OpenSSL
OpenSSL is an open source software package that implements SSL (secure sockets layer) and TLS (transport layer security) protocols for conducting secure communication over digital networks. Its development began in 1998, and today it is used by two-thirds of all web servers on the Internet. It supports many different cryptographic algorithms, including AES (Advanced Encryption Standard), Blowfish, DES (Data Encryption Standard), IDEA (International Data Encryption Algorithm), Triple DES, RSA (Rivest, Shamir, Adelman), DSA (Digital Signature Algorithm), Diffie-Hellman, elliptic curve, MD5, SHA-1, and SHA-2. It also supports Perfect Forward Secrecy using elliptic curve Diffie-Helman public-key cryptography.
OpenSSL is available for most major operating systems, including Windows, OS X, Linux, Solaris, and OpenVMS. It is actively maintained by the OpenSSL Project.