all equivalent to PGP. + +If you use MSDOS and expect to send or receive traffic in western +European languages, set CHARSET = "CP850". This will make PGP +convert incoming canonical text messages from LATIN1 to CP850 after +decryption. If you use the -t (textmode) option to convert to +canonical text, PGP will convert your CP850 text to LATIN1 before +encrypting it. + +For further details, see the section "Sending ASCII Text Files Across +Different Machine Environments". + + +ARMOR - Enable ASCII Armor Output +--------------------------------- + +Default setting: ARMOR = off + +The configuration parameter ARMOR is equivalent to the -a command +line option. If enabled, it causes PGP to emit ciphertext or keys in +ASCII Radix-64 format suitable for transporting through E-mail +channels. Output files are named with the ".asc" extension. + +If you intend to use PGP primarily for E-mail purposes, you should +turn ARMOR=ON. + +For further details, see the section "Sending Ciphertext Through +E-mail Channels: Radix-64 Format" in the Essential Topics volume. + + +ARMORLINES - Size of ASCII Armor Multipart Files +------------------------------------------------ + +Default setting: ARMORLINES = 720 + +When PGP creates a very large ".asc" radix-64 file for sending +ciphertext or keys through the E-mail, it breaks the file up into +separate chunks small enough to send through Internet mail +utilities. Normally, Internet mailers prohibit files larger than +about 50000 bytes, which means that if we restrict the number of +lines to about 720, we'll be well within the limit. The file chunks +are named with suffixes ".as1", ".as2", ".as3", ... + +The configuration parameter ARMORLINES specifies the maximum number +of lines to make each chunk in a multipart ".asc" file sequence. If +you set it to zero, PGP will not break up the file into chunks. + +Fidonet email files usually have an upper limit of about 32K bytes, +so 450 lines would be appropriate for Fidonet environments. + +For further details, see the section "Sending Ciphertext Through +E-mail Channels: Radix-64 Format" in the Essential Topics volume. + + +KEEPBINARY - Keep Binary Ciphertext Files After Decrypting +---------------------------------------------------------- + +Default setting: KEEPBINARY = off + +When PGP reads a ".asc" file, it recognizes that the file is in +radix-64 format and will convert it back to binary before processing +as it normally does, producing as a by-product a ".pgp" ciphertext +file in binary form. After further processing to decrypt the ".pgp" +file, the final output file will be in normal plaintext form. + +You may want to delete the binary ".pgp" intermediate file, or you +may want PGP to delete it for you automatically. You can still rerun +PGP on the original ".asc" file. + +The configuration parameter KEEPBINARY enables or disables keeping +the intermediate ".pgp" file during decryption. + +For further details, see the section "Sending Ciphertext Through +E-mail Channels: Radix-64 Format" in the Essential Topics volume. + + +COMPRESS - Enable Compression +----------------------------- + +Default setting: COMPRESS = on + +The configuration parameter COMPRESS enables or disables data +compression before encryption. It is used mainly for debugging PGP. +Normally, PGP attempts to compress the plaintext before it encrypts +it. Generally, you should leave this alone and let PGP attempt to +compress the plaintext. + + +COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed +------------------------------------------------------------------ + +Default setting: COMPLETES_NEEDED = 1 + +The configuration parameter COMPLETES_NEEDED specifies the minimum +number of completely trusted introducers required to fully certify a +public key on your public key ring. This gives you a way of tuning +PGP's skepticism. + +For further details, see the section "How Does PGP Keep Track of +Which Keys are Valid?" in the Essential Topics volume. + + +MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed +------------------------------------------------------------------ + +Default setting: MARGINALS_NEEDED = 2 + +The configuration parameter MARGINALS_NEEDED specifies the minimum +number of marginally trusted introducers required to fully certify a +public key on your public key ring. This gives you a way of tuning +PGP's skepticism. + +For further details, see the section "How Does PGP Keep Track of +Which Keys are Valid?" in the Essential Topics volume. + + +CERT_DEPTH - How Deep May Introducers Be Nested +----------------------------------------------- + +Default setting: CERT_DEPTH = 4 + +The configuration parameter CERT_DEPTH specifies how many levels deep +you may nest introducers to certify other introducers to certify +public keys on your public key ring. For example, If CERT_DEPTH is +set to 1, there may only be one layer of introducers below your own +ultimately-trusted key. If that were the case, you would be required +to directly certify the public keys of all trusted introducers on +your key ring. If you set CERT_DEPTH to 0, you could have no +introducers at all, and you would have to directly certify each and +every key on your public key ring in order to use it. The minimum +CERT_DEPTH is 0, the maximum is 8. + +For further details, see the section "How Does PGP Keep Track of +Which Keys are Valid?" in the Essential Topics volume. + + +BAKRING - Filename for Backup Secret Keyring +-------------------------------------------- + +Default setting: BAKRING = "" + +All of the key certification that PGP does on your public key ring +ultimately depends on your own ultimately-trusted public key (or +keys). To detect any tampering of your public key ring, PGP must +check that your own key has not been tampered with. To do this, PGP +must compare your public key against a backup copy of your secret key +on some tamper-resistant media, such as a write-protected floppy +disk. A secret key contains all the information that your public key +has, plus some secret components. This means PGP can check your +public key against a backup copy of your secret key. + +The configuration parameter BAKRING specifies what pathname to use +for PGP's trusted backup copy of your secret key ring. On MSDOS, you +could set it to "a:\secring.pgp" to point it at a write-protected +backup copy of your secret key ring on your floppy drive. This check +is performed only when you execute the PGP -kc option to check your +whole public key ring. + +If BAKRING is not defined, PGP will not check your own key against +any backup copy. + +For further details, see the sections "How to Protect Public Keys +from Tampering" and "How Does PGP Keep Track of Which Keys are +Valid?" in the Essential Topics volume. + + +PUBRING - Filename for Your Public Keyring +------------------------------------------ + +Default setting: PUBRING = "$PGPPATH/pubring.pgp" + +You may want to keep your public keyring in a directory separate from +your config.txt file in the directory specified by your $PGPPATH +environmental variable. You may specify the full path and filename +for your public keyring by setting the PUBRING parameter. For +example, on an MSDOS system, you might want to keep your public +keyring on a floppy disk by: + + PUBRING = "a:pubring.pgp" + +This feature is especially handy for specifying an alternative +keyring on the command line. + + +SECRING - Filename for Your Secret Keyring +------------------------------------------ + +Default setting: SECRING = "$PGPPATH/secring.pgp" + +You may want to keep your secret keyring in a directory separate from +your config.txt file in the directory specified by your $PGPPATH +environmental variable. This comes in handy for putting your secret +keyring in a directory or device that is more protected than your +public keyring. You may specify the full path and filename for your +secret keyring by setting the SECRING parameter. For example, on an +MSDOS system, you might want to keep your secret keyring on a floppy +disk by: + + SECRING = "a:secring.pgp" + + +RANDSEED - Filename for Random Number Seed +------------------------------------------ + +Default setting: RANDSEED = "$PGPPATH/randseed.bin" + +You may want to keep your random number seed file (for generation of +session keys) in a directory separate from your config.txt file in +the directory specified by your $PGPPATH environmental variable. +This comes in handy for putting your random number seed file in a +directory or device that is more protected than your public keyring. +You may specify the full path and filename for your random seed file +by setting the RANDSEED parameter. For example, on an MSDOS system, +you might want to keep it on a floppy disk by: + + RANDSEED = "a:randseed.bin" + + +PAGER - Selects Shell Command to Display Plaintext Output +--------------------------------------------------------- + +Default setting: PAGER = "" + +PGP lets you view the decrypted plaintext output on your screen (like +the Unix-style "more" command), without writing it to a file, if you +use the -m (more) option while decrypting. This displays the +decrypted plaintext display on your screen one screenful at a time. + +If you prefer to use a fancier page display utility, rather than +PGP's built-in one, you can specify the name of a shell command that +PGP will invoke to display your plaintext output file. The +configuration parameter PAGER specifies the shell command to invoke +to display the file. For example, on MSDOS systems, you might want +to use the popular shareware program "list.com" to display your +plaintext message. Assuming you have a copy of "list.com", you may +set PAGER accordingly: + + PAGER = "list" + +However, if the sender specified that this file is for your eyes +only, and may not be written to