GnuPG - The GNU Privacy Guard suite of programs
GnuPG is a set of programs for public key encryption and digital signatures. The program most users will want to use is the OpenPGP command line tool, named gpg2. gpgvis a stripped down version of gpg2 with no encryption functionality, used only to verify signatures against a trusted keyring. gpgsm is the X.509/CMS (for S/MIME) counterpart of gpg2. gpg-agent is a passphrase and private key daemon which may also emulate the ssh-agent.
The full documentation for this tool is maintained as a Texinfo manual. If GnuPG and the info program are properly installed at your site, the command
should give you access to the complete manual including a menu structure and an index.