CSC 580: Cryptography and Security in Computing

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Homework 11 – Due Tuesday, April 18

  1. In this problem, we revisit the DAA MAC scheme from the previous homework: if the input message is D1,...,DN, and E(K,M) is the encryption function for any block cipher, such as DES or AES, we first compute
    O1 = E(K, D1)
    O2 = E(K, D2 XOR O1)
    ON = E(K, DN XOR ON-1)

    The final MAC is ON. If we know that the MAC of a single block message D1 is T, it is possible to figure out what the MAC of the two block message D1, T XOR D1 is, even if you don't know the key. What is it? Justify your answer (show your work).

  2. There are two main authenticated encryption techniques described in the book, CCM and GCM. Describe at least two advantages of GCM over CCM.

  3. Both MACs and digital signature schemes are designed with the goal of being resistant to "existential forgery." Describe what this means and why this is an important property.