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1.1     ! root        1:           PGP 2.2 Application Note:  Integrating PGP with mailers
        !             2:                     Derek Atkins <[email protected]>
        !             3:                              6-Mar-93
        !             4: 
        !             5:        This document will try to describe how to create an
        !             6: application around PGP version 2.2, in order to incorporate PGP into
        !             7: mailers and news readers.  There are a number of changes in 2.2 which
        !             8: make it easier to write wrappers for PGP.  In writing this, I may have
        !             9: inadvertantly left out some functionality, some mailers, or some
        !            10: machine types.  I should warn readers now that I am a UNIX person, and
        !            11: that most of this document will probably be in UNIX terms.
        !            12: 
        !            13:        This document references scripts which are in the contrib
        !            14: directory of PGP.  Within the contrib area are scripts contributed to
        !            15: the release.  A lot of these scripts were written for PGP 2.1, so they
        !            16: do not handle multiple recipients, or some of the new PGP features.
        !            17: 
        !            18:        The first thing to remember is that PGP usually needs to have
        !            19: a controlling TTY in order to gather information, like the password,
        !            20: or for the user to answer questions, like to whether to add or sign
        !            21: keys.  This is useful for text-based mail agents.  In fact, it is most
        !            22: useful for agents which use files for messages, like MH.
        !            23: 
        !            24:        One example for this is in the emacs directory.  The e-lisp
        !            25: pgp.el1 winds up saving the buffer to encrypt or decrypt to a file,
        !            26: and then runs pgp on that file using the emacs interactive mode.  It
        !            27: looks the same as if you type "pgp filename" at the shell.
        !            28: 
        !            29:        However, this is not very interesting.  While it is easy to to
        !            30: this, it means that there are extra files being created on the disk.
        !            31: This can be a security risk, since wiping files off disk is not always
        !            32: successful, and by having the plain-text go to a file means more time
        !            33: for a possible attacker to get ahold of the plain-text of the message.
        !            34: A better way to accomplish this is to use filter_mode, which is the -f
        !            35: option.  This tells PGP to read the message from stdin, and to put the
        !            36: output onto stdout.  Unfortunately, in this manner, the signature
        !            37: information is output onto stderr, so you will either lose it, or it
        !            38: and all other PGP output will be put in the same stream with the
        !            39: message, but this depends on your piping ability.
        !            40: 
        !            41:        An example of how to use this is the mailx script.  This
        !            42: script is supposed to run in place of the UNIX /bin/mail program.  It
        !            43: reads from stdin and will do the proper encryption and then execute
        !            44: /bin/mail.  This works by specifying a special user for the recipient
        !            45: to encrypt and signature.  This works by calling PGP with the
        !            46: following arguments, and uses stdin and stdout for the mail input and
        !            47: output.  However, this script only is useful for sending mail, not
        !            48: reading it.
        !            49: 
        !            50:  To just sign the message, in clear-text mode:
        !            51:        pgp -fast +clearsig=on 
        !            52:  or to sign and encrypt:
        !            53:        pgp -feast user1 user2 user3...
        !            54: 
        !            55:        This works well when dealing with a command-line mailer, or a
        !            56: mailer that is run in a terminal.  There are problems, however, if you
        !            57: do not have a TTY in which to get a password to decrypt or sign
        !            58: messages.  I'm not sure of a way around this, but then again, PEM is
        !            59: going to have this same problem.  (An example that I can think of is
        !            60: integrating with xmh).
        !            61: 
        !            62:        There is a way around this, however, in some cases.  One way,
        !            63: which is not recommended, but can be used, is to use the "-z" option
        !            64: to set the passphrase.  Again, this is *NOT* recommended, since some
        !            65: operating systems will not allow the program to erase the process
        !            66: table, and someone can retreive the pass phrase from there.  A similar
        !            67: way to get the pass phrase in is to use the PGPPASS environment
        !            68: variable.  Again, this has the same problems.  An example of this
        !            69: usage is:
        !            70: 
        !            71:        pgp -sat +clearsig=on -z "This is my pass phrase" inputfile
        !            72: 
        !            73:        There is a better way of doing this in PGP 2.2, which is an
        !            74: environment variable called "PGPPASSFD".  If this is set, it means
        !            75: that the FIRST thing PGP will do is read the pass phrase from this
        !            76: file descriptor.  So, for example, one can set PGPPASSFD to "0"
        !            77: (zero), and then PGP will read the pass phrase from stdin as the first
        !            78: thing.  
