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

From: Eric Biggers <ebiggers@kernel.org>
To: Stephan Mueller <smueller@chronox.de>
Cc: herbert@gondor.apana.org.au, mathew.j.martineau@linux.intel.com,
	dhowells@redhat.com, linux-crypto@vger.kernel.org,
	linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org,
	keyrings@vger.kernel.org
Subject: Re: [PATCH 3/5] crypto: add RFC5869 HKDF
Date: Thu, 7 Jan 2021 10:53:42 -0800	[thread overview]
Message-ID: <X/dYtjA6OJAKc3Pc@gmail.com> (raw)
In-Reply-To: <a5c50afa7e11329ea301e64bc03951b38f4e1eda.camel@chronox.de>

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