fscrypt: add support for AES-128-CBC

fscrypt provides facilities to use different encryption algorithms which
are selectable by userspace when setting the encryption policy. Currently,
only AES-256-XTS for file contents and AES-256-CBC-CTS for file names are
implemented. This is a clear case of kernel offers the mechanism and
userspace selects a policy. Similar to what dm-crypt and ecryptfs have.

This patch adds support for using AES-128-CBC for file contents and
AES-128-CBC-CTS for file name encryption. To mitigate watermarking
attacks, IVs are generated using the ESSIV algorithm. While AES-CBC is
actually slightly less secure than AES-XTS from a security point of view,
there is more widespread hardware support. Using AES-CBC gives us the
acceptable performance while still providing a moderate level of security
for persistent storage.

Especially low-powered embedded devices with crypto accelerators such as
CAAM or CESA often only support AES-CBC. Since using AES-CBC over AES-XTS
is basically thought of a last resort, we use AES-128-CBC over AES-256-CBC
since it has less encryption rounds and yields noticeable better
performance starting from a file size of just a few kB.

Signed-off-by: Daniel Walter <dwalter@sigma-star.at>
[david@sigma-star.at: addressed review comments]
Signed-off-by: David Gstir <david@sigma-star.at>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
index 08b46e6e3995..02b7d91c9231 100644
--- a/fs/crypto/Kconfig
+++ b/fs/crypto/Kconfig
@@ -7,6 +7,7 @@ config FS_ENCRYPTION
 	select CRYPTO_XTS
 	select CRYPTO_CTS
 	select CRYPTO_CTR
+	select CRYPTO_SHA256
 	select KEYS
 	help
 	  Enable encryption of files and directories.  This