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In cryptography, a secure channel is a method or technique assumed to provide means by which data can be transferred from one place or user to another without risk of interception or tampering.

The analytic purpose is to presume something (the capability of such secure transfer) which does not bear directly on the instant discussion of, say, the cryptanalytic vulnerability of a new block cipher design or a new digital signature protocol proposal.

To be brief, there are no secure channels in the real world. There are, at best, only ways to make insecure channels (eg, couriers, homing pigeons, diplomatic bags, etc) less insecure. Padlocks (between courier wrists and a briefcase), loyalty tests, 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). It makes possible very much greater confidence than ever before that, after performing the protocol, both parties have the same key, and that no one else knows it. However, there are workable attack techniques against D-H, and against most such protocols. None are known to be unconditionally secure, which is precisely what the "secure channel" of cryptography discussions is assumed to be.

It is important to note that many 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, for instance. An actually secure channel will not be required if an insecure channel works securely to 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 rigs, and workarounds.

Researchers have proposed, and actually demonstrated in real circumstances quantum cryptography in order to create a secure channel. There is at least one commercial company offering a product embodying it. One aspect of quantum cryptography is absolutely secure data exchange, that is guaranteed (if we understand the physics correctly, and most everyone thinks we do in this case) exchange of uneavesdroppable, non-interceptable, non-tamperable, data. The mechanism is related to quantum uncertainty (the uncertainty relation).

It is not now clear whether the special conditions under which it can be made to work are practical in the real world of noise, dirt, and imperfection in which most everything is required to function. Thus far, actual implementation of the technique is exquisitely finicky and expensive, limiting it to very special purpose applications indeed. It may also be vulnerable to attacks specific to particular implementations and imperfections in the optical components of which the quantum cryptographic equipment is built. While implementations of classical cryptographic algorithms have received the worldwide scrutiny over the years, only a limited amount of public research has been done to assess security of the present-day implementations of quantum cryptosystems (mostly because they are not in the widespread use now).