Question

In CMAC, which scenario is a different key K2 is used instead of K1?

(a) If the tag is larger than the key length

(b) If the tag is shorter than the key length

(c) In the last step of the algorithm

(d) If the plaintext/message is not an integer multiple of the cipher clock length

The question was posed to me in an interview for internship.

Origin of the question is HMAC, DAA and CMAC in portion Message Authentication Codes, Whirlpool Algorithm and Key Management and Distribution of Cryptograph & Network Security

Answer 1

Correct option is (d) If the plaintext/message is not an integer multiple of the cipher clock length

To explain I would say: If the plaintext/message is not an integer multiple of the cipher clock length, then K2 is used.