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> > > > > Add an implementation of HKDF (RFC 5869) to fscrypt, for the purpose of > > deriving additional key material from the fscrypt master keys for v2 > > encryption policies. HKDF is a key derivation function built on top of > > HMAC. We choose SHA-512 for the underlying unkeyed hash, and use an > > "hmac(sha512)" transform allocated from the crypto API. > > > > We'll be using this to replace the AES-ECB based KDF currently used to > > derive the per-file encryption keys. While the AES-ECB based KDF is > > believed to meet the original security requirements, it is nonstandard > > and has problems that don't exist in modern KDFs such as HKDF: > > > > 1. It's reversible. Given a derived key and nonce, an attacker can > > easily compute the master key. This is okay if the master key and > > derived keys are equally hard to compromise, but now we'd like to be > > more robust against threats such as a derived key being compromised > > through a timing attack, or a derived key for an in-use file being > > compromised after the master key has already been removed. > > > > 2. It doesn't evenly distribute the entropy from the master key; each 16 > > input bytes only affects the corresponding 16 output bytes. > > > > 3. It isn't easily extensible to deriving other values or keys, such as > > a public hash for securely identifying the key, or per-mode keys. > > Per-mode keys will be immediately useful for Adiantum encryption, for > > which fscrypt currently uses the master key directly, introducing > > unnecessary usage constraints. Per-mode keys will also be useful for > > hardware inline encryption, which is currently being worked on. > > > > 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. 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@xxxxxxxxxxxxxxxxxxx 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. Also, its correctness is tested by the ciphertext verification xfstests.] - Eric
