GnuTLS is a secure communications library implementing the SSL, TLS and DTLS
protocols and technologies around them.  It provides a simple C language
application programming interface (API) to access the secure communications
protocols as well as APIs to parse and write X.509, PKCS #12, OpenPGP and
other required structures.  It is aimed to be portable and efficient with
focus on security and interoperability.

WWW: https://gnutls.org/