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

"Blumenthal, Uri" <uri.blumenthal@intel.com> Tue, 25 October 2005 20:09 UTC

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Subject: RE: [Cfrg] Fwd: Hash-Based Key Derivation
Date: Tue, 25 Oct 2005 16:07:23 -0400
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Thread-Topic: [Cfrg] Fwd: Hash-Based Key Derivation
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From: "Blumenthal, Uri" <uri.blumenthal@intel.com>
To: David Wagner <daw-usenet@taverner.CS.Berkeley.EDU>, cfrg@ietf.org
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> > Is there a proof somewhere that pre-hashing then MACing makes a good
KDF?
>
> In the random oracle model, it is easy to prove that pre-hashing then
> applying a PRF makes a good PRF. 

What properties of hash are assumed in this proof?

> In other words, let
>     F'(S,X) = F(H(S),X)
> where F is a PRF (when its key-input is uniformly distributed) and H
is
> a random oracle.  Then F' is a PRF, even when its secret-input S is
not
> uniformly distributed, so long as S has sufficient min-entropy.

Is it proven that hash function output is uniformly distributed?
Are most cryptographers OK with modelling hash functions as random
oracles?

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