disk, PGP always uses its own +built-in display function. + +For further details, see the section "Displaying Decrypted Plaintext +on Your Screen". + + +SHOWPASS - Echo Pass Phrase to User +----------------------------------- + +Default setting: SHOWPASS = off + +Normally, PGP does not let you see your pass phrase as you type it +in. This makes it harder for someone to look over your shoulder +while you type and learn your pass phrase. But some typing-impaired +people have problems typing their pass phrase without seeing what +they are typing, and they may be typing in the privacy of their own +homes. So they asked if PGP can be configured to let them see what +they type when they type in their pass phrase. + +The configuration parameter SHOWPASS enables PGP to echo your typing +during pass phrase entry. + + +TZFIX - Timezone Adjustment +--------------------------- + +Default setting: TZFIX = 0 + +PGP provides timestamps for keys and signature certificates in +Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC), which +means the same thing for our purposes. When PGP asks the system for +the time of day, the system is supposed to provide it in GMT. + +But sometimes, because of improperly configured MSDOS systems, the +system time is returned in US Pacific Standard Time time plus 8 +hours. Sounds weird, doesn't it? Perhaps because of some sort of US +west-coast jingoism, MSDOS presumes local time is US Pacific time, +and pre-corrects Pacific time to GMT. This adversely affects the +behavior of the internal MSDOS GMT time function that PGP calls. +However, if your MSDOS environmental variable TZ is already properly +defined for your timezone, this corrects the misconception MSDOS has +that the whole world lives on the US west coast. + +The configuration parameter TZFIX specifies the number of hours to +add to the system time function to get GMT, for GMT timestamps on +keys and signatures. If the MSDOS environmental variable TZ is +defined properly, you can leave TZFIX=0. Unix systems usually +shouldn't need to worry about setting TZFIX at all. But if you are +using some other obscure operating system that doesn't know about +GMT, you may have to use TZFIX to adjust the system time to GMT. + +On MSDOS systems that do not have TZ defined in the environment, you +should make TZFIX=0 for California, -1 for Colorado, -2 for Chicago, +-3 for New York, -8 for London, -9 for Amsterdam. In the summer, +TZFIX should be manually decremented from these values. What a mess. + +It would be much cleaner to set your MSDOS environmental variable TZ +in your AUTOEXEC.BAT file, and not use the TZFIX correction. Then +MSDOS gives you good GMT timestamps, and will handle daylight savings +time adjustments for you. Here are some sample lines to insert into +AUTOEXEC.BAT, depending on your time zone: + +For Los Angeles: SET TZ=PST8PDT +For Denver: SET TZ=MST7MDT +For Arizona: SET TZ=MST7 + (Arizona never uses daylight savings time) +For Chicago: SET TZ=CST6CDT +For New York: SET TZ=EST5EDT +For London: SET TZ=GMT0BST +For Amsterdam: SET TZ=MET-1DST +For Moscow: SET TZ=MSK-3MSD +For Aukland: SET TZ=NZT-13 + + +CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text +------------------------------------------------------------------ + +Default setting: CLEARSIG = on + +Normally, unencrypted PGP signed messages have a signature +certificate prepended in binary form. To send this through a 7-bit +E-mail channel, radix-64 ASCII armor is applied (see the ARMOR +parameter), rendering the message unreadable to casual human eyes, +even though the message is not actually encrypted. The recipient +must use PGP to strip the armor off before reading the message. + +If the original plaintext message is in text (not binary) form, there +is a way to send it through an E-mail channel in such a way that the +ASCII armor is applied only to the binary signature certificate, but +not to the plaintext message. This makes it possible to read the +signed message with human eyes, without the aid of PGP. Of course, +you still need PGP to actually check the signature. + +The CLEARSIG flag is preset to "on" beginning with PGP version 2.5. +To enable the full CLEARSIG behavior, the ARMOR and TEXTMODE flags +must also be turned on. Set ARMOR=ON (or use the -a option), and set +TEXTMODE=ON (or use the -t option). If your config file has CLEARSIG +turned off, you can turn it back on again directly on the command +line, like so: + + pgp -sta +clearsig=on message.txt + +This message representation is analogous to the MIC-CLEAR message type +used in Internet Privacy Enhanced Mail (PEM). It is important to +note that since this method only applies ASCII armor to the binary +signature certificate, and not to the message text itself, there is +some risk that the unarmored message may suffer some accidental +molestation while en route. This can happen if it passes through +some E-mail gateway that performs character set conversions, or in +some cases extra spaces may be added to or stripped from the ends of +lines. If this occurs, the signature will fail to verify, which may +give a false indication of intentional tampering. But since PEM +lives under a similar vulnerability, it seems worth having this +feature despite the risks. + +Beginning with PGP version 2.2, trailing blanks are ignored on each +line in calculating the signature for text in CLEARSIG mode. + + +VERBOSE - Quiet, Normal, or Verbose Messages +-------------------------------------------- + +Default setting: VERBOSE = 1 + +VERBOSE may be set to 0, 1, or 2, depending on how much detail you +want to see from PGP diagnostic messages. The settings are: + +0 - Display messages only if there is a problem. Unix fans wanted +this "quiet mode" setting. + +1 - Normal default setting. Displays a reasonable amount of detail +in diagnostic or advisory messages. + +2 - Displays maximum information, usually to help diagnose problems +in PGP. Not recommended for normal use. Besides, PGP doesn't have +any problems, right? + + +INTERACTIVE - Ask for Confirmation for Key Adds +----------------------------------------------- + +Default Setting: INTERACTIVE = off + +Enabling this mode will mean that if you add a key file containing +multiple keys to your key ring, PGP will ask for confirmation for +each key before adding it to your key ring. + + + +Protecting Against Bogus Timestamps +=================================== + +A somewhat obscure vulnerability of PGP involves dishonest users +creating bogus timestamps on their own public key certificates and +signatures. You can skip over this section if you are a casual user +and aren't deeply into obscure public key protocols. + +There's nothing to stop a dishonest user from altering the date and +time setting of his own system's clock, and generating his own public +key certificates and signatures that appear to have been created at a +different time. He can make it appear that he signed something +earlier or later than he actually did, or that his public/secret key +pair was created earlier or later. This may have some legal or +financial benefit to him, for example by creating some kind of +loophole that might allow him to repudiate a signature. + +A remedy for this could involve some trustworthy Certifying Authority +or notary that would create notarized signatures with a trustworthy +timestamp. This might not necessarily require a centralized +authority. Perhaps any trusted introducer or disinterested party +could serve this function, the same way real notary publics do now. +A public key certificate could be signed by the notary, and the +trusted timestamp in the notary's signature would have some legal +significance. The notary could enter the signed certificate into a +special certificate log controlled by the notary. Anyone can read +this log. + +The notary could also sign other people's signatures, creating a +signature certificate of a signature certificate. This would serve +as a witness to the signature the same way real notaries do now with +paper. Again, the notary could enter the detached signature +certificate (without the actual whole document that was signed) into +a log controlled by the notary. The notary's signature would have a +trusted timestamp, which might have greater credibility than the +timestamp in the original signature. A signature becomes "legal" if +it is signed and logged by the notary. + +This problem of certifying signatures with notaries and trusted +timestamps warrants further discussion. This can of worms will not +be fully covered here now. There is a good treatment of this topic +in Denning's 1983 article in IEEE Computer (see references). There +is much more detail to be worked out in these various certifying +schemes. This will develop further as PGP usage increases and other +public key products develop their own certifying schemes. + + + +A Peek Under the Hood +===================== + +Let's take a look at a few internal features of PGP. + + +Random Numbers +-------------- + +PGP uses a cryptographically strong pseudorandom number generator for +creating temporary conventional session keys. The seed file for this +is called "randseed.bin". It too can be kept in whatever directory +is indicated by the PGPPATH environmental variable. If this random +seed file does not exist, it is automatically created and seeded with +truly random numbers derived from timing your keystroke latencies. + +This generator reseeds the disk file each time it is used by mixing +in new key material partially derived with the time of day and other +truly random sources. It uses the conventional encryption algorithm +as an engine for the random number generator. The seed file contains +both random seed material and random key material to key the +conventional encryption engine