libgnutls

GnuTLS is a project that aims to develop a library which provides a
secure layer, over a reliable transport layer.
Currently the GnuTLS library implements the proposed standards by the
IETF's TLS working group.
Quoting from RFC2246 - the TLS 1.0 protocol specification:
The TLS protocol provides communications privacy over the Internet.
The protocol allows client/server applications to communicate in a way
that is designed to prevent eavesdropping, tampering, or message
forgery.