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RE: Encrypting IV in ECB

From: Blair Dillaway <blaird@microsoft.com>
Date: Mon, 12 Nov 2001 08:47:11 -0800
Message-ID: <AA19CFCE90F52E4B942B27D42349637902CDCDE7@red-msg-01.redmond.corp.microsoft.com>
To: "Christian Geuer-Pollmann" <geuer-pollmann@nue.et-inf.uni-siegen.de>, "Donald E. Eastlake 3rd" <dee3@torque.pothole.com>
Cc: "XML Encryption WG" <xml-encryption@w3.org>
I agree with Don on this.  Lets not start adding in IV encryption modes.
I disagree with the assertion doing this is a trivial change.  It will
end up creating quite a bit more work for implementors and interop
testing.

The issue Christian describes below is already dealt with through the
use of the optional NONCE value.  By placing a NONCE of length larger
than the alg block size, manipulating the IV can only cause the NONCE to
decrypt incorrectly.  It will not allow one to manipulate the decrypted
value of the original plain-text.

Blair

-----Original Message-----
From: Christian Geuer-Pollmann
[mailto:geuer-pollmann@nue.et-inf.uni-siegen.de] 
Sent: Saturday, November 10, 2001 4:47 AM
To: Donald E. Eastlake 3rd
Cc: XML Encryption WG
Subject: Re: Encrypting IV in ECB


Hi Donald,

I think that other standards do not encrypt such higly structured data
as 
we do. For example, given a schema that allows only particular attribute

values like here:

<a v='1'/>

If we encrypt something like this, imagine the v attribute can only have

the values '1' and '0' by the schema. In such a case, the attacker knows

exactly on which part of the IV he has to mess around - our problem is
that 
XML is not free-choosen text, but restricted by some means.

If we do something like encrypting the IV, it costs us absolutely
nothing 
(but 1 block cipher algo execution), but it removes us one potential
flaw.

Yes, I agree that we say: "We only provide the security service 
'confidentiality' and do _not_ provide 'integrity'. The user has to use
XML 
Signature for integrity." But in this case, I think it makes 
cryptographically sense to add something like this.

Christian

--On Samstag, 10. November 2001 00:17 -0500 "Donald E. Eastlake 3rd" 
<dee3@torque.pothole.com> wrote:

> While this doesn't seem like such a bad idea, I'm not aware of any 
> other standards that do this and I'm not sure we should be the first. 
> This just seems like another case where you want a message integrity 
> check or signature inside the encryption.
>
> Donald
>
> From:  Christian Geuer-Pollmann 
> <geuer-pollmann@nue.et-inf.uni-siegen.de>
> To:  XML Encryption WG <xml-encryption@w3.org>
>
>> about the use of the IV in block encryption in CBC mode: 
>> [Menezes/Orschoot/Vanstone] state in Remark 7.16 (integrity if IV in
>> CBC):
>>
>>  "While the IV in the CBC mode need not be secret, its
>>   integrity should be protected, since malicious
>>   modifications thereof allows an adversary to make
>>   predictable bit changes to the first plaintext
>>   block recovered."
>>
>> Suggestion:
>>
>> If we encrypt the IV in Electronic Codebook Mode (ECB), we ensure 
>> that modifications on the bit layer will break decryption of the 
>> complete block.
>>
>>  "ALGORITHM is used in the Cipher Block Chaining
>>   (CBC) mode with a ALGO_KEY_BIT_LENGTH bit
>>   Initialization Vector (IV). <ADD>The IV is
>>   encrypted in ECB mode.</ADD> The resulting
>>   cipher text is prefixed by the
>>   <ADD>encrypted</ADD> IV."
>>
>> Does this make sense to you?
>>
>> Best regards,
>> Christian
>>
>> [Menezes/Orschoot/Vanstone] Handbook of applied cryptography, page 
>> 230
Received on Monday, 12 November 2001 11:48:00 GMT

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