for the random generator. + +This random seed file should be at least slightly protected from +disclosure, to reduce the risk of an attacker deriving your next or +previous session keys. The attacker would have a very hard time +getting anything useful from capturing this random seed file, because +the file is cryptographically laundered before and after each use. +Nonetheless, it seems prudent to at least try to keep it from falling +into the wrong hands. + +If you feel uneasy about trusting any algorithmically derived random +number source however strong, keep in mind that you already trust the +strength of the same conventional cipher to protect your messages. +If it's strong enough for that, then it should be strong enough to +use as a source of random numbers for temporary session keys. Note +that PGP still uses truly random numbers from physical sources +(mainly keyboard timings) to generate long-term public/secret key +pairs. + + + +PGP's Conventional Encryption Algorithm +--------------------------------------- + +As described earlier, PGP "bootstraps" into a conventional single-key +encryption algorithm by using a public key algorithm to encipher the +conventional session key and then switching to fast conventional +cryptography. So let's talk about this conventional encryption +algorithm. It isn't the DES. + +The Federal Data Encryption Standard (DES) used to be a good +algorithm for most commercial applications. But the Government never +did trust the DES to protect its own classified data, because the DES +key length is only 56 bits, short enough for a brute force attack. +Also, the full 16-round DES has been attacked with some success by +Biham and Shamir using differential cryptanalysis, and by Matsui +using linear cryptanalysis. + +The most devastating practical attack on the DES was described at the +Crypto '93 conference, where Michael Wiener of Bell Northern Research +presented a paper on how to crack the DES with a special machine. He +has fully designed and tested a chip that guesses 50 million DES keys +per second until it finds the right one. Although he has refrained +from building the real chips so far, he can get these chips +manufactured for $10.50 each, and can build 57000 of them into a +special machine for $1 million that can try every DES key in 7 hours, +averaging a solution in 3.5 hours. $1 million can be hidden in the +budget of many companies. For $10 million, it takes 21 minutes to +crack, and for $100 million, just two minutes. With any major +government's budget for examining DES traffic, it can be cracked in +seconds. This means that straight 56-bit DES is now effectively dead +for purposes of serious data security applications. + +A possible successor to DES may be a variation known as "triple DES", +which uses two DES keys to encrypt three times, achieving an +effective key space of 112 bits. But this approach is three times +slower than normal DES. A future version of PGP may support triple +DES as an option. + +PGP does not use the DES as its conventional single-key algorithm to +encrypt messages. Instead, PGP uses a different conventional +single-key block encryption algorithm, called IDEA(tm). + +For the cryptographically curious, the IDEA cipher has a 64-bit block +size for the plaintext and the ciphertext. It uses a key size of 128 +bits. It is based on the design concept of "mixing operations from +different algebraic groups". It runs much faster in software than +the DES. Like the DES, it can be used in cipher feedback (CFB) and +cipher block chaining (CBC) modes. PGP uses it in 64-bit CFB mode. + +The IPES/IDEA block cipher was developed at ETH in Zurich by James L. +Massey and Xuejia Lai, and published in 1990. This is not a +"home-grown" algorithm. Its designers have a distinguished +reputation in the cryptologic community. Early published papers on +the algorithm called it IPES (Improved Proposed Encryption Standard), +but they later changed the name to IDEA (International Data +Encryption Algorithm). So far, IDEA has resisted attack much better +than other ciphers such as FEAL, REDOC-II, LOKI, Snefru and Khafre. +And recent evidence suggests that IDEA is more resistant than the DES +to Biham & Shamir's highly successful differential cryptanalysis +attack. Biham and Shamir have been examining the IDEA cipher for +weaknesses, without success. Academic cryptanalyst groups in +Belgium, England, and Germany are also attempting to attack it, as +well as the military services from several European countries. As +this new cipher continues to attract attack efforts from the most +formidable quarters of the cryptanalytic world, confidence in IDEA is +growing with the passage of time. + +Every once in a while, I get a letter from someone who has just +learned the awful truth that PGP does not use pure RSA to encrypt +bulk data. They are concerned that the whole package is weakened if +we use a hybrid public-key and conventional scheme just to speed +things up. After all, a chain is only as strong as its weakest +link. They demand an explanation for this apparent "compromise" in +the strength of PGP. This may be because they have been caught up in +the public's reverence and awe for the strength and mystique of RSA, +mistakenly believing that RSA is intrinsically stronger than any +conventional cipher. Well, it's not. + +People who work in factoring research say that the workload to +exhaust all the possible 128-bit keys in the IDEA cipher would equal +the factoring workload to crack a 3100-bit RSA key, which is quite a +bit bigger than the 1024-bit RSA key size that most people use for +high security applications. Given this range of key sizes, and +assuming there are no hidden weaknesses in the conventional cipher, +the weak link in this hybrid approach is in the public key algorithm, +not the conventional cipher. + +It is not ergonomically practical to use pure RSA with large keys to +encrypt and decrypt long messages. A 1024-bit RSA key would decrypt +messages about 4000 times slower than the IDEA cipher. Absolutely no +one does it that way in the real world. Many people less experienced +in cryptography do not realize that the attraction of public key +cryptography is not because it is intrinsically stronger than a +conventional cipher-- its appeal is because it helps you manage keys +more conveniently. + +Not only is RSA too slow to use on bulk data, but it even has certain +weaknesses that can be exploited in some special cases of particular +kinds of messages that are fed to the RSA cipher. These special +cases can be avoided by using the hybrid approach of using RSA to +encrypt random session keys for a conventional cipher. So the bottom +line is this: Using pure RSA on bulk data is the wrong approach, +period. It's too slow, it's not stronger, and may even be weaker. If +you find a software application that uses pure RSA on bulk data, it +probably means the implementor does not understand these issues. + + + +Data Compression +---------------- + +PGP normally compresses the plaintext before encrypting it. It's too +late to compress it after it has been encrypted; encrypted data is +incompressible. Data compression saves modem transmission time and +disk space and more importantly strengthens cryptographic security. +Most cryptanalysis techniques exploit redundancies found in the +plaintext to crack the cipher. Data compression reduces this +redundancy in the plaintext, thereby greatly enhancing resistance to +cryptanalysis. It takes extra time to compress the plaintext, but +from a security point of view it seems worth it, at least in my +cautious opinion. + +Files that are too short to compress or just don't compress well are +not compressed by PGP. + +If you prefer, you can use PKZIP to compress the plaintext before +encrypting it. PKZIP is a widely-available and effective MSDOS +shareware compression utility from PKWare, Inc. Or you can use ZIP, +a PKZIP-compatible freeware compression utility on Unix and other +systems, available from Jean-Loup Gailly. There is some advantage in +using PKZIP or ZIP in certain cases, because unlike PGP's built-in +compression algorithm, PKZIP and ZIP have the nice feature of +compressing multiple files into a single compressed file, which is +reconstituted again into separate files when decompressed. PGP will +not try to compress a plaintext file that has already been +compressed. After decrypting, the recipient can decompress the +plaintext with PKUNZIP. If the decrypted plaintext is a PKZIP +compressed file, PGP automatically recognizes this and advises the +recipient that the decrypted plaintext appears to be a PKZIP file. + +For the technically curious readers, the current version of PGP uses +the freeware ZIP compression routines written by Jean-loup Gailly, +Mark Adler, and Richard B. Wales. This ZIP software uses +functionally-equivalent compression algorithms as those used by +PKWare's new PKZIP 2.0. This ZIP compression software was selected +for PGP mainly because of its free portable C source code +availability, and because it has a really good compression ratio, and +because it's fast. + +Peter Gutmann has also written a nice compression utility called +HPACK, available for free from many Internet FTP sites. It encrypts +the compressed archives, using PGP data formats and key rings. He +wanted me to mention that here. + + + +Message Digests and Digital Signatures +-------------------------------------- + +To create a digital signature, PGP encrypts with your secret key. +But PGP doesn't actually encrypt your entire message with your secret +key-- that would take too long. Instead, PGP encrypts a "message +digest". + +The message digest is a compact (128 bit) "distillate" of your +message, similar in concept to a checksum. You can