Re: [Hash] randomized hashes and DSA

"Steven M. Bellovin" <smb@cs.columbia.edu> Thu, 04 August 2005 08:36 UTC

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From: "Steven M. Bellovin" <smb@cs.columbia.edu>
To: Eric Rescorla <ekr@networkresonance.com>
Subject: Re: [Hash] randomized hashes and DSA
In-Reply-To: Your message of "Thu, 04 Aug 2005 00:26:10 PDT." <20050804072610.060AEB813@delta.rtfm.com>
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Date: Thu, 04 Aug 2005 04:35:58 -0400
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In message <20050804072610.060AEB813@delta.rtfm.com>;, Eric Rescorla writes:
>Steven M. Bellovin <smb@cs.columbia.edu>; wrote:
>> At the hash BoF, Ran Canetti suggested using the same random number for 
>> the hash as for the DSA signature.  That left me feeling very uneasy.  
>> I think I can now show that it's a very bad idea.
>> 
>> The problem is that the two have very different properties.  The random 
>> number used for signing must remain confidential; the random number for 
>> hashing need only be unpredictable.  If I receive a signed message, in 
>> order to verify it I need to have the random number to feed to the hash 
>> function.  But before this, the hash module did not need to have any 
>> confidentiality properties.  With this scheme, it does.  This imposes a 
>> signficant new requirement on the modularization of the total system.
>
>I was assuming that Ran meant r, which is computed by generating
>a random k and then computing: (g^k mod p) mod q
>where k is random and secret. r, however, is public and part of
>the signature, and random since it was derived from k.
>

That would certainly be better, though there are still issues with 
modularization.  The signing process would no longer be a simple
pipeline of an hash operator that merely needs to be authentic and a 
signature operator that requires confidentiality.  To give a concrete 
example, in a secure email system the signature function -- DSA, RSA, 
or whatever -- should be in a separate compartment to protect the 
long-term secret key from the vast bulk of the MTA.  This scheme would 
complicate the API to the signature function, and require a different 
API for DSA than for RSA.

		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb



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