On Mon, 2019-07-29 at 13:29 -0700, Eric Biggers wrote: > On Sun, Jul 28, 2019 at 03:39:49PM -0400, Theodore Y. Ts'o wrote: > > On Fri, Jul 26, 2019 at 03:41:34PM -0700, Eric Biggers wrote: > > > From: Eric Biggers <ebiggers@xxxxxxxxxx> [...] > > > HKDF solves all the above problems. > > > > > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > Unless I missed something there's nothing here which is fscrypt > > specific. Granted that it's somewhat unlikely that someone would > > want to implement (the very bloated) IKE from IPSEC in the kernel, > > I wonder if there might be other users of HKDF, and whether this > > would be better placed in lib/ or crypto/ instead of fs/crypto? > > This is standard HKDF-SHA512; only the choice of parameters is > fscrypt-specific. So it could indeed use a common implementation of > HKDF if one were available. > > However, I don't think there are any other HKDF users in the kernel > currently. Well, I'm still trying to add the TPM ones, but they're based on SP800- 108 for arbitrary keys and SP800-56A for elliptic curve ones. These are similar to the RFC5869 except that they do extract/expand in a single operation. Plus, of course, the TPM mandates we use the name algorithm (usually sha256, which is what I hardcoded) as the hash. Note: since you don't use the extract step either in your implementation, effectively you're equivalent to SP800-108 as well. This is effectively the same reason as the TPM: we need deterministic keys, so we've nowhere to get the salt from that would persist. > Also, while there was a patch to support HKDF via the crypto_rng API, > there was no consensus about whether this was actually the best way > to add KDF support: > https://lore.kernel.org/linux-crypto/2423373.Zd5ThvQH5g@xxxxxxxxxxxxx > nox.de > > So for now, to avoid unnecessarily blocking this patchset I think we > should just go with this implementation in fs/crypto/. It can always > be changed later, once we decide on the best way to add KDFs to the > crypto API. > > [To be clear: this patch already uses "hmac(sha512)" from the crypto > API. It's only the actual HKDF part that we're talking about here. Right, once you have the hmac + hash available, the rest is easy, so this is what we have for the TPM KDFa: static void KDFa(u8 *key, int keylen, const char *label, u8 *u, u8 *v, int bytes, u8 *out) { u32 counter; const __be32 bits = cpu_to_be32(bytes * 8); for (counter = 1; bytes > 0; bytes -= SHA256_DIGEST_SIZE, counter++, out += SHA256_DIGEST_SIZE) { SHASH_DESC_ON_STACK(desc, sha256_hash); __be32 c = cpu_to_be32(counter); hmac_init(desc, key, keylen); crypto_shash_update(desc, (u8 *)&c, sizeof(c)); crypto_shash_update(desc, label, strlen(label)+1); crypto_shash_update(desc, u, SHA256_DIGEST_SIZE); crypto_shash_update(desc, v, SHA256_DIGEST_SIZE); crypto_shash_update(desc, (u8 *)&bits, sizeof(bits)); hmac_final(desc, key, keylen, out); } } I honestly think these things are so simplistic with the correct hmac that it would make it more confusing to try to produce a general KDF than it would simply to hard code them where we need them. James
