A receiver must establish that an arriving message really came from the sender it names, and that nobody altered it on the way. Which security service is that?

Answer

Message authentication

Options - A. Message authentication - B. Encryption - C. Traffic-flow confidentiality - D. Electronic codebook mode Why - A. Correct — a procedure that lets communicating parties verify that a received message is authentic: unaltered contents, genuine source. - B. The near-miss, and the one most learners pick. Encryption hides the contents from a reader; it makes no statement about who wrote them or whether they were changed. - C. That conceals who is talking to whom and how much. It is a confidentiality service and says nothing about a message’s origin. - D. ECB is a mode of operation for a block cipher — a way of applying encryption, not a service. It is famously bad at hiding structure and provides no authentication at all. Classify the attacker: modifying or forging a message is an ACTIVE attack, and authentication is the service that answers active attacks.

Stallings & Brown, Computer Security 5e, ch2 §2.2