Mailcrypt is a bunch of extensions written in Emacs Lisp
that mediates between Emacs (actually, probably a mailreader like rmail
within Emacs) and PGP
It supports signing, authentication, encryption, and decryption of text within Emacs text buffers. At least, it did all that way back in 1996 when I was using it. That's basically still all it does, but really, the flexibility of Emacs and the power of PGP encryption seems like a pretty potent combination to me.
Mailcrypt was originally written by Jin Choi and Pat LoPresti; when Phil Zimmerman created a new version of PGP and changed its command-line interface, it broke Mailcrypt, and Len Budney fixed it to work with the current versions of both PGP and GnuPG -- now you can pick your back end. Budney is the current maintainer of Mailcrypt.