fscrypt: allow 256-bit master keys with AES-256-XTS

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>

1 files changed, 5 insertions, 5 deletions

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index d6f6495b56c0..4d5d50dca65c 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -176,11 +176,11 @@ Master Keys
 
 Each encrypted directory tree is protected by a *master key*.  Master
 keys can be up to 64 bytes long, and must be at least as long as the
-greater of the key length needed by the contents and filenames
-encryption modes being used.  For example, if AES-256-XTS is used for
-contents encryption, the master key must be 64 bytes (512 bits).  Note
-that the XTS mode is defined to require a key twice as long as that
-required by the underlying block cipher.
+greater of the security strength of the contents and filenames
+encryption modes being used.  For example, if any AES-256 mode is
+used, the master key must be at least 256 bits, i.e. 32 bytes.  A
+stricter requirement applies if the key is used by a v1 encryption
+policy and AES-256-XTS is used; such keys must be 64 bytes.
 
 To "unlock" an encrypted directory tree, userspace must provide the
 appropriate master key.  There can be any number of master keys, each