also think of it +as a "fingerprint" of the message. The message digest "represents" +your message, such that if the message were altered in any way, a +different message digest would be computed from it. This makes it +possible to detect any changes made to the message by a forger. A +message digest is computed using a cryptographically strong one-way +hash function of the message. It would be computationally infeasible +for an attacker to devise a substitute message that would produce an +identical message digest. In that respect, a message digest is much +better than a checksum, because it is easy to devise a different +message that would produce the same checksum. But like a checksum, +you can't derive the original message from its message digest. + +A message digest alone is not enough to authenticate a message. The +message digest algorithm is publicly known, and does not require +knowledge of any secret keys to calculate. If all we did was attach +a message digest to a message, then a forger could alter a message +and simply attach a new message digest calculated from the new +altered message. To provide real authentication, the sender has to +encrypt (sign) the message digest with his secret key. + +A message digest is calculated from the message by the sender. The +sender's secret key is used to encrypt the message digest and an +electronic timestamp, forming a digital signature, or signature +certificate. The sender sends the digital signature along with the +message. The receiver receives the message and the digital +signature, and recovers the original message digest from the digital +signature by decrypting it with the sender's public key. The +receiver computes a new message digest from the message, and checks +to see if it matches the one recovered from the digital signature. If +it matches, then that proves the message was not altered, and it came +from the sender who owns the public key used to check the signature. + +A potential forger would have to either produce an altered message +that produces an identical message digest (which is infeasible), or +he would have to create a new digital signature from a different +message digest (also infeasible, without knowing the true sender's +secret key). + +Digital signatures prove who sent the message, and that the message +was not altered either by error or design. It also provides +non-repudiation, which means the sender cannot easily disavow his +signature on the message. + +Using message digests to form digital signatures has other advantages +besides being faster than directly signing the entire actual message +with the secret key. Using message digests allows signatures to be +of a standard small fixed size, regardless of the size of the actual +message. It also allows the software to check the message integrity +automatically, in a manner similar to using checksums. And it allows +signatures to be stored separately from messages, perhaps even in a +public archive, without revealing sensitive information about the +actual messages, because no one can derive any message content from a +message digest. + +The message digest algorithm used here is the MD5 Message Digest +Algorithm, placed in the public domain by RSA Data Security, Inc. +MD5's designer, Ronald Rivest, writes this about MD5: + +"It is conjectured that the difficulty of coming up with two messages +having the same message digest is on the order of 2^64 operations, +and that the difficulty of coming up with any message having a given +message digest is on the order of 2^128 operations. The MD5 +algorithm has been carefully scrutinized for weaknesses. It is, +however, a relatively new algorithm and further security analysis is +of course justified, as is the case with any new proposal of this +sort. The level of security provided by MD5 should be sufficient for +implementing very high security hybrid digital signature schemes +based on MD5 and the RSA public-key cryptosystem." + + + +Compatibility with Previous Versions of PGP +=========================================== + +PGP version 2.5 is able to read anything produced by versions 2.3, +2.3a, or 2.4. And PGP version 2.4, 2.3a, 2.3, and 2.2 can read +anything produced by PGP 2.5. But unfortunately, due to data format +limitations imposed by RSAREF, PGP 2.5 cannot interpret any messages +or signatures made with PGP version 2.2 or earlier. Since we have no +choice but to use the new data formats, because of the legal +requirement to switch to RSAREF, we can't do anything about this +problem in PGP 2.5. + +There is compatibility from version 2.0 upwards through 2.4. Because +new features are added, older versions may not always be able to +handle some files created with newer versions. + +PGP version 2.0 (and later) is not compatible with PGP version 1.0. +If you have keys generated with version 1.0, you will have to +generate new keys to use with this version. There were just too many +needed changes and too many new algorithms to make it compatible with +the old formats. This applies to everything -- keys, messages and +signatures. + +We made some effort to design the internal data structures of this +version of PGP to be adaptable to future changes, so that hopefully +you will not be required to discard and regenerate your keys in future +versions. + + +Vulnerabilities +=============== + +No data security system is impenetrable. PGP can be circumvented in +a variety of ways. In any data security system, you have to ask +yourself if the information you are trying to protect is more +valuable to your attacker than the cost of the attack. This should +lead you to protecting yourself from the cheapest attacks, while not +worrying about the more expensive attacks. + +Some of the discussion that follows may seem unduly paranoid, but +such an attitude is appropriate for a reasonable discussion of +vulnerability issues. + + +Compromised Pass Phrase and Secret Key +-------------------------------------- + +Probably the simplest attack is if you leave your pass phrase for +your secret key written down somewhere. If someone gets it and also +gets your secret key file, they can read your messages and make +signatures in your name. + +Don't use obvious passwords that can be easily guessed, such as the +names of your kids or spouse. If you make your pass phrase a single +word, it can be easily guessed by having a computer try all the words +in the dictionary until it finds your password. That's why a pass +phrase is so much better than a password. A more sophisticated +attacker may have his computer scan a book of famous quotations to +find your pass phrase. An easy to remember but hard to guess pass +phrase can be easily constructed by some creatively nonsensical +sayings or very obscure literary quotes. + +For further details, see the section "How to Protect Secret Keys from +Disclosure" in the Essential Topics volume of the PGP User's Guide. + + +Public Key Tampering +-------------------- + +A major vulnerability exists if public keys are tampered with. This +may be the most crucially important vulnerability of a public key +cryptosystem, in part because most novices don't immediately +recognize it. The importance of this vulnerability, and appropriate +hygienic countermeasures, are detailed in the section "How to Protect +Public Keys from Tampering" in the Essential Topics volume. + +To summarize: When you use someone's public key, make certain it has +not been tampered with. A new public key from someone else should be +trusted only if you got it directly from its owner, or if it has been +signed by someone you trust. Make sure no one else can tamper with +your own public key ring. Maintain physical control of both your +public key ring and your secret key ring, preferably on your own +personal computer rather than on a remote timesharing system. Keep a +backup copy of both key rings. + + +"Not Quite Deleted" Files +------------------------- + +Another potential security problem is caused by how most operating +systems delete files. When you encrypt a file and then delete the +original plaintext file, the operating system doesn't actually +physically erase the data. It merely marks those disk blocks as +deleted, allowing the space to be reused later. It's sort of like +discarding sensitive paper documents in the paper recycling bin +instead of the paper shredder. The disk blocks still contain the +original sensitive data you wanted to erase, and will probably +eventually be overwritten by new data at some point in the future. +If an attacker reads these deleted disk blocks soon after they have +been deallocated, he could recover your plaintext. + +In fact this could even happen accidentally, if for some reason +something went wrong with the disk and some files were accidentally +deleted or corrupted. A disk recovery program may be run to recover +the damaged files, but this often means some previously deleted files +are resurrected along with everything else. Your confidential files +that you thought were gone forever could then reappear and be +inspected by whomever is attempting to recover your damaged disk. +Even while you are creating the original message with a word +processor or text editor, the editor may be creating multiple +temporary copies of your text on the disk, just because of its +internal workings. These temporary copies of your text are deleted +by the word processor when it's done, but these sensitive fragments +are still on your disk somewhere. + +Let me tell you a true horror story. I had a friend, married with +young children, who once had a brief and not very serious affair. +She wrote a letter to her lover on her word processor, and deleted +the letter after she sent it. Later, after the affair was over, the +floppy disk got damaged