What property must a hash function possess for message authentication?

• A. It must only work on small data sizes
• B. It must produce variable-length output
• C. It must be computationally infeasible to find multiple blocks with the same hash
• D. It should be easy to reverse-engineer the original data

Answer: (C)

For message authentication, a hash function must ensure that it is computationally infeasible to find multiple blocks of data that produce the same hash output. This property, known as collision resistance, is crucial for maintaining the integrity and authenticity of messages.

When utilizing a hash function for message authentication, the expectation is that the hash value generated from original data will be unique to that data. If it were possible to find two different inputs that produce the same hash value, an attacker could potentially substitute one input for the other without detection. This would compromise the authenticity of the data, as the recipient would believe they are receiving a legitimate message when, in fact, it has been altered.

Other options present characteristics that do not align with the needs for secure hashing in message authentication. For instance, a requirement for variable-length output or the ability to easily reverse-engineer original data does not contribute to the security framework required for effective message authentication. Additionally, asserting that the function should only work on small data sizes limits its usability and practicality in real-world applications where data sizes can vary significantly. Thus, the proper property ensuring security in message authentication relies heavily on the infeasibility of hash collisions.