The interoperability standard for PGP/GPG.
See: 
http://www.openpgp.org/ and http://sunsite.icm.edu.pl/gnupg/rfc2440.html
Recently an attack was discovered on some forms of the OpenPGP message protocol. It's only effective against automated applications of PGP (bots): http://www.pgp.com/library/ctocorner/openpgp.html
Contrast: X509
