Open PGP is an IETF standard for the PGP email encryption system. The current version, as of late 2008, is RFC 4880 , Earlier versions were RFC 1991 and RFC 2440.
Supported public key algorithms are RSA, El Gamal (encrypt-only, not for signatures), and Digital Signature Algorithm. Supported block ciphers are IDEA, Triple DES, CAST-128, AES and Twofish.
Both the current commercial release  of PGP and the open source product GNU Privacy Guard (GPG) are based on this specification.