Subject: RE: [Cfrg] New Proofs for NMAC and HMAC: Security Without Collision-Resistance

From: "Blumenthal, Uri" <[email-masked]>
Date: Tue, 7 Feb 2006 10:41:37 -0500

> This paper by Mihir Bellare seems fairly interesting and
> quite related to some recent threads on this mailing list.
> <http://eprint.iacr.org/2006/043>

Paul,

Thanks for bringing our attention to it. It is more than interesting.

One observation: this paper basically says that we were looking for
wrong properties! Collision resistance (strong or weak) is unnecessary:
if the underlying compression function is a PRF - then keyed MAC is
secure; and if it's not a PRF (Bellare proved that a slightly weaker
assumption is still OK) - then weak collision resistance won't help.

So when we specify requirements for crypto hash - what we really
want/need is Pseudo-Randomness. Then we can safely use it in key
derivation, and then keyed MAC is also secure. 

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From: David McGrew <[email-masked]>
Date: Tue, 7 Feb 2006 10:34:58 -0800

Hi Uri,

> derivation, and then keyed MAC is also secure.

AFAICT, Mihir's work doesn't address the issue of whether or not NMAC  
or HMAC is a secure way of deriving symmetric keys from a Diffie- 
Hellman secret.  The abstract describes the work as showing that the  
functions are good PRFs (or just good MACs) based on some different  
assumptions, but a DH key derivation function can't be shown to be  
secure merely because it is a PRF.

Of course, this is not to suggest that HMAC is a bad KDF.  I suspect  
that it is a good one, but I just don't think that the work cited  
proves anything in that direction.

David