OpenSSL is a general purpose cryptography library that provides an open source implementation of the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols.
The library includes tools for generating RSA private keys and Certificate Signing Requests (CSRs), checksums, managing certificates and performing encryption/decryption. OpenSSL is written in C, but wrappers are available for a wide variety of computer languages.
As of this writing, it’s estimated that 66% of all Web servers use OpenSSL. The OpenSSL toolkit is licensed under an Apache-style license.