linux-kernel - Re: [PATCH 3/5] crypto: add RFC5869 HKDF

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Date:   Thu, 7 Jan 2021 10:53:42 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Stephan Mueller <smueller@...onox.de>
Cc:     herbert@...dor.apana.org.au, mathew.j.martineau@...ux.intel.com,
        dhowells@...hat.com, linux-crypto@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org
Subject: Re: [PATCH 3/5] crypto: add RFC5869 HKDF

On Thu, Jan 07, 2021 at 08:53:15AM +0100, Stephan Mueller wrote:
> > 
> > > RFC5869
> > > allows two optional parameters to be provided to the extract operation:
> > > the salt and additional information. Both are to be provided with the
> > > seed parameter where the salt is the first entry of the seed parameter
> > > and all subsequent entries are handled as additional information. If
> > > the caller intends to invoke the HKDF without salt, it has to provide a
> > > NULL/0 entry as first entry in seed.
> > 
> > Where does "additional information" for extract come from?  RFC 5869 has:
> > 
> >         HKDF-Extract(salt, IKM) -> PRK
> > 
> >         Inputs:
> >               salt     optional salt value (a non-secret random value);
> >                        if not provided, it is set to a string of HashLen
> > zeros.
> >               IKM      input keying material
> > 
> > There's no "additional information".
> 
> I used the terminology from SP800-108. I will update the description
> accordingly. 

For HKDF, it would be better to stick to the terminology used in RFC 5869
(https://tools.ietf.org/html/rfc5869), as generally that's what people are most
familiar with for HKDF.  It also matches the HKDF paper
(https://eprint.iacr.org/2010/264.pdf) more closely.

- Eric