It\'s my understanding that Electronic Code Book (ECB) produces similar cypher t

Question

It's my understanding that Electronic Code Book (ECB) produces similar cypher text for similar plaintext inputs which is not a good thing. To get around that, Cypher Block Chaining (CBC) can be used to increase diffusion within the encrypted message. For example an algorithm could take the previous block's cypher text and XOR that with next block's plaintext. My question is, doesn't that make the beginning of a message easier to decrypt? In my mind it seems like the 1st block of an encrypted message has a closer relationship to the original message (less mathematical permutations involved) than subsequently encrypted blocks?

Explanation / Answer

In the first block, the IV provides the "randomness", and in subsequent blocks you just use the previous block of ciphertext instead. Based on the assumption, that the cipher is not weak and behaves like a pseudorandom permutation, this is basically the same: You XOR something unpredictable on the plaintext, and then encrypt.

As long as the IV is chosen randomly (and therefore, it should never be the same), there is no weak beginning. If you disregard the randomness and always start with the same value for a new message, then yes, the first block can be considered weaker. Because then you can distinguish, if two messages have the same beginning.

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