OpenSSL

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 as well as a full-strength general purpose cryptography library. Apart from being a component of many crypto programs, OpenSSL comes with a lot of command-line tools for encryption, hashing, certificate handling, and more.