Cipher suite

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A cipher suite is a named combination of authentication, encryption, message authentication code (MAC) and key exchange 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.[1] A reference for named cipher suites is provided in the TLS Cipher Suite Registry.[2]


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.[3] First, the client sends 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's list.[4] To test which TLS ciphers a server supports, an SSL/TLS Scanner may be used.

Detailed description[edit]

Each named cipher suite, e.g. TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, defines a key exchange algorithm, a bulk encryption algorithm, a message authentication code (MAC) algorithm, and a pseudorandom function (PRF).[4][5][6]

  • The key exchange algorithm, e.g. ECDHE_RSA, is used to determine if and how the client and server will authenticate during the handshake.[7]
  • The bulk encryption algorithm, e.g. AES_128_GCM, is used to encrypt the message stream. It also includes the key size and the lengths of explicit and implicit initialization vectors (cryptographic nonces).[8]
  • The message authentication code algorithm, e.g. SHA256, is used to create the message digest, a cryptographic hash of each block of the message stream.[8]
  • The pseudorandom function, e.g. TLS 1.2's PRF using the MAC algorithm's hash function, 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.[9]

Examples of algorithms used[edit]

Key exchange/agreement
RSA, Diffie–Hellman, ECDH, SRP, PSK
Block 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.

Programming references[edit]

Programatically, a cipher suite is referred to as:

CipherSuite cipher_suites
a list of the cryptographic options supported by the client[10]
CipherSuite cipher_suite
the cipher suite selected by the server from the client's cipher_suites and revealed in the ServerHello message[11]


  1. ^ RFC 5246
  2. ^ TLS Cipher Suite Registry
  3. ^ RFC 5246, p. 37
  4. ^ a b RFC 5246, p. 40
  5. ^ "CipherSuites and CipherSpecs". IBM. Retrieved 20 November 2009. 
  6. ^ "Cipher Suites in Schannel". Microsoft MSDN. Retrieved 20 November 2009. 
  7. ^ RFC 5246, p. 47
  8. ^ a b RFC 5246, p. 17
  9. ^ RFC 5246, p. 16-17, 26
  10. ^ RFC 5246, p. 41
  11. ^ RFC 5246, p. 42-43, 64