PGP

PGP stands for Pretty Good Privacy. It is an encryption program written by Phillip Zimmerman. PGP allows to encrypt e-mail messages by means of both public-key and secret-key cryptosystems. It uses RSA cryptosystem to encrypt a private-key which is used to encrypt the message. The message is encrypted by using the IDEA cryptosystem. More updated versions use a different algorithm to encrypt the private-key. PGP is a freeware program. It is a de facto standard for e-mail encryption by now. However the main problem is the export restriction due to the use of RSA public-key cryptosystem. To know more about PGP, you should read the PGP FAQ. You could get a copy from:

ftp://ftp.csn.net/mpj/getpgp.asc
http://www.prairienet.org/~jalicqui/pgpfaq.txt
ftp://ftp.prairienet.org/pub/providers/pgp/

You should also look at: The International PGP Home Page