DiffieHellmanKeyExchange is an Algorithm to bootstrap from authentication to cryptography. It is dynamic so it cannot be used to send Email or other asynchronous communications (unless many Emails are sent in each direction prior to the first payload). The negotiation can be in the clear, but it must be signed to prevent ManInTheMiddle attacks.
The SSL protocol uses DiffieHellmanKeyExchange to move from authentication provided by keys to a shared secret, a session key which can be used in ConventionalCryptography.
One page links to DiffieHellmanKeyExchange:
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