To be considered a strong algorithm, an encryption algorithm must be which of the following?

Keeping an algorithm secret is 'security through obscurity,' which is not considered a strong security property since discovery of the algorithm immediately breaks the system.

Kerckhoffs's principle, a foundational tenet of modern cryptography, requires that an algorithm's security must rest entirely on the secrecy of the key, not on the algorithm itself being hidden. Well-known algorithms like AES and RSA are publicly scrutinized by the global cryptographic community, which strengthens confidence in their security by exposing any weaknesses. Proprietary or secret algorithms cannot be vetted, making undiscovered flaws far more likely.

Confidential is equivalent to secret in this context - relying on algorithm secrecy for security violates Kerckhoffs's principle.

Proprietary algorithms are not publicly audited, meaning vulnerabilities may exist that have never been discovered or disclosed, undermining trust.