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

"Tom Shrimpton" <teshrim@cs.pdx.edu> Tue, 25 October 2005 18:58 UTC

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From: Tom Shrimpton <teshrim@cs.pdx.edu>
To: 'Simon Blake-Wilson' <sblakewilson@bcisse.com>, cfrg@ietf.org
Subject: RE: [Cfrg] Fwd: Hash-Based Key Derivation
Date: Tue, 25 Oct 2005 11:57:58 -0700
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> 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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