Question: 27. The CBC-MAC uses a block cipher n cbc-mode to create a hash function. Is this secure? When is secure and when is it not?

27. The CBC-MAC uses a block cipher n cbc-mode to create

27. The CBC-MAC uses a block cipher n cbc-mode to create a hash function. Is this secure? When is secure and when is it not? 28. Suppose we propose the following MAC: given a message broken into blocks mi, mi, .., mp, we compute fori 1.., where k is a shared secret key andok. The MAC tag is the value c. Is this a secure MAC? Can you create any forgeries? Does it matter if we insist that the message length be a multiple of the block length? Does it matter if we have to pad the last block? 29. EMAC is the encrypted CBC-MAC. How does it differ from cbc-mac? Is this secure? What flaw in CBC-MAC does EMAC address? 27. The CBC-MAC uses a block cipher n cbc-mode to create a hash function. Is this secure? When is secure and when is it not? 28. Suppose we propose the following MAC: given a message broken into blocks mi, mi, .., mp, we compute fori 1.., where k is a shared secret key andok. The MAC tag is the value c. Is this a secure MAC? Can you create any forgeries? Does it matter if we insist that the message length be a multiple of the block length? Does it matter if we have to pad the last block? 29. EMAC is the encrypted CBC-MAC. How does it differ from cbc-mac? Is this secure? What flaw in CBC-MAC does EMAC address

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