Kerckhoffs' Principle

[Accessing]

Kerckhoffs' Principle states that the security of a cryptosystem must lie in the choice of its keys only; everything else (including the algorithm itself) should be considered public knowledge.

I recently came across this studying for something else entirely but thought it to be interesting. It is that no system is secured in obscurity and essentially nothing that is obscure is secured. This can apply to a number of different things but is introduced in cryptosystems because its likely someone knows everything about a system outside of the key. 

[Argon143521]

This reminds me of Kaprekar's constant (6174) for some reason. Interesting read if you're into interesting mathematical tid-bits.