a) The keyspace for a cipher is the set all of possible keys that can be used for the cipher. A DES key consists of 56 (independent) bits.

i) Express the size of the DES keyspaceto the nearest power of 10, and find three examples of physical quantities, or other known values, that are approximately the same size.

ii) Consider the collection of passwords defined by combinations of digits and letters (both upper and lower case). What password length would be required so that the number of possible passwords is at least the size of the DES keyspace?

b) Encryption is only useful if there is a matching reversible operation called decryption. In aFeistel cipher, the F-function or round function need not be reversible.

i) Show that F-function of DES is not reversible

ii) Explain how a Feistelcipher whose main component is not reversible can actually support decryption.