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A cipher suite is a named combination of authentication, encryption, and message authentication code (MAC) algorithms used to negotiate the security settings for a network connection using the Transport Layer Security (TLS) / Secure Sockets Layer (SSL) network protocol.
The structure and use of the cipher suite concept is defined in the documents that define the protocol (RFC 5246 standard for TLS version 1.2). A reference for named cipher suites is provided in the TLS Cipher Suite Registry.
When a TLS connection is established, a handshaking, known as the TLS Handshake Protocol, occurs. Within this handshake, a client hello (ClientHello) and a server hello (ServerHello) message are passed. (RFC 5246, p. 37) First, the client sends a cipher suite list, a list of the cipher suites that it supports, in order of preference. Then the server replies with the cipher suite that it has selected from the client cipher suite list. (RFC 5246, p. 40) In order to test which TLS ciphers that a server supports an SSL/TLS Scanner may be used.
- The key exchange algorithm is used to determine if and how the client and server will authenticate during the handshake. (RFC 5246, p. 47).
- The bulk encryption algorithm is used to encrypt the message stream. It also includes the key size and the lengths of explicit and implicit initialization vectors (cryptographic nonces). (RFC 5246, p. 17)
- The message authentication code (MAC) algorithm is used to create the message digest, a cryptographic hash of each block of the message stream. (RFC 5246, p. 17)
- The pseudorandom function (PRF) is used to create the master secret, a 48-byte secret shared between the two peers in the connection. The master secret is used as a source of entropy when creating session keys, such as the one used to create the MAC. (RFC 5246, p. 16-17, 26)
Examples of algorithms used
- key exchange/agreement
- RSA, Diffie-Hellman, ECDH, SRP, PSK
- RSA, DSA, ECDSA
- bulk ciphers
- RC4, Triple DES, AES, IDEA, DES, or Camellia. In older versions of SSL, RC2 was also used.
- message authentication
- for TLS, a Hash-based Message Authentication Code using MD5 or one of the SHA hash functions is used. For SSL, SHA, MD5, MD4, and MD2 are used.
Programatically, a cipher suite is referred to as:
- CipherSuite cipher_suites
- a list of the cryptographic options supported by the client (RFC 5246, p. 41)
- CipherSuite cipher_suite
- the cipher suite selected by the server and revealed in the ServerHello message (RFC 5246, p. 42-43, 64)