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i have a question about CBC operating mode. what problem happen if we always use IV Zero in CBC Mode?


marked as duplicate by otus, Gilles 'SO- stop being evil', SEJPM, CodesInChaos Nov 15 '15 at 17:44

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It become a deterministic encryption key: Same message encrypted twice, produces the same ciphertext. This has negative consequences on the security of the cipher, because an adversary know if you encrypt the same message, which means she can distinguish encryptions.

  • $\begingroup$ i understand that , if we use any fix IV the problem that you say would be happen and it does not depend on Zero IV or other number. yes? $\endgroup$ – Mahsa ehsani Nov 15 '15 at 16:12
  • $\begingroup$ The property that we need for the IV is to be a random number , that changes per encryption $\endgroup$ – curious Nov 15 '15 at 16:17
  • $\begingroup$ If $0$ comes as a result of a random generation then it is fine, but every time it should change $\endgroup$ – curious Nov 15 '15 at 16:23
  • $\begingroup$ Do not forget to mark the post as a valid answer $\endgroup$ – curious Nov 15 '15 at 18:06
  • $\begingroup$ @curious : ​ ​ ​ The actual requirement for CBC mode is that the IV be unpredictable. ​ If the IVs are chosen randomly but, after the adversary sees challenge ciphertext's IV, the adversary learns an IV before choosing the corresponding plaintext, then the adversary can check a guess of the challenge plaintext's initial block. ​ On the other hand, (for example) the middle two bits of the IV always being 1 wouldn't be a problem. ​ ​ ​ ​ ​ ​ ​ ​ $\endgroup$ – user991 Nov 16 '15 at 3:20

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