ITPEC FE Morning October 2019 Question 38

To confirm that the message has not been falsified — a message authentication code (MAC) checks integrity and the sender, not secrecy.

The sender computes a short code from the message and a secret key shared with the receiver (HMAC, for example, uses a hash function to make the digest) and sends it with the message. The receiver recomputes the code with the same key; if the two match, the message was not altered on the way and came from someone holding the key.

The message itself travels as it is: a MAC does not encrypt anything.

Answer (c)

Why not others:

• •(a) — a MAC says nothing about how, or whether, a message was encrypted; it is computed over the message whatever the encryption
• •(b) — a digest is not a readable overview; it is a fixed-length value that reveals nothing about the content
• •(d) — confidentiality comes from encryption; a message with a MAC can still be read by anyone who intercepts it

Key rule: Encryption protects confidentiality; a MAC or a digital signature protects integrity, and only a signature, made with a private key, also gives non-repudiation.

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