The primary purpose of using HMAC is:

• A. To encrypt data
• B. To uniquely identify a user
• C. To ensure integrity and authenticity
• D. To compress data

Answer: (C)

HMAC provides integrity and authenticity for a message by combining a cryptographic hash function with a secret key. The sender creates a tag (the MAC) from the message and the shared key, then sends the message alongside this tag. The receiver, who also knows the secret key, recomputes the tag from the received message and compares it to the sent tag. If they match, you can trust that the message wasn’t altered in transit (integrity) and that it came from someone who possesses the shared key (authenticity). HMAC does not encrypt the message, so it doesn’t provide confidentiality. It isn’t about identifying a user, but about ensuring that a message genuinely comes from a party that shares the secret key and hasn’t been tampered with.