gnupg -- A GNU utility for secure communication and data storage.
http://www.gnupg.org/
GnuPG (GNU Privacy Guard) is a GNU utility for encrypting data
and creating digital signatures. GnuPG has advanced key management
capabilities and is compliant with the proposed OpenPGP Internet standard
described in RFC2440. Since GnuPG doesn't use any patented algorithm,
it is not compatible with any version of PGP2 (PGP2.x uses only IDEA
for symmetric-key encryption, which is patented worldwide).