Most message-by-message authentication methods provide message integrity as a by-product.

• A. True
• B. False
• C. Not Always
• D. Depends

Answer: (A)

At the heart of message-by-message authentication is the idea that each message carries proof (a MAC or digital signature) that it came from the claimed sender and that the content has not been altered. When a recipient verifies that tag with the correct key, they inherently confirm two things: authenticity of the sender and integrity of the message. If the tag verifies, the message wasn’t modified in transit; if it doesn’t, tampering or forgery is detected. Because integrity is produced by the very act of computing and verifying the authentication tag, integrity is a by-product of these methods. That’s why this statement is true.

In practice, this isn’t about occasional lapses or depending on circumstances; properly implemented message-by-message authentication provides integrity as part of its standard function.