gpgsm

GnuPG's S/MIME (gpgsm) is a tool that provides cryptographic functions for the S/MIME standard, which is used for secure email communication. It is part of the GnuPG (GNU Privacy Guard) suite of tools, which also includes gpg for OpenPGP. Gpgsm is designed to work with X.509 certificates, which are commonly used in public key infrastructure (PKI) systems.

Gpgsm can be used to sign, encrypt, and decrypt email messages, as well as verify the signatures of received messages. It supports a variety of cryptographic algorithms, including RSA, DSA, and ECDSA for signing, and AES, 3DES, and Camellia for encryption. The tool can also handle certificate management, including the generation of certificate signing requests (CSRs) and the import and export of certificates.

Gpgsm is a command-line tool, and it can be integrated with email clients such as Thunderbird and Evolution through plugins. It is also used in various other applications and services that require secure email communication. The tool is open-source and is distributed under the GNU General Public License (GPL).

Gpgsm is part of the GnuPG project, which is maintained by the Free Software Foundation (FSF). The project aims to provide a complete and free implementation of the OpenPGP standard as defined by RFC 4880. Gpgsm is available for various operating systems, including Linux, Windows, and macOS.