2. For n-bit block size, there are 2" possible plaintext blocks and 2" possible ciphertext blocks. If the plaintext or ciphertext is represented by an integer, then its value ranges from 0 to 2" - 1. a. How many different n-bit block to n-bit block mappings are there? b. How many different reversible n-bit block to n-bit block mappings are there? c. If a reversible n-bit block mapping is used as a key for the substitution, then what would its minimum bit-length be? [Hint: If there are 64 keys altogether, then each key can be represented by log (64) = 8 bits. This is the minimum bit length.] d. In class we considered the representation of a reversible 4-bit mapping that was a 64-bit string (that was the key length). How much shorter is the key length computed in c. when n=4?