somehow and she had to recover it because it +contained other important documents. She asked her husband to +salvage the disk, which seemed perfectly safe because she knew she +had deleted the incriminating letter. Her husband ran a commercial +disk recovery software package to salvage the files. It recovered +the files alright, including the deleted letter. He read it, which +set off a tragic chain of events. + +The only way to prevent the plaintext from reappearing is to somehow +cause the deleted plaintext files to be overwritten. Unless you know +for sure that all the deleted disk blocks will soon be reused, you +must take positive steps to overwrite the plaintext file, and also +any fragments of it on the disk left by your word processor. You can +overwrite the original plaintext file after encryption by using the +PGP -w (wipe) option. You can take care of any fragments of the +plaintext left on the disk by using any of the disk utilities +available that can overwrite all of the unused blocks on a disk. For +example, the Norton Utilities for MSDOS can do this. + +Even if you overwrite the plaintext data on the disk, it may still be +possible for a resourceful and determined attacker to recover the +data. Faint magnetic traces of the original data remain on the disk +after it has been overwritten. Special sophisticated disk recovery +hardware can sometimes be used to recover the data. + + +Viruses and Trojan Horses +------------------------- + +Another attack could involve a specially-tailored hostile computer +virus or worm that might infect PGP or your operating system. This +hypothetical virus could be designed to capture your pass phrase or +secret key or deciphered messages, and covertly write the captured +information to a file or send it through a network to the virus's +owner. Or it might alter PGP's behavior so that signatures are not +properly checked. This attack is cheaper than cryptanalytic attacks. + +Defending against this falls under the category of defending against +viral infection generally. There are some moderately capable +anti-viral products commercially available, and there are hygienic +procedures to follow that can greatly reduce the chances of viral +infection. A complete treatment of anti-viral and anti-worm +countermeasures is beyond the scope of this document. PGP has no +defenses against viruses, and assumes your own personal computer is a +trustworthy execution environment. If such a virus or worm actually +appeared, hopefully word would soon get around warning everyone. + +Another similar attack involves someone creating a clever imitation +of PGP that behaves like PGP in most respects, but doesn't work the +way it's supposed to. For example, it might be deliberately crippled +to not check signatures properly, allowing bogus key certificates to +be accepted. This "Trojan horse" version of PGP is not hard for an +attacker to create, because PGP source code is widely available, so +anyone could modify the source code and produce a lobotomized zombie +imitation PGP that looks real but does the bidding of its diabolical +master. This Trojan horse version of PGP could then be widely +circulated, claiming to be from me. How insidious. + +You should make an effort to get your copy of PGP from a reliable +source, whatever that means. Or perhaps from more than one +independent source, and compare them with a file comparison utility. + +There are other ways to check PGP for tampering, using digital +signatures. If someone you trust signs the executable version of +PGP, vouching for the fact that it has not been infected or tampered +with, you can be reasonably sure that you have a good copy. You +could use an earlier trusted version of PGP to check the signature on +a later suspect version of PGP. But this will not help at all if +your operating system is infected, nor will it detect if your +original copy of PGP.EXE has been maliciously altered in such a way +as to compromise its own ability to check signatures. This test also +assumes that you have a good trusted copy of the public key that you +use to check the signature on the PGP executable. + + +Physical Security Breach +------------------------ + +A physical security breach may allow someone to physically acquire +your plaintext files or printed messages. A determined opponent +might accomplish this through burglary, trash-picking, unreasonable +search and seizure, or bribery, blackmail or infiltration of your +staff. Some of these attacks may be especially feasible against +grassroots political organizations that depend on a largely volunteer +staff. It has been widely reported in the press that the FBI's +COINTELPRO program used burglary, infiltration, and illegal bugging +against antiwar and civil rights groups. And look what happened at +the Watergate Hotel. + +Don't be lulled into a false sense of security just because you have +a cryptographic tool. Cryptographic techniques protect data only +while it's encrypted-- direct physical security violations can still +compromise plaintext data or written or spoken information. + +This kind of attack is cheaper than cryptanalytic attacks on PGP. + + +Tempest Attacks +--------------- + +Another kind of attack that has been used by well-equipped opponents +involves the remote detection of the electromagnetic signals from +your computer. This expensive and somewhat labor-intensive attack is +probably still cheaper than direct cryptanalytic attacks. An +appropriately instrumented van can park near your office and remotely +pick up all of your keystrokes and messages displayed on your +computer video screen. This would compromise all of your passwords, +messages, etc. This attack can be thwarted by properly shielding all +of your computer equipment and network cabling so that it does not +emit these signals. This shielding technology is known as "Tempest", +and is used by some Government agencies and defense contractors. +There are hardware vendors who supply Tempest shielding commercially, +although it may be subject to some kind of Government licensing. Now +why do you suppose the Government would restrict access to Tempest +shielding? + + +Exposure on Multi-user Systems +------------------------------ + +PGP was originally designed for a single-user MSDOS machine under +your direct physical control. I run PGP at home on my own PC, and +unless someone breaks into my house or monitors my electromagnetic +emissions, they probably can't see my plaintext files or secret keys. + +But now PGP also runs on multi-user systems such as Unix and VAX/VMS. +On multi-user systems, there are much greater risks of your plaintext +or keys or passwords being exposed. The Unix system administrator or +a clever intruder can read your plaintext files, or perhaps even use +special software to covertly monitor your keystrokes or read what's +on your screen. On a Unix system, any other user can read your +environment information remotely by simply using the Unix "ps" +command. Similar problems exist for MSDOS machines connected on a +local area network. The actual security risk is dependent on your +particular situation. Some multi-user systems may be safe because +all the users are trusted, or because they have system security +measures that are safe enough to withstand the attacks available to +the intruders, or because there just aren't any sufficiently +interested intruders. Some Unix systems are safe because they are +only used by one user-- there are even some notebook computers +running Unix. It would be unreasonable to simply exclude PGP from +running on all Unix systems. + +PGP is not designed to protect your data while it is in plaintext +form on a compromised system. Nor can it prevent an intruder from +using sophisticated measures to read your secret key while it is +being used. You will just have to recognize these risks on +multi-user systems, and adjust your expectations and behavior +accordingly. Perhaps your situation is such that you should consider +only running PGP on an isolated single-user system under your direct +physical control. That's what I do, and that's what I recommend. + + +Traffic Analysis +---------------- + +Even if the attacker cannot read the contents of your encrypted +messages, he may be able to infer at least some useful information by +observing where the messages come from and where they are going, the +size of the messages, and the time of day the messages are sent. +This is analogous to the attacker looking at your long distance phone +bill to see who you called and when and for how long, even though the +actual content of your calls is unknown to the attacker. This is +called traffic analysis. PGP alone does not protect against traffic +analysis. Solving this problem would require specialized +communication protocols designed to reduce exposure to traffic +analysis in your communication environment, possibly with some +cryptographic assistance. + + +Cryptanalysis +------------- + +An expensive and formidable cryptanalytic attack could possibly be +mounted by someone with vast supercomputer resources, such as a +Government intelligence agency. They might crack your RSA key by +using some new secret factoring breakthrough. Perhaps so, but it is +noteworthy that the US Government trusts the RSA algorithm enough in +some cases to use it to protect its own nuclear weapons, according to +Ron Rivest. And civilian academia has been intensively attacking it +without success since 1978. + +Perhaps the Government has some classified methods of cracking the +IDEA(tm) conventional encryption algorithm used in PGP. This is +every cryptographer's worst nightmare. There can be no absolute +security guarantees in practical cryptographic implementations. + +Still, some optimism seems justified. The