19. Suppose that you have a secure block cipher, but no hash function. Also, no key is available. For simplicity, assume that the block cipher has key length and block length both equal to n.

a. How can you use the block cipher as a cryptographic hash function, assuming that you only need to hash one block of exactly n bits?

b. How can you use the block cipher as a cryptographic hash function when the message consists of multiple n-bit blocks?