a. In Section 3.1, under the subsection on the motivation for the Feistel cipher structure, it was stated that, for a block of n bits, the number of different reversible mappings for the ideal block cipher is 2^n !. Justify. b. In that same discussion, it was stated that for the ideal block cipher, which allows all possible reversible mappings, the size of the key is n ×2^n bits. But, if there are 2^n ! possible mappings, it should take log2 2^n ! bits to discriminate among the different mapping