IDEA algorithm's designers +are among the best cryptographers in Europe. It has had extensive +security analysis and peer review from some of the best cryptanalysts +in the unclassified world. It appears to have some design advantages +over the DES in withstanding differential cryptanalysis, which has +been used to crack the DES. + +Besides, even if this algorithm has some subtle unknown weaknesses, +PGP compresses the plaintext before encryption, which should greatly +reduce those weaknesses. The computational workload to crack it is +likely to be much more expensive than the value of the message. + +If your situation justifies worrying about very formidable attacks of +this caliber, then perhaps you should contact a data security +consultant for some customized data security approaches tailored to +your special needs. Boulder Software Engineering, whose address and +phone are given at the end of this document, can provide such +services. + + +In summary, without good cryptographic protection of your data +communications, it may have been practically effortless and perhaps +even routine for an opponent to intercept your messages, especially +those sent through a modem or E-mail system. If you use PGP and +follow reasonable precautions, the attacker will have to expend far +more effort and expense to violate your privacy. + +If you protect yourself against the simplest attacks, and you feel +confident that your privacy is not going to be violated by a +determined and highly resourceful attacker, then you'll probably be +safe using PGP. PGP gives you Pretty Good Privacy. + + +Legal Issues +============ + + +Trademarks, Copyrights, and Warranties +-------------------------------------- + +"Pretty Good Privacy", "Phil's Pretty Good Software", and the "Pretty +Good" label for computer software and hardware products are all +trademarks of Philip Zimmermann and Phil's Pretty Good Software. PGP +is (c) Copyright Philip R. Zimmermann, 1990-1994. All rights +reserved. Philip Zimmermann also holds the copyright for the PGP +User's Manual, as well as any foreign language translations of the +manual or the software, and all derivative works. All rights +reserved. + +The author assumes no liability for damages resulting from the use of +this software, even if the damage results from defects in this +software, and makes no representations concerning the merchantability +of this software or its suitability for any specific purpose. It is +provided "as is" without express or implied warranty of any kind. +Because certain actions may delete files or render them +unrecoverable, the author assumes no responsibility for the loss or +modification of any data. + + +Patent Rights on the Algorithms +------------------------------- + +The RSA public key cryptosystem was developed at MIT, which holds a +patent on it (U.S. patent #4,405,829, issued 20 Sep 1983). A company +in California called Public Key Partners (PKP) holds the exclusive +commercial license to sell and sub-license the RSA public key +cryptosystem. MIT distributes a freeware version of PGP under the +terms of the RSAREF 2.0 license dated 16 March 1994, from RSA Data +Security, Inc (RSADSI). + +Non-US users of earlier versions of PGP should note that the RSA +patent does not apply outside the US, and at least at the time of +this writing, the author is not aware of any RSA patent in any other +country. Federal agencies may use the RSA algorithm, because the +Government paid for the development of RSA with grants from the +National Science Foundation and the Navy. But despite the fact of +Government users having free access to the RSA algorithm, Government +use of PGP has additional restrictions imposed by the agreement I +have with ViaCrypt, as explained later. + +I wrote my PGP software from scratch, with my own independently +developed implementation of the RSA algorithm. Before publishing +PGP, I got a formal written legal opinion from a patent attorney with +extensive experience in software patents. I'm convinced that +publishing PGP the way I did does not violate patent law. + +Not only did PKP acquire the exclusive patent rights for the RSA +cryptosystem, but they also acquired the exclusive rights to three +other patents covering other public key schemes invented by others at +Stanford University, also developed with federal funding. This +essentially gives one company a legal lock in the USA on nearly all +practical public key cryptosystems. They even appear to be claiming +patent rights on the very concept of public key cryptography, +regardless of what clever new original algorithms are independently +invented by others. I find such a comprehensive monopoly troubling, +because I think public key cryptography is destined to become a +crucial technology in the protection of our civil liberties and +privacy in our increasingly connected society. At the very least, +it places these vital tools at risk by affording to the Government +a single pressure point of influence. + +Beginning with PGP version 2.5 (distributed by MIT, the holders of the +original RSA patent), the freeware version of PGP uses the RSAREF 2.0 +subroutine library to perform its RSA calculations, under the RSAREF +license of 16 March 1994, which allows noncommercial use in the USA. +RSAREF is a subroutine package from RSA Data Security Inc, that +implements the RSA algorithm. The RSAREF subroutines are used instead +of PGP's original subroutines to implement the RSA functions in PGP. +See the RSAREF 2.0 license of 16 March 1994 for terms and conditions of +use of RSAREF applications. + +The PGP 2.0 release was a joint effort of an international team of +software engineers, implementing enhancements to the original PGP +with design guidance from me. It was released by Branko Lankester in +The Netherlands and Peter Gutmann in New Zealand, out of reach of US +patent law. Although released only in Europe and New Zealand, it +spontaneously spread to the USA without help from me or the PGP +development team. + +The IDEA(tm) conventional block cipher used by PGP is covered by a +patent in Europe, held by ETH and a Swiss company called Ascom-Tech +AG. The US Patent number is US005214703, and the European patent +number is EP 0 482 154 B1. IDEA(tm) is a trademark of Ascom-Tech AG. +There is no license fee required for noncommercial use of IDEA. +Commercial users of IDEA may obtain licensing details from Dieter +Profos, Ascom Tech AG, Teleservices Section, Postfach 151, 4502 +Solothurn, Switzerland, Tel +41 65 242885, Fax +41 65 235761. + +Ascom-Tech AG has granted permission for the freeware version PGP to +use the IDEA cipher in non-commercial uses, everywhere. In the US +and Canada, all commercial or Government users must obtain a licensed +version from ViaCrypt, who has a license from Ascom-Tech for the IDEA +cipher. Ascom-Tech has recently been changing its policies regarding +the use of IDEA in PGP for commercial use outside the US, and that +policy still seems to be in flux. + +The ZIP compression routines in PGP come from freeware source code, +with the author's permission. I'm not aware of any patents on the +compression algorithms used in the ZIP routines, but you're welcome to +check into that question yourself. + + +Licensing and Distribution +-------------------------- + +In the USA, PGP 2.5 is available from the Massachusetts Institute of +Technology, under the terms of the RSAREF 2.0 license dated 16 March +1994. I have no objection to anyone freely using or distributing the +freeware version of PGP, without payment of fees to me, as long as it +is for personal non-commercial use. For commercial use, contact +ViaCrypt in Phoenix, Arizona (phone 602-944-0773). You must keep the +copyright, patent, and trademark notices on PGP and keep all the +documentation with it. + +NOTE: Regardless of the complexities and partially overlapping +restrictions from all the other terms and conditions imposed by the +various patent and copyright licenses (RSA, RSAREF, and IDEA) from +various third parties, an additional overriding restriction on PGP +usage is imposed by my own agreement with ViaCrypt: The freeware +version of PGP is only for personal, noncommercial use -- all other +users in the USA and Canada must obtain a fully licensed version of +PGP from ViaCrypt. + +I had to make an agreement with ViaCrypt in the summer of 1993 to +license the exclusive commercial rights to PGP, so that there would +be a legally safe way for corporations to use PGP without risk of a +patent infringement lawsuit from PKP. For PGP to succeed in the long +term as a viable industry standard, the legal stigma associated with +the RSA patent rights had to be resolved. ViaCrypt had already +obtained a patent license from PKP to make, use, and sell products +that practice the RSA patents. ViaCrypt offered a way out of the +patent quagmire for PGP to penetrate the corporate environment. They +could sell PGP, if I licensed it to them under these terms. So we +entered into an agreement to do that, opening the door for PGP's +future in the commercial sector, which was necessary for PGP's +political future. + +PGP is not shareware, it's freeware. Published as a community service. +Giving PGP away for free will encourage far more people to use it, which +hopefully will have a greater social impact. This could lead to +widespread awareness and use of the RSA public key cryptosystem. + +Feel free to disseminate the complete PGP release package as widely +as possible, but be careful not to violate U.S. export controls if +you live in the USA. Give it to all your friends. If you have +access to any electronic Bulletin Boards Systems, please upload the +complete PGP executable object release package to as many BBS's as +possible. The freeware version of PGP