PGP® or Pretty Good Privacy® is a powerful cryptographic product family that enables people to securely exchange messages, and to secure files, disk volumes and network connections with both privacy and strong authentication.
Privacy means that only the intended recipient of a message can read it. By providing the ability to encrypt messages, PGP provides protection against anyone eavesdropping on the network. Even if the information is intercepted, it is completely unreadable to the snooper. Authentication identifies the origin of the information, certainty that it is authentic, and that it has not been altered. Authentication also provides an extremely valuable tool in network security: verification of the identity of an individual. In addition to secure messaging, PGP also provides secure data storage, enabling you to encrypt files stored on your computer. Version 6.5.8 also includes PGPnet - a powerful VPN client which enables secure peer-to-peer IP-based network connections - and Self-Decrypting Archives (SDAs) which allow you to exchange information securely even with those who do not have PGP.

Free PGP software from MIT