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Kerckhoffs's principle

Level: 101 · for anyone starting from zero

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: A cryptosystem should stay secure even if everything about it except the key is public knowledge. Stated in 1883; still the field's founding rule, and still the one most often violated by people who think they have found an exception.

The questions this page has to answer

  • Why is a secret algorithm a liability rather than an extra layer?
  • What is the difference between this principle and "security through obscurity is useless"? (They are not the same claim, and the second one as usually stated is too strong.)
  • How does the principle explain ROT13 ↗ having a keyspace of one?
  • Why did AES and SHA-3 get chosen by public competition, and what did that buy?
  • Shannon's restatement — "the enemy knows the system" — and why that is a design assumption, not a prediction.

See also