fscrypt: write CBC-CTS instead of CTS-CBC

Calling CBC with ciphertext stealing "CBC-CTS" seems to be more common
than calling it "CTS-CBC".  E.g., CBC-CTS is used by OpenSSL, Crypto++,
RFC3962, and RFC6803.  The NIST SP800-38A addendum uses CBC-CS1,
CBC-CS2, and CBC-CS3, distinguishing between different CTS conventions
but similarly putting the CBC part first.  In the interest of avoiding
any idiosyncratic terminology, update the fscrypt documentation and the
fscrypt_mode "friendly names" to align with the more common convention.

Changing the "friendly names" only affects some log messages.  The
actual mode constants in the API are unchanged; those call it simply
"CTS".  Add a note to the documentation that clarifies that "CBC" and
"CTS" in the API really mean CBC-ESSIV and CBC-CTS, respectively.

Link: https://lore.kernel.org/r/20240224053550.44659-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2024-02-23 21:35:49 -08:00
parent d3a7bd4200
commit 2f944c66ae
2 changed files with 18 additions and 15 deletions

View File

@ -338,11 +338,14 @@ Supported modes
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,
@ -351,11 +354,11 @@ contents encryption uses a block cipher in `XTS mode
`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`_.
@ -364,7 +367,7 @@ upgrades the filenames encryption to use a wide-block cipher. (A
*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>`_.
@ -375,13 +378,13 @@ the work is done by XChaCha12, which is much faster than AES when AES
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
@ -393,7 +396,7 @@ Kernel config options
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
@ -407,7 +410,7 @@ kernel crypto API (see `Inline encryption support`_); in that case,
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
@ -433,7 +436,7 @@ API, but the filenames mode still does.
- 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
@ -521,7 +524,7 @@ alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
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

View File

@ -23,7 +23,7 @@ struct fscrypt_mode fscrypt_modes[] = {
.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_256_XTS,
},
[FSCRYPT_MODE_AES_256_CTS] = {
.friendly_name = "AES-256-CTS-CBC",
.friendly_name = "AES-256-CBC-CTS",
.cipher_str = "cts(cbc(aes))",
.keysize = 32,
.security_strength = 32,
@ -38,7 +38,7 @@ struct fscrypt_mode fscrypt_modes[] = {
.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_128_CBC_ESSIV,
},
[FSCRYPT_MODE_AES_128_CTS] = {
.friendly_name = "AES-128-CTS-CBC",
.friendly_name = "AES-128-CBC-CTS",
.cipher_str = "cts(cbc(aes))",
.keysize = 16,
.security_strength = 16,
@ -53,7 +53,7 @@ struct fscrypt_mode fscrypt_modes[] = {
.blk_crypto_mode = BLK_ENCRYPTION_MODE_SM4_XTS,
},
[FSCRYPT_MODE_SM4_CTS] = {
.friendly_name = "SM4-CTS-CBC",
.friendly_name = "SM4-CBC-CTS",
.cipher_str = "cts(cbc(sm4))",
.keysize = 16,
.security_strength = 16,