According to Kerckhoffs's Principle, what must remain secret to ensure the security of a cryptosystem?

Kerckhoffs's Principle posits that a cryptosystem should remain secure even if everything about the system except for the secret key is publicly known. This means that the encryption algorithm, while it may be known and analyzed, does not compromise the system's security as long as the key remains confidential. The key is essential for the encryption and decryption processes, and if an unauthorized party obtains it, they can easily decrypt the data regardless of how secure or complex the algorithm is.

While the codebook, encryption algorithms, and plaintext can potentially enhance security or complicate the attacker’s work, they are not the core element that must remain secret according to this principle. The crux of Kerckhoffs's Principle emphasizes that the key must be kept confidential to ensure the integrity and confidentiality of the entire cryptographic system.