Which of the following properties must a hash function NOT have for message authentication?

• A. Collision resistance
• B. Public accessibility
• C. One-way property
• D. Fixed-length output

Answer: (B)

The property of public accessibility is not a requirement for a hash function used in message authentication. In the context of message authentication, the primary focus is on ensuring the integrity and authenticity of the message. This involves characteristics like collision resistance, which prevents two different messages from producing the same hash value, the one-way property that ensures the hash cannot be easily reversed to retrieve the original message, and fixed-length output which standardizes the size of the hash regardless of the input size.

Public accessibility, on the other hand, refers to the ability of anyone to access the hash function – a characteristic that does not impact the effectiveness of the hash function in providing message authentication. A hash function can be secure and effective for message authentication even if it is not publicly accessible. For example, a proprietary hash function used internally within an organization can still maintain message integrity and authenticity without being available to the public. This highlights that public accessibility is not critical for the function to fulfill its role in message authentication.