is available in source code +form, and you may disseminate the source release package too, if you've +got it. NOTE: Under no circumstances should PGP be distributed +without the PGP documentation, including this PGP User's Guide and the +RSAREF 2.0 license agreement dated March 16, 1994. + +The PGP version 2.5 executable object release package for MSDOS contains +the PGP executable software, documentation, RSAREF 2.0 license, sample +key rings including my own public key, and signatures for the software +and this manual, all in one PKZIP compressed file called pgp25.zip. The +PGP source release package for MSDOS contains all the C source files in +one PKZIP compressed file called pgp25src.zip. The filename for the +release package is derived from the version number of the release. + +The primary release site for PGP is the Massachusetts Institute of +Technology, at their FTP site "net-dist.mit.edu", in their /pub/PGP +directory. You may obtain free copies or updates to PGP from this +site, or any other Internet FTP site or BBS that PGP has spread to. +Don't ask me for a copy directly from me, since I'd rather avoid +further legal problems at this time. + +After all this work I have to admit I wouldn't mind getting some fan +mail for PGP, to gauge its popularity. Let me know what you think +about it and how many of your friends use it. Bug reports and +suggestions for enhancing PGP are welcome, too. Perhaps a future PGP +release will reflect your suggestions. + +This project has not been funded and the project has nearly eaten me +alive. This means you can't count on a reply to your mail, unless +you only need a short written reply and you include a stamped +self-addressed envelope. But I do reply to E-mail. Please keep it in +English, as my foreign language skills are weak. If you call and I'm +not in, it's best to just try again later. I usually don't return +long distance phone calls, unless you leave a message that I can call +you collect. If you need any significant amount of my time, I am +available on a paid consulting basis, and I do return those calls. + +The most inconvenient mail I get is for some well-intentioned person +to send me a few dollars asking me for a copy of PGP. I don't send +it to them because I'd rather avoid any legal problems with PKP. Or +worse, sometimes these requests are from foreign countries, and I +would be risking a violation of US cryptographic export control +laws. Even if there were no legal hassles involved in sending PGP to +them, they usually don't send enough money to make it worth my time. +I'm just not set up as a low cost low volume mail order business. I +can't just ignore the request and keep the money, because they +probably regard the money as a fee for me to fulfill their request. +If I return the money, I might have to get in my car and drive down +to the post office and buy some postage stamps, because these +requests rarely include a stamped self-addressed envelope. And I +have to take the time to write a polite reply that I can't do it. If +I postpone the reply and set the letter down on my desk, it might be +buried within minutes and won't see the light of day again for +months. Multiply these minor inconveniences by the number of +requests I get, and you can see the problem. Isn't it enough that +the software is free? It would be nicer if people could try to get +PGP from any of the myriad other sources. If you don't have a modem, +ask a friend to get it for you. If you can't find it yourself, I +don't mind answering a quick phone call. + +If anyone wants to volunteer to improve PGP, please let me know. It +could certainly use some more work. Some features were deferred to +get it out the door. A number of PGP users have since donated their +time to port PGP to Unix on Sun SPARCstations, to Ultrix, to VAX/VMS, +to OS/2, to the Amiga, and to the Atari ST. Perhaps you can help +port it to some new environments. But please let me know if you plan +to port or add enhancements to PGP, to avoid duplication of effort, +and to avoid starting with an obsolete version of the source code. + +Because so many foreign language translations of PGP have been +produced, most of them are not distributed with the regular PGP +release package because it would require too much disk space. +Separate language translation "kits" are available from a number of +independent sources, and are sometimes available separately from the +same distribution centers that carry the regular PGP release +software. These kits include translated versions of the file +LANGUAGE.TXT, PGP.HLP, and the PGP User's Guide. If you want to +produce a translation for your own native language, contact me first +to get the latest information and standard guidelines, and to find +out if it's been translated to your language already. To find out +where to get a foreign language kit for your language, you might +check on the Internet newsgroups, or get it from Mike Johnson +(mpj@csn.org). + +If you have access to the Internet, watch for announcements of new +releases of PGP on the Internet newsgroups "sci.crypt" and PGP's own +newsgroup, "alt.security.pgp". If you want to know where to get PGP, +MIT is the primary FTP distribution site (net-dist.mit.edu). Or ask +Mike Johnson (mpj@csn.org) for a list of Internet FTP sites and BBS +phone numbers. + +Future versions of PGP may have to change the data formats for +messages, signatures, keys and key rings, in order to provide +important new features. This may cause backward compatibility +problems with this version of PGP. Future releases may provide +conversion utilities to convert old keys, but you may have to dispose +of old messages created with the old PGP. + + + +Export Controls +--------------- + +The U.S. Government has made it illegal in most cases to export good +cryptographic technology, and that may include PGP. They regard this +kind of software just like they regard munitions. This is determined +by volatile State Department, Defense Department and Commerce +Department policies, not fixed laws. I will not export this software +out of the US or Canada in cases when it is illegal to do so under US +controls, and I urge other people not to export it on their own. + +If you live outside the US or Canada, I urge you not to violate US +export laws by getting any version of PGP in a way that violates +those laws. Since thousands of domestic users got the first version +after its initial publication, it somehow leaked out of the US and +spread itself widely abroad, like dandelion seeds blowing in the +wind. + +Starting with PGP version 2.0 through version 2.3a, the release point +of the software has been outside the US, on publicly-accessible +computers in Europe. Each release was electronically sent back into +the US and posted on publicly-accessible computers in the US by PGP +privacy activists in foreign countries. There are some restrictions +in the US regarding the import of munitions, but I'm not aware of any +cases where this was ever enforced for importing cryptographic +software into the US. I imagine that a legal action of that type +would be quite a spectacle of controversy. + +ViaCrypt PGP version 2.4 is sold in the United States and Canada and +is not for export. The following language was supplied by the US +Government to ViaCrypt for inclusion in the ViaCrypt PGP +documentation: "PGP is export restricted by the Office of Export +Administration, United States Department of Commerce and the Offices +of Defense Trade Controls and Munitions Control, United States +Department of State. PGP cannot be exported or reexported, directly +or indirectly, (a) without all export or reexport licenses and +governmental approvals required by any applicable laws, or (b) in +violation of any prohibition against the export or reexport of any +part of PGP." The Government may take the position that the freeware +PGP version 2.5 is also subject to those controls. + +The freeware PGP version 2.5 is being released through a posting on a +controlled FTP site maintained by MIT. This site has restrictions +and limitations which have been used on other FTP sites to comply +with export control requirements with respect to other encryption +software such as Kerberos and software from RSA Data Security, Inc. I +urge you not to do anything which would weaken those controls or +facilitate any improper export of ViaCrypt PGP or the freeware PGP +2.5. + +Some foreign governments impose serious penalties on anyone inside +their country for merely using encrypted communications. In some +countries they might even shoot you for that. But if you live in +that kind of country, perhaps you need PGP even more. + + + +Philip Zimmermann's Legal Situation +----------------------------------- + +At the time of this writing, I am the target of a US Customs criminal +investigation in the Northern District of California. My defense +attorney has been told by the Assistant US Attorney that the area of +law of interest to the investigation has to do with the export +controls on encryption software. The federal mandatory sentencing +guidelines for this offense are 41 to 51 months in a federal prison. +US Customs appears to be taking the position that electronic domestic +publication of encryption software is the same as exporting it. The +prosecutor has issued a number of federal grand jury subpoenas. It +may be months before a decision is reached on whether to seek +indictment. This situation may change at any time, so this +description may be out of date by the time you read it. Watch the +news for further developments. If I am indicted and this goes to +trial, it will be a major test case. + +I have a legal defense fund set up for this case. So far, no other +organization is doing the fundraising for me, so I