Editing Secure channel (section)

==Secure channels in the real world==
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There are no perfectly secure channels in the real world. There are, at best, only ways to make [[insecure channel]]s (e.g., couriers, [[homing pigeon]]s, [[diplomatic bag]]s, etc.) less insecure: [[padlock]]s (between courier wrists and a briefcase), [[loyalty test]]s, security investigations, and guns for courier personnel, [[diplomatic immunity]] for diplomatic bags, and so forth.

In 1976, two researchers proposed a key exchange technique (now named after them)—[[Diffie–Hellman key exchange]] (D-H). This protocol allows two parties to generate a [[key (cryptography)|key]] only known to them, under the assumption that a certain mathematical problem (e.g., the [[Diffie–Hellman problem]] in their proposal) is computationally infeasible (i.e., very very hard) to solve, and that the two parties have access to an authentic channel. In short, that an eavesdropper—conventionally termed 'Eve', who can listen to all messages exchanged by the two parties, but who can not modify the messages—will not learn the exchanged key. Such a key exchange was impossible with any previously known cryptographic schemes based on [[symmetric cipher]]s, because with these schemes it is necessary that the two parties exchange a secret key at some prior time, hence they require a confidential channel at that time which is just what we are attempting to build.

Most cryptographic techniques are trivially breakable if keys are not exchanged securely or, if they actually were so exchanged, if those keys become known in some other way— burglary or extortion, for instance. An actually secure channel will not be required if an insecure channel can be used to securely exchange keys, and if burglary, bribery, or threat aren't used. The eternal problem has been and of course remains—even with modern key exchange protocols—how to know when an insecure channel worked securely (or alternatively, and perhaps more importantly, when it did not), and whether anyone has actually been bribed or threatened or simply lost a notebook (or a notebook computer) with key information in it. These are hard problems in the real world and no solutions are known—only expedients, [[jury rig]]s, and [[workaround]]s.