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I know that the security from CBC stems from the fact that since each message block is randomized by xoring it with a block cipher output, each new call to $E_{K}$ is independent from the previous ones and no attacker can succeed unless a random collision occurs. However, assume that $C[i − 1]$ is known when choosing $M[i]$. The independence is clearly lost. But how could one efficiently attack that system compromising real-or-Random (ROR) security?

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  • $\begingroup$ What happens if you pick $M[i] = C[i - 1]$? $\endgroup$ – Squeamish Ossifrage Oct 27 '19 at 15:11

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