am depending on +people like you to contribute directly to this cause. The fund is run +by my lead defense attorney, Phil Dubois, here in Boulder. Please +send your contributions to: + + Philip Dubois + 2305 Broadway + Boulder, Colorado 80304 USA + Phone 303-444-3885 + E-mail: dubois@csn.org + +You can also phone in your donation and put it on Mastercard or Visa. +If you want to be really cool, you can use Internet E-mail to send in +your contribution, encrypting your message with PGP so that no one +can intercept your credit card number. Include in your E-mail +message your Mastercard or Visa number, expiration date, name on the +card, and amount of donation. Then sign it with your own key and +encrypt it with Phil Dubois's public key (his key is included in the +standard PGP distribution package, in the "keys.asc" file). Put a +note on the subject line that this is a donation to my legal defense +fund, so that Mr. Dubois will decrypt it promptly. Please don't send +a lot of casual encrypted email to him -- I'd rather he use his +valuable time to work on my case. + +If you want to read some press stories about this case, see the +following references: + + 1) William Bulkeley, "Cipher Probe", Wall Street Journal, Thursday + April 28th, 1994, front page. + 2) John Cary, "Spy vs. Computer Nerd: The Fight Over Data + Security", Business Week, 4 Oct 1993, page 43. + 3) Jon Erickson, "Cryptography Fires Up the Feds", Dr. Dobb's + Journal, December 1993, page 6. + 4) John Markoff, "Federal Inquiry on Software Examines Privacy + Programs", New York Times, Tuesday 21 Sep 1993, page C1. + 5) Kurt Kleiner, "Punks and Privacy", Mother Jones Magazine, + Jan/Feb 1994, page 17. + 6) John Markoff, "Cyberspace Under Lock and Key", New York Times, + Sunday 13 Feb 1994. + 7) Philip Elmer-DeWitt, "Who Should Keep the Keys", Time, 14 Mar + 1994, page 90. + + +Where to Get a Commercial Version of PGP +---------------------------------------- + +To get a fully licensed version of PGP for use in the USA or Canada, +contact: + + ViaCrypt + 2104 West Peoria Avenue + Phoenix, Arizona 85029 + Phone: 602-944-0773 + Fax: 602-943-2601 + E-mail: viacrypt@acm.org + +ViaCrypt has a version of PGP for MSDOS, and a number of Unix +platforms. Other versions are under development. If you have a need +to use PGP in a commercial or Government setting, and ViaCrypt has a +version of PGP for your hardware platform, you should get ViaCrypt +PGP. + +ViaCrypt has obtained all the necessary licenses from PKP, Ascom-Tech +AG, and Philip Zimmermann to sell PGP for use in commercial or +Government environments. ViaCrypt PGP is every bit as secure as the +freeware PGP, and is entirely compatible in both directions with the +freeware version of PGP. ViaCrypt PGP is the perfect way to get a +fully licensed version of PGP into your corporate environment. + + +Reporting PGP Bugs +------------------ + +Bugs in PGP should be reported via E-mail to MIT, the official +distribution site of PGP. The E-mail address for bug reports is +pgp-bugs@mit.edu. + + + +Computer-Related Political Groups +================================= + +PGP is a very political piece of software. It seems appropriate to +mention here some computer-related activist groups. Full details on +these groups, and how to join them, is provided in a separate +document file in the PGP release package. + +The Electronic Frontier Foundation (EFF) was founded in 1990 to +assure freedom of expression in digital media, with a particular +emphasis on applying the principles embodied in the US Constitution +and the Bill of Rights to computer-based communication. They can be +reached in Washington DC, at (202) 347-5400. Internet E-mail address: +eff@eff.org. + +Computer Professionals For Social Responsibility (CPSR) empowers +computer professionals and computer users to advocate for the +responsible use of information technology and empowers all who use +computer technology to participate in public policy debates on the +impacts of computers on society. They can be reached at: +415-322-3778 in Palo Alto, E-mail address cpsr@csli.stanford.edu. + +The League for Programming Freedom (LPF) is a grass-roots organization +of professors, students, businessmen, programmers and users dedicated +to bringing back the freedom to write programs. They regard patents +on computer algorithms as harmful to the US software industry. They +can be reached at (617) 433-7071. E-mail address: lpf@uunet.uu.net. + +For more details on these groups, see the accompanying document in +the PGP release package. + + +Recommended Introductory Readings +================================= + +1) Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and + Source Code in C", John Wiley & Sons, 1993 + (This book is a watershed work on the subject.) +2) Dorothy Denning, "Cryptography and Data Security", Addison-Wesley, + Reading, MA 1982 +3) Dorothy Denning, "Protecting Public Keys and Signature Keys", + IEEE Computer, Feb 1983 +4) Martin E. Hellman, "The Mathematics of Public-Key Cryptography," + Scientific American, Aug 1979 +5) Steven Levy, "Crypto Rebels", WIRED, May/Jun 1993, page 54. + (This is a "must-read" article on PGP and other related topics.) + +Other Readings +============== + +6) Ronald Rivest, "The MD5 Message Digest Algorithm", MIT Laboratory + for Computer Science, 1991 +7) Xuejia Lai, "On the Design and Security of Block Ciphers", + ETH Series on Information Processing (Ed. J. L. Massey), + Vol. 1, Hartung-Gorre Verlag, Konstanz, Switzerland, 1992 +8) Philip Zimmermann, "A Proposed Standard Format for RSA + Cryptosystems", Advances in Computer Security, Vol III, edited by + Rein Turn, Artech House, 1988 +9) Paul Wallich, "Electronic Envelopes", Scientific American, Feb + 1993, page 30. (This is an article on PGP) +10) William Bulkeley, "Cipher Probe", Wall Street Journal, 28 April + 1994, front page. (This is an article on PGP and Zimmermann) + + +To Contact the Author +===================== + +Philip Zimmermann may be reached at: + +Boulder Software Engineering +3021 Eleventh Street +Boulder, Colorado 80304 USA +Internet: prz@acm.org +Phone 303-541-0140 (voice) (10:00am - 7:00pm Mountain Time) +Fax line available, if you arrange it via voice line. + + + +Appendix A: Where to Get PGP +============================= + +The following describes how to get the freeware public key +cryptographic software PGP (Pretty Good Privacy) from an anonymous +FTP site on Internet, or from other sources. + +PGP has sophisticated key management, an RSA/conventional hybrid +encryption scheme, message digests for digital signatures, data +compression before encryption, and good ergonomic design. PGP is +well featured and fast, and has excellent user documentation. Source +code is free. + +The Massachusetts Institute of Technology is the distributor of PGP +version 2.5, for distribution in the USA only. It is available from +"net-dist.mit.edu," a controlled FTP site that has restrictions and +limitations, similar to those used by RSA Data Security, Inc., to comply +with export control requirements. The software resides in the directory +/pub/PGP. + +A reminder: Set mode to binary or image when doing an FTP transfer. +And when doing a kermit download to your PC, specify 8-bit binary +mode at both ends. + +There are two compressed archive files in the standard release, with +the file name derived from the release version number. For PGP +version 2.5, you must get pgp25.zip which contains the MSDOS binary +executable and the PGP User's Guide, and you can optionally get +pgp25src.zip which contains all the source code. These files can be +decompressed with the MSDOS shareware archive decompression utility +PKUNZIP.EXE, version 1.10 or later. For Unix users who lack an +implementation of UNZIP, the source code can also be found in the +compressed tar file pgp25src.tar.Z. + +If you don't have any local BBS phone numbers handy, here is a BBS +you might try. The Catacombs BBS, operated by Mike Johnson in +Longmont, Colorado, has PGP available for download by people in the US +or Canada only. The BBS phone number is 303-938-9654. Mike +Johnson's voice phone number is 303 772-1773, and his email address +is mpj@csn.org. Mike also has PGP available on an Internet FTP site +for users in the US or Canada only; the site name is csn.org, in +directory /mpj/, and you must read the README.MPJ file to get it. + +To get a fully licensed version of PGP for use in the USA or Canada, +contact ViaCrypt in Phoenix, Arizona. Their phone number is +602-944-0773. ViaCrypt has obtained all the necessary licenses from +PKP, Ascom-Tech AG, and Philip Zimmermann to sell PGP for use in +commercial or Government environments. ViaCrypt PGP is every bit as +secure as the freeware PGP, and is entirely compatible in both +directions with the freeware version of PGP. ViaCrypt PGP is the +perfect way to get a fully licensed version of PGP into your +corporate or Government environment. + +Source and binary distributions of PGP are available from the Canadian +Broadcasting Corporation library, which is open to the public. It has +branches in Toronto, Montreal, and Vancouver. Contact Max Allen, at ++1 416 205-6017 if you have questions. + +Here are a few people and their email addresses or phone numbers you +can contact in some countries to get information on local PGP +availability for versions earlier than 2.5: + +Peter Gutmann Hugh Kennedy +pgut1@cs.aukuni.ac.nz 70042.710@compuserve.com +New Zealand Germany + +Branko Lankester Miguel Angel Gallardo +branko@hacktic.nl gallardo@batman.fi.upm.es ++31 2159 42242 (341) 474 38 09 +The Netherlands Spain + +Hugh Miller Colin Plumb +hmiller@lucpul.it.luc.edu colin@nyx.cs.du.edu +(312) 508-2727 Toronto, Ontario, Canada +USA + +Jean-loup Gailly +jloup@chorus.fr +France +