PGP 2.6 Application Note: Integrating PGP with mailers Derek Atkins 22-May-94 This document will try to describe how to create an application around PGP version 2.6, in order to incorporate PGP into mailers and news readers. There are a number of changes in 2.6 which make it easier to write wrappers for PGP. In writing this, I may have inadvertantly left out some functionality, some mailers, or some machine types. I should warn readers now that I am a UNIX person, and that most of this document will probably be in UNIX terms. The first thing to remember is that PGP usually needs to have a controlling TTY in order to gather information, like the password, or for the user to answer questions, like to whether to add or sign keys. This is useful for text-based mail agents. In fact, it is most useful for agents which use files for messages, like MH. One way to use PGP is to just decrypt the message into a file and display that file. This can be a security risk, since wiping files off disk is not always successful, and by having the plain-text go to a file means more time for a possible attacker to get ahold of the plain-text of the message. A better way to accomplish this is to use filter_mode, which is the -f option. This tells PGP to read the message from stdin, and to put the output onto stdout. Unfortunately, in this manner, the signature information is output onto stderr, so you will either lose it, or it and all other PGP output will be put in the same stream with the message, but this depends on your piping ability. PGP tries to send all "interesting" data to standard out, and error messages to standard error. This lets you pick out the interesting information and discard the rest. This also means that you can use PGP in filter-mode as a back-end to some user interface, and obtain the data in the manner. But remember that the current implementation of PGP uses temporary files to store intermediate data, so you are still at a risk, although it is much less of a risk than just decrypting into a file. This works well when dealing with a command-line mailer, or a mailer that is run in a terminal. There are problems with this approach, however, if you do not have a TTY in which to get a password to decrypt or sign messages. It seems that there would not be a good way around this, but then again, PEM is going to have this same problem. (An example that I can think of is integrating with xmh). However, there is a way around this in some cases. PGP has numerous ways to accept the passphrase other than just promping for it. One way, which is not recommended, is to use the "-z" option to set the passphrase. Again, this is *NOT* recommended, since some operating systems will not allow the program to erase the process table, and someone can retreive the pass phrase from there via a "ps" listing of active processes. A similar way to get the pass phrase in is to use the PGPPASS environment variable. Again, this has the same problems as "-z" with regards to an attacker finding the passphrase in the OS kernel memory of the process table.. An example of this usage is: pgp -sat -z "This is my pass phrase" inputfile There is a better way of doing this in PGP 2.6, which is an environment variable called "PGPPASSFD". If this is set, it means that the FIRST thing PGP will do is read the pass phrase from this file descriptor. So, for example, one can set PGPPASSFD to "0" (zero), and then PGP will read the pass phrase from stdin as the first thing. This allows you to send the passphrase to PGP in a manner invisible to someone armed with the process listing. For example, an emacs utility could grab the block to be encrypted (or decrypted), ask the user for the pass phrase in the mini-buffer, and then do the equivalent of this shell script, using something like: (send-string PROCESS "PassPhrase") (send-region PROCESS (point-min) (point-max)) ---begin--- #!/bin/sh PGPPASSFD=0; export PGPPASSFD (echo "PassPhraseHere"; cat filename ) | pgp -feast recipient1 recipient2... ---end--- I must admit, this is a crude script, since it doesn't strip out stderr, which included the bannerlines and error messages, but that is not difficult to do out of band. This is an example perl script that demonstrates the use of PGPPASSFD: ---begin--- #!/usr/local/bin/perl # # perl example for PGPPASSFD, # encrypts stream with password 'test' # pipe(READER,WRITER); if (!fork) { close(WRITER); $ENV{'PGPPASSFD'}=fileno(READER); # the $^F (Ctrl-F) variable controls close-on-exec of files $=fileno(READER); exec "pgp -acf"; die "can't exec pgp\n"; } close(READER); syswrite(WRITER, "test\n", 5); close(WRITER); wait ---end--- Another feature of 2.6 which can be utilized in mailer scripts is the batchmode feature. This is used in the key-server software (see keyserv.doc), to allow a process to call PGP and have it perform without prompting the user for anything. It will take the default answer to most questions, which may not be what the user wants. This is switched by adding "+batchmode" to the command line. One more mailer I should mention, and this is probably the most important of all of them, is MIME compatibility. In order to use MIME, a user needs to create a proper entry for PGP. Unfortunately there is not yet a standard MIME content-type for PGP-MIME. However there is a recommended set of mailcap entries which would be useful for using metamail: application/pgp; pgp -f < %s | metamail; needsterminal; \ test=test %{encapsulation}=entity application/pgp; pgp %s; needsterminal I hope that this document has helped people understand some of the work being done to integrate PGP with mailers. There is some work going on already to integrate it even more. If you have a mailer for which there is no PGP handler, and you want to write one, please let me know, so that we don't duplicate work. In addition, if you have written a mailer application, and its not included here in the release, again let me know. A second contact for this is Colin Plumb . Have fun! -derek