Yarn is a freeware offline mail/news reader written by Chin Huang. It hasn't been updated in a few years, but it's still my preferred newsreader when I have to use a dial-up account to access Usenet. One of the reasons I like Yarn is that its programmer included a simple but extensible interface to PGP using MS-DOS batch files. This is a collection of batch files I've written over the years to test Yarn's ability to support other encryption formats.
Yarn ships with support for both PGP and MIME (using batch files and metamail, respectively), but doesn't mention the PGP/MIME standard specified in RFC 3156. Some basic PGP/MIME support can be added to yarn with two new batch files.
ME-PGPE.BAT (a replacement for Yarn's usual PGPE.BAT) encrypts PGP messages as RFC 3156-compliant multipart/encrypted messages. (Note: This batch file worked fine when I used it on a true DOS system, but it fails miserably in a Windows DOS box.) SHOWPGP.BAT works with metamail to decrypt the multipart/encrypted and application/pgp-keys message formats.
GPG (GNU Privacy Guard) is an open source implementation of the OpenPGP standard. There is no purely DOS version of GPG, but the Windows version is a command-line program that can be run from a batch file.
RIPEM (Riordan's Privacy Enhanced Mail) was developed in 1991 by Mark Riordan as a freeware demo implementation of the PEM standard. PEM was supseded by MOSS and S/MIME, and development of RIPEM ended around 1997. (RIPEM's source code is still available but the pre-compiled DOS binaries are no longer distributed on the Internet.)
Sifr was a public key encryption program written by C.T. Skinner to demonstrate the LUC algorithm. Skinner seems to have disappeared from the Web, but the DOS version of Sifr is still available at a few European software archives.
SIFRSIG.BAT armor-signs messages (Sifr doesn't do clear-signing), while SIFRD.BAT decrypts/verifies them. There's no batch file for encryption because Sifr requires a command-line parameter (the filename of the recipient's public key) that Yarn doesn't know.