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RE: [Cfrg] Fwd: Hash-Based Key Derivation

"Simon Blake-Wilson" <sblakewilson@bcisse.com> Tue, 25 October 2005 19:28 UTC

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From: Simon Blake-Wilson <sblakewilson@bcisse.com>
To: cfrg@ietf.org
Subject: RE: [Cfrg] Fwd: Hash-Based Key Derivation
Date: Tue, 25 Oct 2005 15:27:31 -0400
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Hi Tom,

But a PRF also assumes random or pseudorandom, rather than merely
unpredictable input, doesn't it?

Best regards. Simon

> -----Original Message-----
> From: cfrg-bounces@ietf.org [mailto:cfrg-bounces@ietf.org] On 
> Behalf Of Tom Shrimpton
> Sent: Tuesday, October 25, 2005 2:58 PM
> To: 'Simon Blake-Wilson'; cfrg@ietf.org
> Subject: RE: [Cfrg] Fwd: Hash-Based Key Derivation
> 
> 
> > Is it really true that you can build a KDF like this based on
> > standard assumptions about a MAC?
> 
> Technically speaking, I believe you need a PRF, not a MAC.   (A PRF is
> a MAC, but not the other way around.)  A MAC is *not* 
> required to provide any sort of secrecy or "randomness", only 
> authenticity.  Say MAC_K(X) is a 
> good MAC, and let MAC'_K(X) = 0^{128} || MAC_K(X).  The tags 
> created by MAC' won't look anything like random bits, but 
> it's still hard to forge a new tag.
> 
> I believe that David is suggesting to construct a good PRF, 
> although he doesn't say so in his post.
> 
> -Tom  
> 
> 
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