Consider a mac technique called cbc mac- Computer Network Security
Informally speaking, a MAC is secure if no efficient forger can generate any valid (message, tag) pair, even if the forger is given a tagging oracle. (As we discussed in class a tagging oracle is an algorithm that will tag arbitrary messages for an attacker.)
Consider a MAC technique called CBCMAC. The algorithm takes a message, m, a secret key, k, and runs CBC mode encryption on the blocks of the message. For purposes of this problem the initialization vector will always be zero. The tag is the final block of the ciphertext.
Demonstrate that this MAC scheme is not secure for variable length messages. Write pseudocode for an algorithm, F, that, given a tagging oracle Tk(m), can easily generate a (message, tag) pair for a message that was not sent to the tagging oracle.
