linux-kernel - Re: [PATCH v2 0/7] Add KDF implementations to crypto API

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Message-ID: <4083606.ejJDZkT8p0@positron.chronox.de>
Date:   Sun, 24 Jan 2021 15:34:02 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Eric Biggers <ebiggers@...nel.org>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        David Howells <dhowells@...hat.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        keyrings@...r.kernel.org, simo@...hat.com
Subject: Re: [PATCH v2 0/7] Add KDF implementations to crypto API

Am Sonntag, 24. Januar 2021, 15:23:29 CET schrieb Ard Biesheuvel:

Hi Ard,

> On Sun, 24 Jan 2021 at 15:10, Stephan Müller <smueller@...onox.de> wrote:
> > Hi,
> > 
> > The key derviation functions are considered to be a cryptographic
> > operation. As cryptographic operations are provided via the kernel
> > crypto API, this patch set consolidates the KDF implementations into the
> > crypto API.
> > 
> > The KDF implementations are provided as service functions. Yet, the
> > interface to the two provided KDFs are identical with the goal to allow
> > them to be transformed into a crypto API template eventually.
> 
> Why? There are typically two reasons to use the crypto API abstractions:
> - the algorithm is not known at compile time, so we need the runtime
> dispatch that the crypto API implements,
> - the algorithm may be implemented by a h/w accelerator which is
> discovered at runtime via the driver stack
> 
> In other cases, a library API is much more suitable, even in the case
> where we may provide arch-specific accelerated implementations of such
> an algorithm.

In case your "why" refers to why I stated that the KDF implementations are 
similar to eventually consolidate them into a template eventually:

A KDF is conceptually a logic on top of a (hash) algorithm like a block 
chaining mode on top of a block cipher or a deterministic RNG on top of an 
underlying cipher.

So, conceptually with the kernel crypto API, we would have a KDF template that 
can be used like hkdf(sha256) or hkdf(sha256-avx2).

The behavior of a KDF is identical to a deterministic RNG. Thus, a long time 
ago, I had a patch developed that adds a very small addition to the existing 
RNG API to allow the KDFs to be used. See [1]. Yet, that was not desired at 
the time due to different reasons.

Yet, the crypto API as it stands today knows of templates and basic 
algorithms. Having a separate library API providing a crypto algorithm is new 
to the crypto API. You see that with the test manager which works well with 
the templates / algorithms but does not provide any helpers for some "library 
APIs".

In case your "why" refers to whether I am not using a template to begin with:

Some time back I provided the patch using a template (see [1] for example). At 
that time, Herbert wanted to have a service API instead.

[1] http://www.chronox.de/kdf.html

Ciao
Stephan