According to Kerchoff's principle, what must be kept secret?

Kerckhoffs's principle, which is a fundamental concept in cryptography, states that a cryptographic system should be secure even if everything about the system, except for the key, is public knowledge. This means that the security of the encryption method does not rely on keeping the algorithm secret, but rather on the secrecy of the encryption key.

Consequently, in the context of Kerckhoffs's principle, only the key must remain confidential. The rationale is that if the algorithm is known (which it often is in practice), the security of the information is still intact as long as the key remains confidential. Thus, the correct understanding is that the security of the cryptographic system revolves around the protection of the key.

In contrast, other choices relate to specifics that are not directly tied to Kerckhoffs's principle. The algorithm's complexity and the parameters such as the number of rounds in a cipher or minimum key size are important considerations for overall security but do not pertain to what must specifically be kept secret under this principle.