PGP (Pretty Good Privacy)
acronymcryptographyprivacy
The encryption program Phil Zimmermann released free in 1991, combining public-key and symmetric cryptography for mail and files, and the OpenPGP standard and implementations - notably GnuPG - that grew from it.
PGP introduced most people to public-key cryptography and to the web of trust, a decentralised alternative to certificate authorities in which users sign each other's keys. It also demonstrated the model's limits: key management proved too hard for general use, and its long-lived keys and lack of forward secrecy are why modern messaging moved to different designs. It remains widely used for signing software releases and for journalist contact, and it is the tool whose distribution triggered a three-year criminal investigation of its author.
Also known as: pretty good privacy, gpg, openpgp, web of trust