Alice has four blocks of plaintext, P_0, P_1, P_2, P_3, which she encrypts using

Question

Alice has four blocks of plaintext, P_0, P_1, P_2, P_3, which she encrypts using CBC mode to obtain C_0, C_1, C_2, C_3. 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 C_3 as the final ciphertext block, he will trust the integrity of the plaintext. a. Suppose that Trudy changes C_1 to X, leaving all other blocks and the IV unchanged. Will Bob detect that the data lacks integrity? b. Suppose that Trudy changes C_3 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?

Explanation / Answer

Encryption Decryption

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