Question: Before HMAC, it was common to define a MAC for arbitrary-length messages by Macs,k (m) H8(A 11 m), where H is a collision-resistant hash function

Before HMAC, it was common to define a MAC for arbitrary-length

Before HMAC, it was common to define a MAC for arbitrary-length messages by Macs,k (m) H8(A 11 m), where H is a collision-resistant hash function 1. Show that this is never a secure MAC when H is constructed via the Merkle- Damgrd transform. (Assume that the hash key s is known to the attacker, and only k is kept secret.) 2. Prove that this is a secure MAC if H is modeled as a random oracle

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