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[Cfrg] Fwd: Hash-Based Key Derivation

David Wagner <daw@cs.berkeley.edu> Tue, 25 October 2005 18:41 UTC

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From: David Wagner <daw@cs.berkeley.edu>
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Subject: [Cfrg] Fwd: Hash-Based Key Derivation
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Date: Tue, 25 Oct 2005 11:41:14 -0700
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Simon Blake-Wilson writes:
>Is it really true that you can build a KDF like this based on standard
>assumptions about a MAC?
>[...] MACs are by design secure only if the
>key is pseudorandom, aren't they?

Yes, in general, you are right.  The fix is to pre-hash the key.
So you might use F(S,X) = SHA256-HMAC(SHA256(S), X), where S is the
secret.

For HMAC, it might just happen to be the case that pre-hashing is
unnecessary -- I don't know, and I haven't tried to do the analysis.

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