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How Key Exchange Works Today

Current hybrid cryptosystems like SSL/TLS use symmetric key algorithms (they are generally faster than asymmetric algorithms.) Symmetric key algorithms require a shared secret, exchanged via key-exchange algorithm.

The most famous cryptographic protocol for key exchange is Diffie–Hellman, published in 1976 by Whitfield Diffie and Martin Hellman. Diffie–Hellman allows the creation of a shared secret between a sender and receiver. This shared secret is unable to be deduced by an eavesdropper who is observing the messages between the sender and receiver, except via a brute force attack. If the keyspace for the shared secret is large enough and the secret generated is sufficiently random, brute force attacks become nearly impossible.

How Key Exchange Works Today - Transport Layer Security
How Key Exchange Works Today {#tls-key-exchange}