GnuPG

GnuPG, short for GNU Privacy Guard, is a free, open-source implementation of the OpenPGP standard. It provides cryptographic privacy and authentication through encryption, decryption, digital signatures, and secure key management. Designed to be compatible with PGP and OpenPGP ecosystems, GnuPG runs on major platforms and is widely used for email, file, and software signing.

Developed within the GNU project by Werner Koch and released in the late 1990s, GnuPG offers a free alternative when PGP was proprietarily licensed. It centers on the gpg command-line tool and the libgcrypt library, and is distributed under the GNU General Public License.

Core features include public-key and symmetric encryption, key generation and management, and support for revocation, expiration, and subkeys. It implements a trust model rooted in the OpenPGP Web of Trust and can distribute public keys via a network of keyservers. It supports a diverse set of algorithms and can sign and verify data, emails, and software packages.

Usage and integration: GnuPG is commonly used with mail user agents to secure messages, and with frontends such as Kleopatra or Seahorse. It also plays a central role in software distribution and backup encryption, and supports hardware tokens for security.

Impact: As a reference OpenPGP implementation, GnuPG has helped popularize free cryptography and interoperability across open-source projects and services.