Question: Define a message authentication function PHMAC (Prepend Hash MAC ) via for any message M = PIIRI . . . IPL. Suppose an attacker knows

Define a message authentication function PHMAC (Prepend Hash MAC ) via

Define a message authentication function PHMAC (Prepend Hash MAC ) via for

for any message M = PIIRI . . . IPL. Suppose an attacker knows a message/PHMAC pair (Mi, PHMACK (Mi)). Let X be an arbitrary n-bit block and put M2 = MIX. Show how an attacker can compute PHMACK(M2) without knowledge of K, thereby defeating computation resistance for PHMAC Hint: Suppose M1 consists of L blocks. Tracing through the ITHASH algorithm, com pare the outputs of the first L +1 rounds of PHMACK(Mi) and PHMACK(M2)

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