3. Suppose that I propose a new version of DES called Quadruple DES, which uses 4 56-bit keys instead of the 3 used by Triple DES. The structure is similar to that of Triple DES, with an additional encryption with the fourth key: Plaintext DES DES DES DES Ciphertext Key 1 Key 2 Key 3 Key 4 Since this uses four keys, I argue that this is more secure than Triple DES, which uses 3 keys. Explain why it is not more secure than Triple DES; specifically • Why adding a fourth key does not create any additional security over having just three keys, and • Where in the above structure you would perform an attack. Big hint: Why is triple DES used? That is, why is double DES not used? If you don't know this, be sure to view the lecture on triple DES again!

Fig: 1