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

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Date:   Wed, 6 Jan 2021 23:30:50 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Stephan Müller <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 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?

> 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".

> 
> 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?

- Eric