key agreement (algorithm or protocol)

A key establishment method (especially one involving asymmetric cryptography) by which two or more entities, without prior arrangement except a public exchange of data (such as public keys), each can generate the same key value. That is, the method does not send a secret from one entity to the other; instead, both entities, without prior arrangement except a public exchange of data, can compute the same secret value, but that value cannot be computed by other, unauthorized entities. (See: Diffie-Hellman- Merkle, key establishment, KEA, MQV. Compare: key transport.)

"A method for negotiating a key value on line without transferring the key, even in an encrypted form, e.g., the Diffie- Hellman technique." [X509] (See: Diffie-Hellman-Merkle.)

"The procedure whereby two different parties generate shared symmetric keys such that any of the shared symmetric keys is a function of the information contributed by all legitimate participants, so that no party [alone] can predetermine the value of the key." [A9042]

Example: A message originator and the intended recipient can each use their own private key and the other's public key with the Diffie-Hellman-Merkle algorithm to first compute a shared secret value and, from that value, derive a session key to encrypt the message.