        !            79: 
        !            80:        For example, an emacs utility could grab the block to be
        !            81: encrypted (or decrypted), ask the user for the pass phrase in the
        !            82: mini-buffer, and then do the equivalent of this shell script, using
        !            83: something like:
        !            84: 
        !            85:        (send-string PROCESS "PassPhrase") 
        !            86:        (send-region PROCESS (point-min) (point-max))
        !            87: 
        !            88: ---begin---
        !            89: #!/bin/sh
        !            90: 
        !            91: PGPPASSFD=0;export PGPPASSFD
        !            92: 
        !            93: (echo "PassPhraseHere"; cat ) | pgp -feast recipient1 recipient2...
        !            94: ---end---
        !            95: 
        !            96:        I must admit, this is a crude script, since it doesn't strip
        !            97: out stderr, which included the bannerlines and error messages, but
        !            98: that is not difficult to do out of band.
        !            99: 
        !           100: This is an example perl script that demonstrates the use of PGPPASSFD:
        !           101: 
        !           102: ---begin---
        !           103: #!/usr/local/bin/perl
        !           104: #
        !           105: # perl example for PGPPASSFD,
        !           106: # encrypts stream with password 'test'
        !           107: #
        !           108: 
        !           109: pipe(READER,WRITER);
        !           110: 
        !           111: if (!fork) {
        !           112:        close(WRITER);
        !           113:        $ENV{'PGPPASSFD'}=fileno(READER);
        !           114: # the $^F (Ctrl-F) variable controls close-on-exec of files
        !           115:        $=fileno(READER);
        !           116:        exec "pgp -acf";
        !           117:        die "can't exec pgp\n";
        !           118: }
        !           119: close(READER);
        !           120: syswrite(WRITER, "test\n", 5);
        !           121: close(WRITER);
        !           122: wait
        !           123: ---end---
        !           124: 
        !           125:        Another feature of 2.2 which can be utilized in mailer scripts
        !           126: is the batchmode feature.  This is used in the key-server software
        !           127: (see key-server.doc), which is not included in the release to allow a
        !           128: process to call PGP, and have it perform without prompting the user
        !           129: for anything.  It will take the default answer to most questions,
        !           130: which may not be what the user wants.  This is switched by adding
        !           131: "+batchmode" to the command line.
        !           132: 
        !           133:        One more mailer I should mention, and this is probably the
        !           134: most important of all of them, is MIME compatibility.  In order to use
        !           135: MIME, a user needs to create a proper entry for PGP.  Unfortunately
        !           136: there hasn't, yet, been a standard MIME content-type created.  One
        !           137: possible mailcap entry would be:
        !           138: 
        !           139:        application/x-pgp:      cat %s | pgp -f
        !           140: 
        !           141: although there are a lot of possibilities.  There is another
        !           142: suggestion given in the mime directory in the contrib area, which I
        !           143: haven't tested.
        !           144: 
        !           145:        I hope that this document has helped people understand some of
        !           146: the work being done to integrate PGP with mailers.  There is some work
        !           147: going on already to integrate it even more.  If you have a mailer for
        !           148: which there is no PGP handler, and you want to write one, please let
        !           149: me know, so that we don't duplicate work.  In addition, if you have
        !           150: written a mailer application, and its not included here in the
        !           151: release, again let me know.
        !           152: 
        !           153:        A second contact for this is Colin Plumb <[email protected]>.
        !           154: 
        !           155:        Have fun!
        !           156: 
        !           157: -derek         <[email protected]>

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