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PGP is an encryption and digital signature utility for adding privacy to electronic mail and stored data. Phil Zimmermann designed PGP in the early 1990s on the principle that e-mail, like conversations, should be private. In addition, both sender and receiver need assurances that messages are from an authentic source, that messages have not been altered or corrupted, and that the sender cannot repudiate (disown) the message. PGP can assure privacy and nonrepudiation. It also provides a tool to encrypt information on disk.

PGP was expanded over the years by an all-volunteer collaborative effort guided by Zimmermann. It is discussed in RFC 1991 (PGP Message Exchange Formats, August 1996). RFC 2015 (MIME Security with Pretty Good Privacy, October 1996) also discusses PGP. Open PGP is also discussed in RFC 2440 (Open PGP Message Format, November 1998).

PGP is an alternative to RSA's S/MIME (Secure MIME). S/MIME uses RSA (Rivest, Shamir, Adleman) public-key algorithms, while PGP uses Diffie-Hellman public-key management algorithms.