Question: Consider a CBC-MAC and suppose that the encryption on key k1 (i.e., the last encryption block) is omitted. Show that such the MAC system is

Consider a CBC-MAC and suppose that the encryption on key k1 (i.e., the last encryption block) is omitted. Show that such the MAC system is not secure. Hint: Choose an arbitrary one-block message m E X, query the challenger for m and obtain its tag t = F(k,m). Now, find a two-block message m, such that t is a valid tag for m as well.

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