Arguably, the most secure encryption $/$ decryption algorithm, called Rijndael, that is used for the advanced encryption system $($AES$)$ in USA is not a secret. It is available in public domain which makes it more secure because the adversary can try breaking it if they can. Having an algorithm in public domain like this is an example of the security design practice called

minimization of implementation.

authorization.

open design.

least astonishment.