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Subject: Re: [PATCH 3/5] crypto: add RFC5869 HKDF
From:   Stephan Mueller <smueller@chronox.de>
To:     Eric Biggers <ebiggers@kernel.org>
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
Date:   Thu, 07 Jan 2021 08:53:15 +0100
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Am Mittwoch, dem 06.01.2021 um 23:30 -0800 schrieb Eric Biggers:
> On Mon, Jan 04, 2021 at 10:49:13PM +0100, Stephan Müller wrote:
> > RFC5869 specifies an extract and expand two-step key derivation
> > function. The HKDF implementation is provided as a service function that
> > operates on a caller-provided HMAC cipher handle.
> 
> HMAC isn't a "cipher".
> 
> > The extract function is invoked via the crypto_hkdf_setkey call.
> 
> Any reason not to call this crypto_hkdf_extract(), to match the
> specification?

I named it to match the other KDF implementation. But you are right, I will
name it accordingly.

> 
> > 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. 
> 
> > 
> > The expand function is invoked via the crypto_hkdf_generate and can be
> > invoked multiple times. This function allows the caller to provide a
> > context for the key derivation operation. As specified in RFC5869, it is
> > optional. In case such context is not provided, the caller must provide
> > NULL / 0 for the info / info_nvec parameters.
> 
> Any reason not to call this crypto_hkdf_expand() to match the specification?

I will update the function name.

Thanks
Stephan
> 
> - Eric