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During CBC mode decryption, each ciphertext block is first decrypted with the block cipher, and then the first decrypted block is XORed with the IV, while later blocks are XORed with the previous ciphertext block: Thus, the first block of plaintext is computed as: $$P_1 = IV \oplus D_K(C_1).$$ Equivalently, we may solve this equation for $IV$ to obtain: ...


If the MAC'ing is done right (after the encryption) and if you pad the CBC data correctly there's no security risk. Howver changing to CTR or even better to GCM would be better, as both don't operate on whole blocks and GCM even provides authentication.


Better is a subjective term. However for the choice between ECB and CBC, the choice should be CBC for almost all situations. Although ECB and CBC are modes of operation of a block cipher, you could also turn this way of thinking around and see the block cipher as a configuration option for the mode of operation. The mode of operation has a big influence on ...

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