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Diffstat (limited to 'Documentation/filesystems/fscrypt.rst')
| -rw-r--r-- | Documentation/filesystems/fscrypt.rst | 22 |
1 files changed, 17 insertions, 5 deletions
diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst index 2e9aaa295125..5ba5817c17c2 100644 --- a/Documentation/filesystems/fscrypt.rst +++ b/Documentation/filesystems/fscrypt.rst @@ -337,6 +337,7 @@ Currently, the following pairs of encryption modes are supported: - AES-256-XTS for contents and AES-256-CTS-CBC for filenames - AES-128-CBC for contents and AES-128-CTS-CBC for filenames - Adiantum for both contents and filenames +- AES-256-XTS for contents and AES-256-HCTR2 for filenames (v2 policies only) If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair. @@ -357,6 +358,17 @@ To use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast implementations of ChaCha and NHPoly1305 should be enabled, e.g. CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM. +AES-256-HCTR2 is another true wide-block encryption mode that is intended for +use on CPUs with dedicated crypto instructions. AES-256-HCTR2 has the property +that a bitflip in the plaintext changes the entire ciphertext. This property +makes it desirable for filename encryption since initialization vectors are +reused within a directory. For more details on AES-256-HCTR2, see the paper +"Length-preserving encryption with HCTR2" +(https://eprint.iacr.org/2021/1441.pdf). To use AES-256-HCTR2, +CONFIG_CRYPTO_HCTR2 must be enabled. Also, fast implementations of XCTR and +POLYVAL should be enabled, e.g. CRYPTO_POLYVAL_ARM64_CE and +CRYPTO_AES_ARM64_CE_BLK for ARM64. + New encryption modes can be added relatively easily, without changes to individual filesystems. However, authenticated encryption (AE) modes are not currently supported because of the difficulty of dealing @@ -404,11 +416,11 @@ 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 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 does not have this -weakness, as it is a wide-block encryption mode. +With CTS-CBC, 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 +wide-block encryption modes. All supported filenames encryption modes accept any plaintext length >= 16 bytes; cipher block alignment is not required. However, |