QGpgme provides a very high level Qt API around GpgMEpp.

The stunnel program is designed to work as an SSL encryption wrapper
between a remote client and the local (inetd-startable) or remote
server. The concept is that by having non-SSL aware daemons running on
your system, you can easily set them to communicate with clients over a
secure SSL channel. Stunnel can be used to add SSL functionality to
commonly used inetd daemons, such as POP-2, POP-3, and IMAP servers,
without any changes to the program code.

The GnuTLS library provides a secure layer over a reliable transport
layer. Currently the GnuTLS library implements the proposed standards
of the IETF's TLS working group.

Nettle is a cryptographic library that is designed to fit easily in more or
less any context: In crypto toolkits for object-oriented languages (C++,
Python, Pike, ...), in applications like LSH or GNUPG, or even in kernel space.

This is the ASN.1 library used by GNUTLS. More up to date information
can be found at http://www.gnu.org/software/gnutls and
http://www.gnutls.org

The OpenSSL Project is a collaborative effort to develop a robust,
commercial-grade, full-featured, and open source toolkit implementing
the Secure Sockets Layer (SSL v2/v3) and Transport Layer Security (TLS
v1) protocols with full-strength cryptography. The project is managed
by a worldwide community of volunteers that use the Internet to
communicate, plan, and develop the OpenSSL toolkit and its related
documentation.

OpenSSL is a software library to be used in applications that need to
secure communications over computer networks against eavesdropping or
need to ascertain the identity of the party at the other end.
OpenSSL contains an implementation of the SSL and TLS protocols.

OpenSSL Provider for asymmetric operations with private PKCS#11 keys

Staging project for testing upcomming OpenSSL release before pushing it to security:tls and Factory