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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>
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Eric Biggers
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@ -338,11 +338,14 @@ Supported modes
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Currently, the following pairs of encryption modes are supported:
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- AES-256-XTS for contents and AES-256-CTS-CBC for filenames
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- AES-256-XTS for contents and AES-256-CBC-CTS for filenames
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- AES-256-XTS for contents and AES-256-HCTR2 for filenames
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- Adiantum for both contents and filenames
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- AES-128-CBC-ESSIV for contents and AES-128-CTS-CBC for filenames
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- SM4-XTS for contents and SM4-CTS-CBC for filenames
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- AES-128-CBC-ESSIV for contents and AES-128-CBC-CTS for filenames
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- SM4-XTS for contents and SM4-CBC-CTS for filenames
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Note: in the API, "CBC" means CBC-ESSIV, and "CTS" means CBC-CTS.
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So, for example, FSCRYPT_MODE_AES_256_CTS means AES-256-CBC-CTS.
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Authenticated encryption modes are not currently supported because of
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the difficulty of dealing with ciphertext expansion. Therefore,
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@ -351,11 +354,11 @@ contents encryption uses a block cipher in `XTS mode
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`CBC-ESSIV mode
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<https://en.wikipedia.org/wiki/Disk_encryption_theory#Encrypted_salt-sector_initialization_vector_(ESSIV)>`_,
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or a wide-block cipher. Filenames encryption uses a
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block cipher in `CTS-CBC mode
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block cipher in `CBC-CTS mode
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<https://en.wikipedia.org/wiki/Ciphertext_stealing>`_ or a wide-block
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cipher.
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The (AES-256-XTS, AES-256-CTS-CBC) pair is the recommended default.
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The (AES-256-XTS, AES-256-CBC-CTS) pair is the recommended default.
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It is also the only option that is *guaranteed* to always be supported
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if the kernel supports fscrypt at all; see `Kernel config options`_.
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@ -364,7 +367,7 @@ upgrades the filenames encryption to use a wide-block cipher. (A
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*wide-block cipher*, also called a tweakable super-pseudorandom
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permutation, has the property that changing one bit scrambles the
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entire result.) As described in `Filenames encryption`_, a wide-block
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cipher is the ideal mode for the problem domain, though CTS-CBC is the
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cipher is the ideal mode for the problem domain, though CBC-CTS is the
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"least bad" choice among the alternatives. For more information about
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HCTR2, see `the HCTR2 paper <https://eprint.iacr.org/2021/1441.pdf>`_.
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@ -375,13 +378,13 @@ the work is done by XChaCha12, which is much faster than AES when AES
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acceleration is unavailable. For more information about Adiantum, see
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`the Adiantum paper <https://eprint.iacr.org/2018/720.pdf>`_.
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The (AES-128-CBC-ESSIV, AES-128-CTS-CBC) pair exists only to support
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The (AES-128-CBC-ESSIV, AES-128-CBC-CTS) pair exists only to support
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systems whose only form of AES acceleration is an off-CPU crypto
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accelerator such as CAAM or CESA that does not support XTS.
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The remaining mode pairs are the "national pride ciphers":
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- (SM4-XTS, SM4-CTS-CBC)
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- (SM4-XTS, SM4-CBC-CTS)
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Generally speaking, these ciphers aren't "bad" per se, but they
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receive limited security review compared to the usual choices such as
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@ -393,7 +396,7 @@ Kernel config options
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Enabling fscrypt support (CONFIG_FS_ENCRYPTION) automatically pulls in
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only the basic support from the crypto API needed to use AES-256-XTS
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and AES-256-CTS-CBC encryption. For optimal performance, it is
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and AES-256-CBC-CTS encryption. For optimal performance, it is
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strongly recommended to also enable any available platform-specific
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kconfig options that provide acceleration for the algorithm(s) you
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wish to use. Support for any "non-default" encryption modes typically
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@ -407,7 +410,7 @@ kernel crypto API (see `Inline encryption support`_); in that case,
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the file contents mode doesn't need to supported in the kernel crypto
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API, but the filenames mode still does.
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- AES-256-XTS and AES-256-CTS-CBC
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- AES-256-XTS and AES-256-CBC-CTS
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- Recommended:
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- arm64: CONFIG_CRYPTO_AES_ARM64_CE_BLK
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- x86: CONFIG_CRYPTO_AES_NI_INTEL
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@ -433,7 +436,7 @@ API, but the filenames mode still does.
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- x86: CONFIG_CRYPTO_NHPOLY1305_SSE2
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- x86: CONFIG_CRYPTO_NHPOLY1305_AVX2
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- AES-128-CBC-ESSIV and AES-128-CTS-CBC:
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- AES-128-CBC-ESSIV and AES-128-CBC-CTS:
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- Mandatory:
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- CONFIG_CRYPTO_ESSIV
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- CONFIG_CRYPTO_SHA256 or another SHA-256 implementation
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@ -521,7 +524,7 @@ alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
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inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
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Thus, IV reuse is limited to within a single directory.
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With CTS-CBC, the IV reuse means that when the plaintext filenames share a
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With CBC-CTS, the IV reuse means that when the plaintext filenames share a
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common prefix at least as long as the cipher block size (16 bytes for AES), the
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corresponding encrypted filenames will also share a common prefix. This is
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undesirable. Adiantum and HCTR2 do not have this weakness, as they are
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@ -23,7 +23,7 @@ struct fscrypt_mode fscrypt_modes[] = {
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.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_256_XTS,
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},
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[FSCRYPT_MODE_AES_256_CTS] = {
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.friendly_name = "AES-256-CTS-CBC",
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.friendly_name = "AES-256-CBC-CTS",
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.cipher_str = "cts(cbc(aes))",
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.keysize = 32,
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.security_strength = 32,
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@ -38,7 +38,7 @@ struct fscrypt_mode fscrypt_modes[] = {
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.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_128_CBC_ESSIV,
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},
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[FSCRYPT_MODE_AES_128_CTS] = {
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.friendly_name = "AES-128-CTS-CBC",
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.friendly_name = "AES-128-CBC-CTS",
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.cipher_str = "cts(cbc(aes))",
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.keysize = 16,
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.security_strength = 16,
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@ -53,7 +53,7 @@ struct fscrypt_mode fscrypt_modes[] = {
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.blk_crypto_mode = BLK_ENCRYPTION_MODE_SM4_XTS,
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},
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[FSCRYPT_MODE_SM4_CTS] = {
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.friendly_name = "SM4-CTS-CBC",
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.friendly_name = "SM4-CBC-CTS",
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.cipher_str = "cts(cbc(sm4))",
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.keysize = 16,
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.security_strength = 16,
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