Kerckhoffs' principle

In cryptography, Kerckhoffs' principle (also called Kerckhoffs' desideratum, Kerckhoffs' assumption, axiom, or law) was stated by Dutch cryptographer Auguste Kerckhoffs in the 19th century: A cryptosystem should be secure even if everything about the system, except the key, is public knowledge.

Kerckhoffs' principle was reformulated (or perhaps independently formulated) by American mathematician Claude Shannon as "the enemy knows the system", i.e., "one ought to design systems under the assumption that the enemy will immediately gain full familiarity with them". In that form, it is called Shannon's maxim. In contrast to "security through obscurity", it is widely embraced by cryptographers.

In 1883 Auguste Kerckhoffs  wrote two journal articles on La Cryptographie Militaire, in which he stated six design principles for military ciphers. Translated from French, they are:

Some are no longer relevant given the ability of computers to perform complex encryption, but his second axiom, now known as Kerckhoffs's principle, is still critically important.

Kerckhoffs viewed cryptography as a rival, and a better alternative than steganographic encoding, which was common in the nineteenth century for hiding the meaning of military messages. One problem with encoding schemes is that they rely on humanly-held secrets such as "dictionaries" which disclose for example, the secret meaning of words. Stenographic-like dictionaries, once revealed, permanently compromise a corresponding encoding system. Another problem is that the risk of exposure increases as the number of users holding the secret(s) increases.

Nineteenth century cryptography in contrast used simple tables which provided for the transposition of alphanumeric characters, generally given row-column intersections which could be modified by keys which were generally short, numeric, and could be committed to human memory. The system was considered "indecipherable" because tables and keys do not convey meaning by themselves. Secret messages can be compromised only if a matching set of table, key, and message falls into enemy hands in a relevant time frame. Kerckhoffs viewed tactical messages as only having a few hours of relevance. Systems are not necessarily compromised, because their components (i.e. alphanumeric character tables and keys) can be easily changed.

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