Pretty Good Privacy
(PGP) A high security RSA public-key encryption application for MS-DOS, Unix, VAX/VMS, and other computers. It was written by Philip R. Zimmermann <email@example.com> of Phil's Pretty Good(tm) Software and later augmented by a cast of thousands, especially including Hal Finney, Branko Lankester, and Peter Gutmann.
PGP was distributed as "guerrilla freeware". The authors don't mind if it is distributed widely, just don't ask Philip Zimmermann to send you a copy. PGP uses a public-key encryption algorithm claimed by US patent #4,405,829. The exclusive rights to this patent are held by a California company called Public Key Partners, and you may be infringing this patent if you use PGP in the USA. This is explained in the PGP User's Guide, Volume II.
PGP allows people to exchange files or messages with privacy and authentication. Privacy and authentication are provided without managing the keys associated with conventional cryptographic software. No secure channels are needed to exchange keys between users, which makes PGP much easier to use. This is because PGP is based on public-key cryptography.
In December 1994 Philip Zimmermann faced prosecution for "exporting" PGP out of the United States but in January 1996 the US Goverment dropped the case. A US law prohibits the export of encryption software out of the country. Zimmermann did not do this, but the US government hoped to establish the proposition that posting an encryption program on a BBS or on the Internet constitutes exporting it - in effect, stretching export control into domestic censorship. If the government had won it would have had a chilling effect on the free flow of information on the global network, as well as on everyone's privacy from government snooping.
["Protect Your Privacy: A Guide for PGP Users", William Stallings, Prentice-Hall, ISBN 0-13-185596-4].