Kerkchoff's principle

https://en.wikipedia.org/wiki/Kerckhoffs%27s_principle

Kerckhoffs's principle (also called Kerckhoffs's desideratum, assumption, axiom, doctrine or law) of cryptography was stated by Netherlands born 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's principle was reformulated (or possibly 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. This concept is widely embraced by cryptographers, in contrast to 'security through obscurity', which is not.