The GNU Transport Layer Security Library
[Overview]

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."
      

The core library licensed under the GNU Lesser General Public License (LGPL). The LGPL license is compatible with a wide range of free licenses, and even permit you to use GnuTLS in non-free proprietary programs.

• Support for TLS 1.1, TLS 1.0 and SSL 3.0 protocols
  (Since SSL 2.0 is insecure it is not supported).
• Support for TLS extensions, including server name and max record size.
• Support for authentication using the SRP protocol.
• Support for authentication using both X.509 certificates and OpenPGP keys.
• Support for TLS Pre-Shared-Keys (PSK) extension.
• Support for Inner Application (TLS/IA) extension.
• Support for X.509 and OpenPGP certificate handling.
• Supports all the strong encryption algorithms.
• Supports compression.
• Runs on most Unix platforms and Windows.
• GPL compatible license.