Kerckhoffs' Principle

Jean-Guillame-Hubert-Victor-Francois-Alexandre-Auguste Kerckhoffs von Niewenhof, whose full name might make a start at a minimally strong polyalphabetic key, was usually known as Auguste Kerckhoffs. In his 1883 book, La Cryptographie Militaire, he stated six axioms of cryptography. Some are no longer relevant given the ability of computers to perform complex encryption, but the second is the most critical, and, perhaps, counterintuitive:

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If the method of encipherment becomes known to one's adversary, this should not prevent one from continuing to use the cipher as long as the key remains unknown

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That is, the security should depend only on the secrecy of the key. Any serious enemy — one with strong motives and plentiful resources — will learn all the other details. In war, the enemy will capture some of your equipment and some of your people, and will use spies. If your method involves software, enemies will do memory dumps, run it under the control of a debugger, and so on. If it is hardware, they will buy or steal some and build whatever programs or gadgets they need to test them. Or in any of these cases, they may bribe, blackmail or threaten your staff to learn your secrets. One way or another, sooner or later they will know exactly how it all works.

Is your system secure when the enemy knows everything except the key? If not, then at some point it is certain to become worthless. Since a security analyst cannot know when that point might come, the analysis can be simplified to The system is worthless if it cannot withstand an attacker that knows all its internal details.