 Kerckhoffs's principle

Basically the opposite of security through obscurity, though slightly more focused on cryptography.

Kerckhoffs's principle is a fundamental concept in cryptography, formulated by Auguste Kerckhoffs in 1883. The principle states that a cryptographic system should be secure even if everything about the system, except the secret key, is public knowledge. In other words, the security of a cryptographic algorithm should not depend on the secrecy of the algorithm itself but rather on the secrecy of the key used within the algorithm.