4. (10 points) A co-worker proposes the following protocol: Each plaintext block

Question

4. (10 points) A co-worker proposes the following protocol: Each plaintext block of 128 bits is encrypted with AES, and then XORed with the previous block of ciphertext. For the first block, the result is instead XORed with a randomly chosen initialization vector (IV). More formally: Cl E(Pl, K) 21 Then (IV, CO, CI, , CN will be transmitted across the network. The intended recipient is expected to have the key K, and therefore can decrypt easily What are some potential issues with this protocol?

Explanation / Answer

Ans. The AES protocol described here is quite secured. It takes about 5 x 1021 years to crack AES encryption. But some issues may be addressed:

1. The Random Input Value generator should be very good and it should generate large IV values. For small ones, it becomes a bit insecure. And also, the randomness should suffice.

2. The middle rounds involving XOR with previous block should be carried out multiple times so as to make the algorithm a bit more secure.

3. The network by which the information is being transferred should also be very secure. The data should not get intercepted.

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