26. Explain why H(m h(m and H(m) h(mk) are not secure MACs when the un- derlying hash function uses the Merkle-Damgrd constructions? Go through the attacks for 27. The CBC-MAC uses a block cipher in cbe-mode to create a hash fnction. Is this secure? 28. Suppose we propose the following MAC: given a message broken into blocks m, mi, ..., each. When is sccure and when is it not? mr, we compute G = AESk(m)4-1 for i L.r, where k is a shared secret key and co k. 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 cbe-mac? Is this secure? What flaw in CBC-MAC does EMAC address