Transport Layer Security (TLS) and its predecessor, SSL, are cryptographic protocols which provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. There are slight differences between SSL 3.0 and TLS 1.0, but the protocol remains substantially the same. The term “TLS” as used here applies to both protocols unless clarified by context.

The TLS protocol(s) allow applications to communicate across a network in a way designed to prevent eavesdropping, tampering, and message forgery. TLS provides endpoint authentication and communications privacy over the Internet using cryptography. Typically, only the server is authenticated (i.e., its identity is ensured) while the client remains unauthenticated; this means that the end user (whether an individual or an application, such as a Web browser) can be sure with whom they are communicating. The next level of security—in which both ends of the “conversation” are sure with whom they are communicating—is known as mutual authentication. Mutual authentication requires public key infrastructure (PKI) deployment to clients.