Which property ensures that a hash function will not produce the same hashed value for two different messages?

Collision resistance means no two different messages M1 and M2 should yield the same hash output, making it computationally infeasible for an attacker to find such a pair. This property is critical for digital signatures and data integrity, because a collision would allow an attacker to substitute one message for another while preserving the same hash value. It is one of the three core security properties required of a cryptographic hash function alongside preimage resistance and second preimage resistance.

Bit length describes the size of the hash output such as 256 bits for SHA-256, which influences brute-force difficulty but does not define the collision-avoidance property.

Key strength is a concept applied to symmetric and asymmetric encryption keys, and is not a property of hash functions, which are keyless by design.

Entropy measures randomness and unpredictability in data and is relevant to key generation and password quality, not to whether a hash function produces unique outputs.