Secure Sockets Layer (SSL), developed by Netscape Communications, and Transport Layer Security (TLS), the open-standard replacement for SSL from the Internet Engineering Task Force, are the two protocols that add encryption and authentication to TCP/IP. This article summarizes the basic concepts of how the two protocols work and how Apache implements these protocols so that one can transmit information securely over HTTP.

SSL and TLS have two main features: ciphers, which enable the encryption of data between the client and server; and digital certificates, which provide a method of authentication of a client and server. SSL uses both symmetric (a.k.a. secret-key) and asymmetric (a.k.a. public-key) ciphers to encrypt information in a secure and efficient manner. Digital certificates, which are based on the public-key encryption technique, provide the method for authentication.

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