# Not getting idea how to formally chose chosen cipher attack so adversay wins with non-negligible probability

I was going through this question and intuition came that same message can be used to break security but I an not able formalize it well...can someone help how to create chosen ciphertext attack formally so that adversary wins with significant probability.

• Hint: consider the malleability of countermode; that is, given the encryption of $E_k(P_1)$, you can convert it into the encryption $E_k(P_2)$ (assuming that you know both $P_1$ and $P_2$, and they're the same length...) – poncho Mar 22 at 21:09
• can you please more elaborate.. – Sam Mar 22 at 21:21
• If this is homework, you're supposed to figure it out... – poncho Mar 22 at 21:51
• it is not a homework i am solving chapter 9 of boneh shoup book and it is 9.4 of that book .....just that i am stuck at this ..i thought for 1 day.....but i am not getting idea of it – Sam Mar 22 at 22:06