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B.4 Key as initialization vector

The des-cbc-crc algorithm (see Cryptographic Overview) uses the DES key as the initialization vector. This is problematic in general (see below5), but may be mitigated in Kerberos by the CRC checksum that is also included.

From daw@espresso.CS.Berkeley.EDU Fri Mar  1 13:32:34 PST 1996
Article: 50440 of sci.crypt
Path: agate!daw
From: daw@espresso.CS.Berkeley.EDU (David A Wagner)
Newsgroups: sci.crypt
Subject: Re: DES-CBC and Initialization Vectors
Date: 29 Feb 1996 21:48:16 GMT
Organization: University of California, Berkeley
Lines: 31
Message-ID: <4h56v0$>
References: <4h39li$>

In article <4h39li$>,
Nair Venugopal <> wrote:
> Is there anything wrong in using the key as the I.V. in DES-CBC mode?

Yes, you're open to a chosen-ciphertext attack which recovers the key.

Alice is sending stuff DES-CBC encrypted with key K to Bob.  Mary is an
active adversary in the middle.  Suppose Alice encrypts some plaintext
blocks P_1, P_2, P_3, ... in DES-CBC mode with K as the IV, and sends off
the resulting ciphertext
	A->B: C_1, C_2, C_3, ...
where each C_j is a 8-byte DES ciphertext block.  Mary wants to discover
the key K, but doesn't even know any of the P_j's.  She replaces the above
message by
	M->B: C_1, 0, C_1
where 0 is the 8-byte all-zeros block.  Bob will decrypt under DES-CBC,
recovering the blocks
	Q_1, Q_2, Q_3
	Q_1 = DES-decrypt(K, C_1) xor K = P_1
	Q_2 = DES-decrypt(K, C_2) xor C_1 = (some unimportant junk)
	Q_3 = DES-decrypt(K, C_1) xor 0 = P_1 xor K
Bob gets this garbage-looking message Q_1,Q_2,Q_3 which Mary recovers
(under the chosen-ciphertext assumption: this is like a known-plaintext
attack, which isn't too implausible).  Notice that Mary can recover K by
	K = Q_1 xor Q_3;
so after this one simple active attack, Mary gets the key back!

So, if you must use a fixed IV, don't use the key-- use 0 or something
like that.  Even better, don't use a fixed IV-- use the DES encryption
of a counter, or something like that.



The post is copyrighted by David Wagner, included here with permission, the canonical location is

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