Re: [Cfrg] Comments Requested on Deterministic DSA and ECDS draft

David McGrew <mcgrew@cisco.com> Wed, 13 April 2011 13:51 UTC

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From: David McGrew <mcgrew@cisco.com>
To: Jim Schaad <ietf@augustcellars.com>, pornin@bolet.org
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Date: Wed, 13 Apr 2011 06:51:24 -0700
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Subject: Re: [Cfrg] Comments Requested on Deterministic DSA and ECDS draft
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Hi Jim,

thanks for forwarding; I copied Thomas as well.

Hi Thomas,

this is a well-written and interesting draft.  I have some questions  
on the motivations.   As I understand it, there are two claimed  
advantages for deterministic DSA/ECDSA over the usual vanilla-flavored  
DSA/ECDSA: it requires fewer bits of random data to be provided as  
input, and it is easier to validate an implementation.   I don't quite  
understand how it requires fewer bits of random data, though.  If an  
implementation of DSA/ECDSA can instantiate a strong pseudorandom  
source (such as a DRBG from SP 800-90) using a single random seed, it  
can use that source to generate private keys and k-values.  For that  
implementation, the use of deterministic DSA/ECDSA would not be  
significantly different than the usual DSA/ECDSA, since no additional  
random data is needed. On the other hand, an implementation that has  
no random source whatsoever can only generate DSA/ECDSA signatures if  
it imports a private key.  If that implementation could import a  
random seed for use in a pseudorandom source, it could perform the  
usual DSA/ECDSA signatures; otherwise, deterministic DSA/ECDSA would  
be useful.

On validation, it seems like a nice property that implementations of  
deterministic signature algorithms are amenable to known-answer tests,  
and thus can be easily and directly tested.  To my thinking this is an  
appealing property in a design, but as I see it more analysis that  
showed that this is a real advantage would be needed to warrant  
recommending the adoption of this variant over the existing standard,  
which is already well established.  The questions that come to my  
mind: are deterministic signatures demonstrably more robust against  
implementation failure?  Does the known-answer-test of a deterministic  
signature algorithm provide a higher assurance level than the testing  
that can be done on a randomized algorithm?   I think these questions  
are worth some consideration (in the context of your draft, and in  
general).

Security wise, the assumption that deterministic DSA/ECDSA relies on  
is that the HMAC_DRBG is a random oracle.   The reference [LN2009]  
"stresses the importance of studying the actual security of schemes  
proven in the ROM [random oracle model]".  Has this sort of study been  
performed for deterministic DSA/ECDSA?

An attacker in a chosen-message model can control more aspects of the  
internal computation during signature generation.  Does that have an  
impact on the efficacy of fault-injection attacks, say?

regards,

David

On Apr 12, 2011, at 9:33 PM, Jim Schaad wrote:

> I am currently in the process of reviewing the document draft-pornin- 
> deterministic-dsa-00.txt which has been submitted as independent  
> submission.  As part of this process I am soliciting for comments  
> from the cryptographic community about the soundness of the concept  
> presented.  The abstract for the document is as follows:
>
>   This document defines a deterministic digital signature generation
>   procedure.  Such signatures are compatible with standard DSA and
>   ECDSA digital signatures, and can be processed with unmodified
>   verifiers, which need not be aware of the procedure described
>   therein.  Deterministic signatures retain the cryptographic security
>   features associated with digital signatures, but can be more easily
>   implemented in various environments since they do not need access to
>   a source of high quality randomness.
>
> As the document points out there are potential attacks on DSA in the  
> event that k is not randomly chosen and this document  then outlines  
> a way to choose k deterministically and hopefully free from that  
> problem.
>
> Comments are requested so that we can complete our review process.
>
> Thanks
>
> Jim
>
>
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