Using CBC mode, Alice encrypts four blocks of plaintext, P0, P1, P2, P3 and she sends the resulting ciphertext blocks, C0, C1, C2, C3, and the IV to Bob. Suppose that Trudy (an attacker) is able to change any of the ciphertex blocks before they are received by Bob. If Trudy knows P1, show that she can replace P1 with X.

Hint: Determine C so that if Trudy replaces C0 with C, when Bob decrypts C1, he will obtain X instead of P1.