Currently, the following pairs of encryption modes are supported:
-- AES-256-XTS for contents and AES-256-CTS-CBC for filenames
+- AES-256-XTS for contents and AES-256-CBC-CTS for filenames
- AES-256-XTS for contents and AES-256-HCTR2 for filenames
- Adiantum for both contents and filenames
-- AES-128-CBC-ESSIV for contents and AES-128-CTS-CBC for filenames
-- SM4-XTS for contents and SM4-CTS-CBC for filenames
+- AES-128-CBC-ESSIV for contents and AES-128-CBC-CTS for filenames
+- SM4-XTS for contents and SM4-CBC-CTS for filenames
+
+Note: in the API, "CBC" means CBC-ESSIV, and "CTS" means CBC-CTS.
+So, for example, FSCRYPT_MODE_AES_256_CTS means AES-256-CBC-CTS.
Authenticated encryption modes are not currently supported because of
the difficulty of dealing with ciphertext expansion. Therefore,
`CBC-ESSIV mode
<https://en.wikipedia.org/wiki/Disk_encryption_theory#Encrypted_salt-sector_initialization_vector_(ESSIV)>`_,
or a wide-block cipher. Filenames encryption uses a
-block cipher in `CTS-CBC mode
+block cipher in `CBC-CTS mode
<https://en.wikipedia.org/wiki/Ciphertext_stealing>`_ or a wide-block
cipher.
-The (AES-256-XTS, AES-256-CTS-CBC) pair is the recommended default.
+The (AES-256-XTS, AES-256-CBC-CTS) pair is the recommended default.
It is also the only option that is *guaranteed* to always be supported
if the kernel supports fscrypt at all; see `Kernel config options`_.
*wide-block cipher*, also called a tweakable super-pseudorandom
permutation, has the property that changing one bit scrambles the
entire result.) As described in `Filenames encryption`_, a wide-block
-cipher is the ideal mode for the problem domain, though CTS-CBC is the
+cipher is the ideal mode for the problem domain, though CBC-CTS is the
"least bad" choice among the alternatives. For more information about
HCTR2, see `the HCTR2 paper <https://eprint.iacr.org/2021/1441.pdf>`_.
acceleration is unavailable. For more information about Adiantum, see
`the Adiantum paper <https://eprint.iacr.org/2018/720.pdf>`_.
-The (AES-128-CBC-ESSIV, AES-128-CTS-CBC) pair exists only to support
+The (AES-128-CBC-ESSIV, AES-128-CBC-CTS) pair exists only to support
systems whose only form of AES acceleration is an off-CPU crypto
accelerator such as CAAM or CESA that does not support XTS.
The remaining mode pairs are the "national pride ciphers":
-- (SM4-XTS, SM4-CTS-CBC)
+- (SM4-XTS, SM4-CBC-CTS)
Generally speaking, these ciphers aren't "bad" per se, but they
receive limited security review compared to the usual choices such as
Enabling fscrypt support (CONFIG_FS_ENCRYPTION) automatically pulls in
only the basic support from the crypto API needed to use AES-256-XTS
-and AES-256-CTS-CBC encryption. For optimal performance, it is
+and AES-256-CBC-CTS encryption. For optimal performance, it is
strongly recommended to also enable any available platform-specific
kconfig options that provide acceleration for the algorithm(s) you
wish to use. Support for any "non-default" encryption modes typically
the file contents mode doesn't need to supported in the kernel crypto
API, but the filenames mode still does.
-- AES-256-XTS and AES-256-CTS-CBC
+- AES-256-XTS and AES-256-CBC-CTS
- Recommended:
- arm64: CONFIG_CRYPTO_AES_ARM64_CE_BLK
- x86: CONFIG_CRYPTO_AES_NI_INTEL
- x86: CONFIG_CRYPTO_NHPOLY1305_SSE2
- x86: CONFIG_CRYPTO_NHPOLY1305_AVX2
-- AES-128-CBC-ESSIV and AES-128-CTS-CBC:
+- AES-128-CBC-ESSIV and AES-128-CBC-CTS:
- Mandatory:
- CONFIG_CRYPTO_ESSIV
- CONFIG_CRYPTO_SHA256 or another SHA-256 implementation
inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
Thus, IV reuse is limited to within a single directory.
-With CTS-CBC, the IV reuse means that when the plaintext filenames share a
+With CBC-CTS, the IV reuse means that when the plaintext filenames share a
common prefix at least as long as the cipher block size (16 bytes for AES), the
corresponding encrypted filenames will also share a common prefix. This is
undesirable. Adiantum and HCTR2 do not have this weakness, as they are
projects / linux-2.6-microblaze.git / blobdiff