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From:   Stephan Mueller <smueller@chronox.de>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     James Bottomley <James.Bottomley@hansenpartnership.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "Lee, Chun-Yi" <joeyli.kernel@gmail.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, keyrings@vger.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Chen Yu <yu.c.chen@intel.com>,
        Oliver Neukum <oneukum@suse.com>,
        Ryan Chen <yu.chen.surf@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Giovanni Gherdovich <ggherdovich@suse.cz>,
        Randy Dunlap <rdunlap@infradead.org>,
        Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler
Date:   Wed, 09 Jan 2019 11:17:45 +0100
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References: <20190103143227.9138-1-jlee@suse.com> <1894062.aDvIuj92vB@tauon.chronox.de> <20190109082103.GA8586@sol.localdomain>
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Am Mittwoch, 9. Januar 2019, 09:21:04 CET schrieb Eric Biggers:

Hi Eric,
> 
> FWIW, it's been very slow going since I've been working on other projects
> and I also need to be very sure to get the API changes right, but I still
> plan to change the KDF in fscrypt (a.k.a. ext4/f2fs/ubifs encryption) to
> HKDF-SHA512 as part of a larger set of improvements to how fscrypt
> encryption keys are managed. I sent the last patchset a year ago
> (https://marc.info/?l=linux-fsdevel&m=150879493206257) but I'm working to
> revive it.  In the work-in-progress version in my git tree, this is the
> commit that adds a HKDF implementation as fs/crypto/hkdf.c:
> https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/commit/?i
> d=e8a78767131c9717ee838f0c4e307948d65a4427 It basically just wraps a
> crypto_shash for "hmac(sha512)".
> 
> I'd be fine with using a common implementation instead, provided that it
> gives the same functionality, including supporting user-specified salt and
> application-specific info strings, and isn't slower or more complex to use.
> 
> (This comment is solely on the tangential discussion about KDF
> implementations; I've not looked at the hibernation image encryption stuff
> yet.)

Thanks for the clarification. I have started a generic HKDF implementation for 
the kernel crypto API which lead to the questions above. I would then also try 
to provide a HKDF proposal.

To use the (H)KDF, I currently envision 2 calls apart from alloc/free. The 
following code would serve as an example.

 * Example without proper error handling:
 *      char *keying_material = "\x00\x11\x22\x33\x44\x55\x66\x77";
 *      char *label_context = "\xde\xad\xbe\xef\x00\xde\xad\xbe\xef";
 *      kdf = crypto_alloc_rng(name, 0, 0);
 *      crypto_rng_reset(kdf, keying_material, 8);
 *      crypto_rng_generate(kdf, label_context, 9, outbuf, outbuflen);

That hopefully should be simple enough.

For HKDF, as mentioned, I would envision to use a struct instead of a char * 
for the label_context to communicate IKM, Salt, and the label/info 
information.

Ciao
Stephan