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>
Currently <crypto/sha.h> contains declarations for both SHA-1 and SHA-2,
and <crypto/sha3.h> contains declarations for SHA-3.
This organization is inconsistent, but more importantly SHA-1 is no
longer considered to be cryptographically secure. So to the extent
possible, SHA-1 shouldn't be grouped together with any of the other SHA
versions, and usage of it should be phased out.
Therefore, split <crypto/sha.h> into two headers <crypto/sha1.h> and
<crypto/sha2.h>, and make everyone explicitly specify whether they want
the declarations for SHA-1, SHA-2, or both.
This avoids making the SHA-1 declarations visible to files that don't
want anything to do with SHA-1. It also prepares for potentially moving
sha1.h into a new insecure/ or dangerous/ directory.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Instead of manually allocating a 'struct shash_desc' on the stack and
calling crypto_shash_digest(), switch to using the new helper function
crypto_shash_tfm_digest() which does this for us.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Constify the struct fscrypt_hkdf parameter to fscrypt_hkdf_expand().
This makes it clearer that struct fscrypt_hkdf contains the key only,
not any per-request state.
Link: https://lore.kernel.org/r/20191209204054.227736-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
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.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Eric Biggers