GnuPG

In short: GnuPG is the free implementation of the OpenPGP standard for encrypting and signing data and e-mails.

In more detail: Werner Koch started the project in 1997 as a free alternative to PGP. The command-line program is called gpg.

In Depth

Features

Generate key pairs, encrypt and decrypt files and mails, create and verify digital signatures. It uses asymmetric methods (RSA, elliptic curves).

Interfaces

Kleopatra (Windows), Thunderbird with built-in OpenPGP. See GPG, public key.

See also: GPG, Kleopatra, Public Key, Asymmetric Encryption