crypto: essiv - create wrapper template for ESSIV generation

Implement a template that wraps a (skcipher,shash) or (aead,shash) tuple
so that we can consolidate the ESSIV handling in fscrypt and dm-crypt and
move it into the crypto API. This will result in better test coverage, and
will allow future changes to make the bare cipher interface internal to the
crypto subsystem, in order to increase robustness of the API against misuse.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

crypto/Kconfig

@@ -568,6 +568,34 @@ config CRYPTO_ADIANTUM
 
 	  If unsure, say N.
 
+config CRYPTO_ESSIV
+	tristate "ESSIV support for block encryption"
+	select CRYPTO_AUTHENC
+	help
+	  Encrypted salt-sector initialization vector (ESSIV) is an IV
+	  generation method that is used in some cases by fscrypt and/or
+	  dm-crypt. It uses the hash of the block encryption key as the
+	  symmetric key for a block encryption pass applied to the input
+	  IV, making low entropy IV sources more suitable for block
+	  encryption.
+
+	  This driver implements a crypto API template that can be
+	  instantiated either as a skcipher or as a aead (depending on the
+	  type of the first template argument), and which defers encryption
+	  and decryption requests to the encapsulated cipher after applying
+	  ESSIV to the input IV. Note that in the aead case, it is assumed
+	  that the keys are presented in the same format used by the authenc
+	  template, and that the IV appears at the end of the authenticated
+	  associated data (AAD) region (which is how dm-crypt uses it.)
+
+	  Note that the use of ESSIV is not recommended for new deployments,
+	  and so this only needs to be enabled when interoperability with
+	  existing encrypted volumes of filesystems is required, or when
+	  building for a particular system that requires it (e.g., when
+	  the SoC in question has accelerated CBC but not XTS, making CBC
+	  combined with ESSIV the only feasible mode for h/w accelerated
+	  block encryption)
+
 comment "Hash modes"
 
 config CRYPTO_CMAC

crypto/Makefile

@@ -147,6 +147,7 @@ obj-$(CONFIG_CRYPTO_USER_API_AEAD) += algif_aead.o
 obj-$(CONFIG_CRYPTO_ZSTD) += zstd.o
 obj-$(CONFIG_CRYPTO_OFB) += ofb.o
 obj-$(CONFIG_CRYPTO_ECC) += ecc.o
+obj-$(CONFIG_CRYPTO_ESSIV) += essiv.o
 
 ecdh_generic-y += ecdh.o
 ecdh_generic-y += ecdh_helper.o