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Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06

Viktor Dukhovni <viktor1dane@dukhovni.org> Wed, 15 April 2015 05:57 UTC

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From: Viktor Dukhovni <viktor1dane@dukhovni.org>
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Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
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On Thu, Apr 09, 2015 at 08:11:50PM -0400, Greg Hudson wrote:

> > In general, though, it's designed to make sense - you can't use SHA-256
> > to get a 192 bit key because a SHA-256 output has at most 128 bits of
> > security no matter how you slice it.
> 
> I think it very much does matter how you slice it.  For key derivation
> using an HMAC, collision resistance is not required, so it is entirely
> reasonable to use SHA-256 to generate a 192-bit key.  In fact, the key
> derivation function used in this draft (from SP 800-108 section 5.1) can
> work securely even if more keying material is required than the HMAC
> output size.

Yes.  A n-bit HMAC used as a key-derivation PRF is sound for deriving
n-bit keys.  The n/2 collision resistance is not in scope.  Combine
your nonces with the raw DH secret and key-purpose-specific salt,
and away you go.

-- 
	Viktor.

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