Come up with as efficient an encoding as you can to specify a completely general
ID: 3651356 • Letter: C
Question
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. 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? How many DES keys on the average encrypt a particular plaintext block to a particular ciphertext block? Make an argument as to why the initial permutation of the bits of the DES key cannot have any security value.Explanation / Answer
3.
There are 256 possible keys and 264 possible ciphertext blocks for a particularplaintext block.So only about 256-64=1/256 of the possible ciphertext blocks can be obtained by with aDES key.