GPG

Gnu Privacy Guard, commonly abbreviated GPG, is a free software implementation of the OpenPGP standard. It provides cryptographic privacy and authentication for data communication and storage by offering encryption, decryption, digital signatures, and key management. GPG is part of the GNU Project and was originally written by Werner Koch; the project first released GPG in 1999 as an open-source replacement for the proprietary PGP software.

It uses public-key cryptography and key pairs stored in a keyring. Users generate a key pair, distribute their public key, and keep their private key secure. With public keys, others can encrypt data or verify digital signatures; with a private key, the user can decrypt data or create signatures. GPG supports importing, exporting, signing, and revoking keys, as well as managing trust through signatures and certifications. Public keys can be shared via key servers or distributed directly; OpenPGP-compatible software can verify signatures and decrypt messages from others.

For algorithms, GPG supports multiple cryptographic algorithms for key exchange and signatures, including RSA and Elliptic Curve variants, and supports various symmetric ciphers for data confidentiality. It adheres to the OpenPGP standard (RFC 4880), ensuring interoperability with other OpenPGP implementations.

While primarily a command-line tool, GPG is extensible through libraries such as GPGME and is integrated with many email clients and desktop environments through graphical frontends like Seahorse, Kleopatra, and GPA. Its usage spans email encryption, file encryption, and software signing to verify integrity of distributed software.