Pretty Good Privacy

Pretty Good Privacy or PGP is a hybrid cryptosystem for email security, originally developed by Phil Zimmerman.

All versions use a public key cryptosystem to provide digital signatures and to manage keys for a block cipher which does the actual message encryption.

The original PGP used a block cipher called Bassomatic, devised by Zimmerman. This was quickly shown to be weak, and replaced with IDEA in version 2.0. In version 3.0. they switched to CAST-128 which, unlike IDEA, was free of patent restrictions.