Messages in this thread | | | Date | Wed, 9 Jan 2019 09:34:55 -0800 | From | Eric Biggers <> | Subject | Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler |
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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 > > (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/?id=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 >
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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