GNU Privacy Guard

The GNU Privacy Guard (GnuPG or GPG) is a free software replacement for the PGP suite of cryptographic software, released under the GNU General Public License। It is a part of the Free Software Foundation's GNU software project, and has received major funding from the German government. GnuPG is completely compliant with RFC 2440, the IETF standard for OpenPGP. Current versions of PGP (and Veridis' Filecrypt) are interoperable with GnuPG and other OpenPGP-compliant systems. Although some older versions of PGP are also interoperable, not all features of newer software are supported by the older software.

http://en.wikipedia.org/wiki/GNU_Privacy_Guard