Editing Symmetric-key algorithm (section)

== Key establishment ==
{{ main | Key establishment }}
Symmetric-key algorithms require both the sender and the recipient of a message to have the same secret key. All early cryptographic systems required either the sender or the recipient to somehow receive a copy of that secret key over a physically secure channel.

Nearly all modern cryptographic systems still use symmetric-key algorithms internally to encrypt the bulk of the messages, but they eliminate the need for a physically secure channel by using [[Diffie–Hellman key exchange]] or some other [[public-key cryptography|public-key protocol]] to securely come to agreement on a fresh new secret key for each session/conversation (forward secrecy).