91Ó°ÊÓ

CBC mode: (a) In CBC-mode encryption, if a single bit of the plaintext is changed, which ciphertext blocks are affected (assume the same IV is used)? (b) In CBC-mode decryption, if a single bit of the ciphertext is changed, which plaintext blocks are affected?

Suppose you observe a CBC ciphertext and two of its blocks happen to be identical. What can you deduce about the plaintext? State some non-trivial property of the plaintext that doesn't depend on the encryption key.