# Blockwise-adaptive RoR-CPA Attack against CBC

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?

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