phc-discussions - RE: [PHC] Deriving multiple keys (was RE: Mechanical tests)

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Message-ID: <469160bae77949f2b1e551fed65f068f@BY2PR03MB523.namprd03.prod.outlook.com>
Date: Wed, 9 Apr 2014 22:46:10 +0000
From: Greg Zaverucha <gregz@...rosoft.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] Deriving multiple keys (was RE: Mechanical tests)

> -----Original Message-----
> From: Christian Forler [mailto:christian.forler@...-weimar.de]
> Sent: Wednesday, April 9, 2014 10:30 AM
> To: discussions@...sword-hashing.net
> Subject: Re: [PHC] Deriving multiple keys (was RE: Mechanical tests)
> 
> On 09.04.2014 13:32, Dmitry Khovratovich wrote:
> > Hi Christian,
> >
> >>Let H be a KDF and let K1||K2 = H(PWD || salt).  Are K1 and K2 be
> > considered to be independent?
> >
> > The independency is a not a well-defined notion here. Consider, for
> > instance, distinct bytes of K1. Are they independent?
> >
> > If we think about it for a while, it becomes clear that a single key
> > can be splitted into as many keys as its length without any security loss.
> >
> >>y= H(PWD || salt)
> > K1' =   F(y, 1)
> > K2' =   F(y, 2)
> >
> > This method is clearly equivalent to the first one. Indeed, consider
> > H'(x) = F(H(x)||1)||F(H(x)||2). Then the second method turns into the
> > first one with H = H'.
> 
> 
> Your observations are fine if H is a secure KDF. But it is easy to construct an
> insecure KDF out of a (preimage) secure password scrambler.
> 
> Let G be a slow and memory-consuming cryptographic hash function and let
> F be a cryptographic hash function. Suppose H is a password scrambler and
> we have
> 
> y = H(x) := G(x) || F(G(x)).
> 
> H can be considered to be s secure password scrambler  since it is hard to
> recover x from y. Nevertheless, it is a bad idea to use y = y1||y2 directly as a
> key since you can derive y2 from y1 by just computing F(y1).
> 
> Therefore, it might be a good idea to output
> 
> F(y, 1) || F(y, 2) instead of y.
> 
> Best regards,
> Christian

This is basically how Catena-KG works, right? (y is computed as the output of Catena(pwd, salt, ...) and then the counter KDF is applied to y.) How would I use Catena-KG to generate multiple keys? Suppose it was instantiated with SHA-256 and in case (a) I need two 256-bit keys and in case (b) I need two 192-bit keys.  Could I use a single call to Catena-KG in both cases (with 512 and 384 as the length input respectively)?  Do you think your Theorem 7.1 generalizes to other password scramblers (so that the construction of Algorithm 5 could be used with the entries which aren't a KDF)

I'll also comment that the counter construction used in Catena-KG is very similar to the one section 5.8.1 of [http://csrc.nist.gov/publications/nistpubs/800-56A/SP800-56A_Revision1_Mar08-2007.pdf] which *does* allow multiple keys to be derived with one call to the KDF (if I interpreted the text at the beginning of section 5.8 correctly).

Thanks
Greg