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fscrypt currently requires a 512-bit master key when AES-256-XTS is used, since AES-256-XTS keys are 512-bit and fscrypt requires that the master key be at least as long any key that will be derived from it. However, this is overly strict because AES-256-XTS doesn't actually have a 512-bit security strength, but rather 256-bit. The fact that XTS takes twice the expected key size is a quirk of the XTS mode. It is sufficient to use 256 bits of entropy for AES-256-XTS, provided that it is first properly expanded into a 512-bit key, which HKDF-SHA512 does. Therefore, relax the check of the master key size to use the security strength of the derived key rather than the size of the derived key (except for v1 encryption policies, which don't use HKDF). Besides making things more flexible for userspace, this is needed in order for the use of a KDF which only takes a 256-bit key to be introduced into the fscrypt key hierarchy. This will happen with hardware-wrapped keys support, as all known hardware which supports that feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys are wrapped 256-bit AES keys. Moreover, there is interest in fscrypt supporting the same type of AES-256-CMAC based KDF in software as an alternative to HKDF-SHA512. There is no security problem with such features, so fix the key length check to work properly with them. Reviewed-by: Paul Crowley <paulcrowley@google.com> Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com> |
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| .. | ||
| bio.c | ||
| crypto.c | ||
| fname.c | ||
| fscrypt_private.h | ||
| hkdf.c | ||
| hooks.c | ||
| inline_crypt.c | ||
| Kconfig | ||
| keyring.c | ||
| keysetup_v1.c | ||
| keysetup.c | ||
| Makefile | ||
| policy.c | ||