Pretty Good Privacy (PGP) is encryption software first released by Phil Zimmermann in 1991. It let an ordinary person encrypt and digitally sign email and files using public-key cryptography, a capability that had previously been the practical preserve of governments, banks, and large institutions. Zimmermann describes its purpose plainly in “Why I Wrote PGP”: to defend personal privacy in an age of electronic communication, because “the only way to hold the line on privacy in the information age is strong cryptography” (https://philzimmermann.com/EN/essays/WhyIWrotePGP.html).
Technically, PGP uses a hybrid design. A message is encrypted with a fast conventional (symmetric) cipher under a one-time session key, and that session key is then encrypted with the recipient’s public key. Public-key cryptography also supports digital signatures, so a recipient can verify both that a message came from the claimed sender and that it was not altered. This combination is what made PGP practical for everyday use rather than a laboratory curiosity.
PGP’s distribution outside the United States soon after release put Zimmermann at the center of a years-long US criminal investigation over cryptography export controls, making the tool a flashpoint in the crypto wars. Its design and file formats were later standardized as OpenPGP, and the broad availability of strong civilian encryption that PGP demonstrated reshaped the policy debate over privacy and surveillance. Zimmermann’s own site documents PGP’s history and his role in it (https://philzimmermann.com/EN/background/index.html).