Receiving the ciphertext, Bob plans to verify the integrity as follows. He'll first decrypt to obtain the putative plaintext, and then he'll re-encrypt this plaintext using CBC mode and the received IV. If he obtains the same C3 as the final ciphertext block, he will trust the integrity of the plaintext.

a. Suppose that Trudy changes C1 to X, leaving all other blocks and the IV unchanged. Will Bob detect that the data lacks integrity?

b. Suppose that Trudy changes C3 to the value Y, leaving all other blocks and the IV unchanged. Will Bob detect that the data lacks integrity?

c. Is Bob's integrity checking method secure? Yes, because the encryption used CBC mode, the rest of the blocks after the block Trudy changed will be affected.