Re: a question about ciphers

[Jerome Etienne <jme@off.net>]

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@epfl.ch                                 *
> * 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/