On Thu, Dec 13, 2001 at 05:11:48PM +0100, Pascal Junod wrote: > On Thu, 13 Dec 2001, Jerome Etienne wrote: > > > suppose you are the attacker and get data encrypted with a block cipher, > > two or more cipher texts block are equal, which information did get > > from it ? > > > If the encryption mode is ECB, you know that both plaintexts are equal. > If the encryption mode is CBC, you know some information about the XOR of > two plaintexts. thanks. just to check i get it... according to my understanding, cbc encryption is Cn = Enc( Cn-1 xor Pn ) and C'n = Enc( C'n-1 xor P'n ). if Cn == C'n, the attacker knows that Cn-1 xor Pn == C'n-1 xor Pn-1 and Pn xor P'n == Cn xor Cn-1. As Cn-1 and C'n-1 are known, he knows exactly the value of Pn xor P'n. is it correct ? do you have any reference where i could learn more about it ? > There exists similar attacks for OFB and CFB, which retrieve some > (Shannon) information. > > A+ > > Pascal > > -- > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > * Pascal Junod, pascal.junod@xxxxxxx * > * Security and Cryptography Laboratory (LASEC) * > * INF 240, EPFL, CH-1015 Lausanne, Switzerland ++41 (0)21 693 76 17 * > * Montétan 13, CH-1004 Lausanne ++41 (0)79 617 28 57 * > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > - > Linux-crypto: cryptography in and on the Linux system > Archive: http://mail.nl.linux.org/linux-crypto/ - Linux-crypto: cryptography in and on the Linux system Archive: http://mail.nl.linux.org/linux-crypto/
