Question: We will now discuss some issues when constructing an efficient MAC. 1. The messages X to be authenticated consists of z independent blocks, so that

We will now discuss some issues when constructing an efficient MAC. 1. The messages X to be authenticated consists of z independent blocks, so that X = x1||x2||...||xz, where every xi consists of |xi| = 8 bits. The input blocks are consecutively put into the compression function ci = h(ci1,xi) = ci1 xi At the end, the MAC value MACk(X) = cz +k mod 2^8 is calculated, where k is a 64-bit long shared key. Describe how exactly the (effective part of the) key k can be calculated with only one known message X. 2. Perform this attack for the following parameters and determine the key k: X = HELLO ALICE! c0 = 111111112 MACk(X) = 100111012

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