PGP lets you sign and encrypt messages using public keys. There is considerable documentation that comes with PGP itself. PGP is to an email message what an envelope is to a letter.
Exmh allows to conveniently use PGP to encrypt or sign your messages and decrypt and check signatures on messages you receive.
To start using PGP, you have to create a key pair. A key pair consists of a public key (that you want to spread around) and a private key (that you don't want anybody else than yourself to have).
The private key is protected by a password. You should be *very* careful not to lose that password *and* not to let anybody get access to it. PGP can offer a very high level of security, but if people can read your file (especially the file that contains the private key) and can find your password (reading keystrokes is very easy once you have access to the Xserver) then all the great work put into the encryption algorithm is totally useless :-)
This button will run PGP to create a key pair. It will create a directory where PGP keys will be kept (by default ~/.pgp), make a key pair (this can be fairly time consuming), sign the key (you should read the PGP documentation to find out why this is done. It doesn't explain why it isn't automatically done by PGP, though :-( ) and finally will send the public key to the keyserver so that other people can find it (in case they want to send you encrypted mail or if they want to check some signature of yours).
You must restart exmh after creating your public key. By default, exmh does not display any PGP-related menus if you are not yet set up for PGP.
Currently exmh has support for PGP versions 2.6, 5.0, 6.5, and GnuPG.
You can use all versions separately or simultaneous from within exmh. You need not have all versions installed to use exmh PGP support.
Having both PGP versions 2.6 and 5.0 on one system can lead to problems, because the main binary of PGP 5.0 is called pgp like the 2.6 one. The pgpe, pgps and pgpv commands are links on that binary. Because calling the pgp binary of 5.0 itself does nothing but print out a version usage summary, one way to disable it, is to put it into a location that is not in PATH and edit the symlinks pgps, pgpe, pgpv to point to this new location. PGP 2.6 and 5.0 can peacefully coexist on the system this way and be used both from within exmh.
Currently, the PGP 6.5 support wants the 'pgp' binary to be called 'pgp6' (rather than the default 'pgp'). PGP 6.5 can co-exist with both 2.6 and 5.0 if this is done.