Wisdom from Cryptography

Although steganography is different from cryptography, we can borrow many of the techniques and much practical wisdom from the latter, a more thoroughly researched discipline. In 1883, Auguste Kerckhoffs enunciated the first principles of cryptographic engineering, in which he advises that we assume the method used to encipher data is known to the opponent, so security must lie only in the choice of key . The history of cryptology since then has repeatedly shown the folly of “security-by-obscurity”—the assumption that the enemy will remain ignorant of the system in use, one of the latest examples being mobile phones.

Applying this wisdom, we obtain a tentative definition of a secure stego-system: one where an opponent who understands the system, but does not know the key, can obtain no evidence (or even grounds for suspicion) that a communication has taken place. It will remain a central principle that steganographic processes intended for wide use should be published, just like commercial cryptographic algorithms and protocols. This teaching of Kerckhoffs holds with particular force for watermarking techniques intended for use in evidence, which “must be designed and certified on the assumption that they will be examined in detail by a hostile expert,” Anderson [39, Prin. 1].

So one might expect that designers of copyright marking systems would publish the mechanisms they use, and rely on the secrecy of the keys employed. Sadly, this is not the case; many purveyors of such systems keep their mechanisms subject to nondisclosure agreements, sometimes offering the rationale that a patent is pending.

That any of these security-by-obscurity systems ever worked was a matter of luck. Yet many steganographic systems available today just embed the “hidden” data in the least significant bits (see Section 3.2) of an audio or video file—which is trivial for a capable opponent to detect and remove.

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