OpenSSL is a popular Transport Layer Security(TLS) library used to develop secure applications. OpenSSL was started in 1998 with the purpose of developing a free general purpose encryption software library.
OpenSSL is most commonly used for securing communications between web browsers and servers. It is the most widely used TLS encryption library. The OpenSSL library is dual-licensed under both an Apache style license and the BSD license.
Many people became familiar with OpenSSL due to the Heartbleed bug in 2014.
For more information about TLS, and developing secure applications, check out our page on TLS for Applications. Or visit the IETF’s Using TLS in Applications (UTA) working group.