SSL provides endpoint authentication and communications privacy over the Internet using cryptography. In typical use, only the server is authenticated (i.e. its identity is ensured) while the client remains unauthenticated; mutual authentication requires public key infrastructure (or PKI) deployment to clients. The protocols allow client/server applications to communicate in a way designed to prevent eavesdropping, tampering, and message forgery.

SSL involves a number of basic phases:

* Peer negotiation for algorithm support
* Public key encryption-based key exchange and certificate-based authentication
* Symmetric cipher-based traffic encryption

During the first phase, the client and server negotiate which cryptographic algorithms will be used. Current implementations support the following choices:

* for public-key cryptography: RSA, Diffie-Hellman, DSA or Fortezza;
* for symmetric ciphers: RC2, RC4, IDEA, DES, Triple DES or AES;
* for one-way hash functions: MD5 or SHA.

The SSL protocol exchanges records; each record can be optionally compressed, encrypted and packed with a message authentication code (MAC). Each record has a content_type field that specifies which upper level protocol is being used.

When the connection starts, the record level encapsulates another protocol, the handshake protocol, which has content_type 22.

The client sends and receives several handshake structures:

* It sends a ClientHello message specifying the list of cipher suites, compression methods and the highest protocol version it supports. It also sends random bytes which will be used later.

* Then it receives a ServerHello, in which the server chooses the connection parameters from the choices offered by the client earlier.

* When the connection parameters are known, client and server exchange certificates (depending on the selected public key cipher). These certificates are currently X.509, but there's also a draft specifying the use of OpenPGP based certificates.

* The server can request a certificate from the client, so that the connection can be mutually authenticated..

* Client and server negotiate a common secret called "master secret", possibly using the result of a Diffie-Hellman exchange, or simply encrypting a secret with a public key that is decrypted with the peer's private key. All other key data is derived from this "master secret" (and the client- and server-generated random values), which is passed through a carefully designed "Pseudo Random Function".

TLS/SSL have a variety of security measures:

* Numbering all the records and using the sequence number in the MACs.
* Using a message digest enhanced with a key (so only with the key can you check the MAC). This is specified in RFC 2104).
* Protection against several known attacks (including man in the middle attacks), like those involving a downgrade of the protocol to previous (less secure) versions, or weaker cipher suites.
* The message that ends the handshake ("Finished") sends a hash of all the exchanged data seen by both parties.
* The pseudo random function splits the input data in 2 halves and processes them with different hashing algorithms (MD5 and SHA), then XORs them together. This way it protects itself in the event that one of these algorithms is found vulnerable.

SSL runs on layers beneath application protocols such as HTTP, SMTP and NNTP and above the TCP transport protocol, which forms part of the TCP/IP protocol suite. While it can add security to any protocol that uses reliable connections (such as TCP), it is most commonly used with HTTP to form HTTPS. HTTPS is used to secure World Wide Web pages for applications such as electronic commerce. It uses public key certificates to verify the identity of endpoints.

While an increasing number of client and server products can support SSL natively, many still do not. In these cases, a user may wish to use standalone SSL products like Stunnel to provide encryption. However, the Internet Engineering Task Force recommended in 1997 that application protocols offer a way to upgrade to TLS from a plaintext connection, rather than use a separate port for encrypted communications - this prevents use of wrappers such as Stunnel.

SSL can also be used to tunnel an entire network stack to create a VPN, as is the case with OpenVPN.

Developed by Netscape, SSL version 3.0 was released in 1996, which later served as a basis to develop TLS version 1.0, an IETF standard protocol first defined in RFC 2246. Visa, MasterCard, American Express and many leading financial institutions have endorsed SSL for commerce over the Internet.

SSL operates in modular fashion: its authors designed it for extendability, with support for forwards and backwards compatibility and negotiation between peers.