fscrypt: remove selection of CONFIG_CRYPTO_SHA256
fscrypt only uses SHA-256 for AES-128-CBC-ESSIV, which isn't the default
and is only recommended on platforms that have hardware accelerated
AES-CBC but not AES-XTS. There's no link-time dependency, since SHA-256
is requested via the crypto API on first use.
To reduce bloat, we should limit FS_ENCRYPTION to selecting the default
algorithms only. SHA-256 by itself isn't that much bloat, but it's
being discussed to move ESSIV into a crypto API template, which would
incidentally bring in other things like "authenc" support, which would
all end up being built-in since FS_ENCRYPTION is now a bool.
For Adiantum encryption we already just document that users who want to
use it have to enable CONFIG_CRYPTO_ADIANTUM themselves. So, let's do
the same for AES-128-CBC-ESSIV and CONFIG_CRYPTO_SHA256.
Acked-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Theodore Ts'o <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 08c23b6..87d4e26 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -191,7 +191,9 @@
If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
AES-128-CBC was added only for low-powered embedded devices with
-crypto accelerators such as CAAM or CESA that do not support XTS.
+crypto accelerators such as CAAM or CESA that do not support XTS. To
+use AES-128-CBC, CONFIG_CRYPTO_SHA256 (or another SHA-256
+implementation) must be enabled so that ESSIV can be used.
Adiantum is a (primarily) stream cipher-based mode that is fast even
on CPUs without dedicated crypto instructions. It's also a true
