GnuPG

GnuPG is a complete, free implementation of the OpenPGP standard for encrypting and signing data and communication. Also known as GPG, it is a command-line tool with a versatile key management system and access modules for public key directories, designed for easy integration with other applications.

Beyond OpenPGP, it adds support for S/MIME and Secure Shell. Public keys and X.509 certificates can be stored in an SQLite database through the keyboxd daemon, while private keys stay as separate files. GnuPG also works with smartcards and can act as an ssh-agent, and Pinentry handles secure passphrase prompts.

A wide range of frontend applications and libraries build on GnuPG, so you can use it directly from a terminal or wire it into mail clients, backup tools, and other software.