GnuPG

GnuPG (GNU Privacy Guard) is a free and open-source implementation of the OpenPGP standard (RFC 4880 and its successors), providing encryption, digital signing, and key management for secure communication and data storage. Created by Werner Koch in 1997 as a free software replacement for PGP (Pretty Good Privacy, which was proprietary at the time), GnuPG has been maintained by g10 Code GmbH and the GnuPG Project for nearly 30 years, with financial support from organizations including the Linux Foundation, German Federal Office for Information Security (BSI), and Meta. GnuPG provides command-line tools including gpg (the primary tool for encryption, decryption, signing, verification, and key management), gpg-agent (a background daemon managing private keys, caching passphrases, and providing SSH agent functionality), gpgsm (X.509 certificate management and CMS/S/MIME support), and dirmngr (directory manager for accessing key servers, LDAP directories, and OCSP responders). Key features include: asymmetric encryption using RSA (up to 4096 bits), DSA, ElGamal, ECC curves (Curve25519, NIST P-256/P-384/P-521, Brainpool, and EdDSA), symmetric encryption with AES-128/192/256, Twofish, CAST5, Camellia, and 3DES using CFB mode, digital signatures with detached, cleartext, and embedded signatures, key management with Web of Trust model (trust signatures, marginal and full trust), key servers (synchronizing via HKP protocol), key revocation certificates, perfect forward secrecy via ECDH, smart card and hardware token support (OpenPGP cards, YubiKey, Nitrokey), TOFU (Trust On First Use) trust model, and GPGME (GnuPG Made Easy) C library for programmatic access.