Alice has four blocks of plaintext, Po,Pi,P2,P3, which she encrypts using CBC mode to obtain C0,C1,C2,CZ. She then sends the IV and ciphertext to Bob. Upon 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.

Is Bob's integrity checking method secure? explain why or why not.