OpenSSL, the Cryptography Lego(TM) Set
OpenSSL is based on its predecessor, SSLeay, which was originally developed as a free implementation of Netscape's Secure Socket Layer by Eric Young and Tim Hudson (both SSLeay and OpenSSL have Apache-style licenses). Both Young and Hudson did such a good job with SSLeay that they were hired by RSA Australia. Enter OpenSSL: OpenSSL builds on the work done on SSLeay, and then some. OpenSSL offers support for SSLv2 and SSLv3 as well as the IETF standard TLSv1. In addition to that, OpenSSL has built a great set of cryptographic libraries.
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