1.	Come up with as efficient an encoding as you can to specify a completely general one-to-one mapping between 64 bit input values and 64-bit output values.
2.	Token cards display a number that changes periodically, perhaps every minute. Each such device has a unique secret key. A human can prove possession of a particular such device by entering the displayed number into a computer system. The computer system knows the secret keys of each authorized device. How would you design such a device?
3.	How many DES keys, on average, encrypt a particular plaintext block to a particular ciphertext block?
4.	Make an argument as to why the initial permutation of the bits of the DES key cannot have any security value.