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Date:   Wed, 9 Jan 2019 09:34:55 -0800
From:   Eric Biggers <>
To:     Stephan Mueller <>
Cc:     James Bottomley <>,
        Andy Lutomirski <>,
        Herbert Xu <>,
        "Lee, Chun-Yi" <>,
        "Rafael J . Wysocki" <>,
        Pavel Machek <>,,,,
        "Rafael J. Wysocki" <>,
        Chen Yu <>,
        Oliver Neukum <>,
        Ryan Chen <>,
        David Howells <>,
        Giovanni Gherdovich <>,
        Randy Dunlap <>,
        Jann Horn <>, Andy Lutomirski <>
Subject: Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler

On Wed, Jan 09, 2019 at 11:17:45AM +0100, Stephan Mueller wrote:
> 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
> > ( 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:
> >
> > 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

That would not meet my performance requirements as I want to precompute
HKDF-Extract, and then do HKDF-Expand many times.  Also the HKDF-Expand part
should be thread-safe and not require allocating memory, especially not a whole
crypto_shash tfm every time.

So presumably with crypto_rng, crypto_rng_reset() would need to take the input
keyring material and salt and do HKDF-Extract (like my fscrypt_init_hkdf()), and
crypto_rng_generate() would need to take the application-specific info string
and do HKDF-Expand (like my fscrypt_hkdf_expand()).

It is ugly though.  Please also consider just having simple crypto_hkdf_*()
helper functions which wrap a HMAC tfm along the lines of my patch, rather than
shoehorning it into the crypto_rng API.

- Eric

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