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}}</ref> so these protocols often combine symmetric cryptography with public-key cryptography in [[Hybrid cryptosystem|hybrid cryptosystems]].
== [[Description]] ==
Before the mid-1970s, all cipher systems used [[symmetric key algorithm]]s, in which the same [[cryptographic key]] is used with the underlying algorithm by both the sender and the recipient, who must both keep it secret. Of necessity, the key in every such system had to be exchanged between the communicating parties in some secure way prior to any use of the system – for instance, via a [[secure channel]]. This requirement is never trivial and very rapidly becomes unmanageable as the number of participants increases, or when secure channels are not available, or when, (as is sensible cryptographic practice), keys are frequently changed. In particular, if messages are meant to be secure from other users, a separate key is required for each possible pair of users.
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