[PATCH v4 8/8] fscrypt: Add HCTR2 support for filename encryption

Nathan Huckleberry
2022-04-12T17:32:35Z

HCTR2 is a tweakable, length-preserving encryption mode. It has the
same security guarantees as Adiantum, but is intended for use on CPUs
with dedicated crypto instructions. It fixes a known weakness with
filename encryption: when two filenames in the same directory share a
prefix of >= 16 bytes, with CTS-CBC their encrypted filenames share a
common substring, leaking information. HCTR2 does not have this
problem.

More information on HCTR2 can be found here: Length-preserving
encryption with HCTR2: https://eprint.iacr.org/2021/1441.pdf

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
---
Documentation/filesystems/fscrypt.rst | 19 ++++++++++++++-----
fs/crypto/fscrypt_private.h | 2 +-
fs/crypto/keysetup.c | 7 +++++++
fs/crypto/policy.c | 4 ++++
include/uapi/linux/fscrypt.h | 3 ++-
tools/include/uapi/linux/fscrypt.h | 3 ++-
6 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 4d5d50dca65c..09915086abd8 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

If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.

@@ -357,6 +358,14 @@ 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. It has the same
+security guarantees as Adiantum, but is intended for use on CPUs with dedicated
+crypto instructions. See the paper "Length-preserving encryption with HCTR2"
+(https://eprint.iacr.org/2021/1441.pdf) for more details. To use 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 +413,11 @@ 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,
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 5b0a9e6478b5..d8617d01f7bd 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -31,7 +31,7 @@
#define FSCRYPT_CONTEXT_V2 2

/* Keep this in sync with include/uapi/linux/fscrypt.h */
-#define FSCRYPT_MODE_MAX FSCRYPT_MODE_ADIANTUM
+#define FSCRYPT_MODE_MAX FSCRYPT_MODE_AES_256_HCTR2

struct fscrypt_context_v1 {
u8 version; /* FSCRYPT_CONTEXT_V1 */
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index eede186b04ce..ae24b581d3d7 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -53,6 +53,13 @@ struct fscrypt_mode fscrypt_modes[] = {
.ivsize = 32,
.blk_crypto_mode = BLK_ENCRYPTION_MODE_ADIANTUM,
},
+ [FSCRYPT_MODE_AES_256_HCTR2] = {
+ .friendly_name = "HCTR2",
+ .cipher_str = "hctr2(aes)",
+ .keysize = 32,
+ .security_strength = 32,
+ .ivsize = 32,
+ },
};

static DEFINE_MUTEX(fscrypt_mode_key_setup_mutex);
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index ed3d623724cd..fa8bdb8c76b7 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -54,6 +54,10 @@ static bool fscrypt_valid_enc_modes(u32 contents_mode, u32 filenames_mode)
filenames_mode == FSCRYPT_MODE_ADIANTUM)
return true;

+ if (contents_mode == FSCRYPT_MODE_AES_256_XTS &&
+ filenames_mode == FSCRYPT_MODE_AES_256_HCTR2)
+ return true;
+
return false;
}

diff --git a/include/uapi/linux/fscrypt.h b/include/uapi/linux/fscrypt.h
index 9f4428be3e36..a756b29afcc2 100644
--- a/include/uapi/linux/fscrypt.h
+++ b/include/uapi/linux/fscrypt.h
@@ -27,7 +27,8 @@
#define FSCRYPT_MODE_AES_128_CBC 5
#define FSCRYPT_MODE_AES_128_CTS 6
#define FSCRYPT_MODE_ADIANTUM 9
-/* If adding a mode number > 9, update FSCRYPT_MODE_MAX in fscrypt_private.h */
+#define FSCRYPT_MODE_AES_256_HCTR2 10
+/* If adding a mode number > 10, update FSCRYPT_MODE_MAX in fscrypt_private.h */

/*
* Legacy policy version; ad-hoc KDF and no key verification.
diff --git a/tools/include/uapi/linux/fscrypt.h b/tools/include/uapi/linux/fscrypt.h
index 9f4428be3e36..a756b29afcc2 100644
--- a/tools/include/uapi/linux/fscrypt.h
+++ b/tools/include/uapi/linux/fscrypt.h
@@ -27,7 +27,8 @@
#define FSCRYPT_MODE_AES_128_CBC 5
#define FSCRYPT_MODE_AES_128_CTS 6
#define FSCRYPT_MODE_ADIANTUM 9
-/* If adding a mode number > 9, update FSCRYPT_MODE_MAX in fscrypt_private.h */
+#define FSCRYPT_MODE_AES_256_HCTR2 10
+/* If adding a mode number > 10, update FSCRYPT_MODE_MAX in fscrypt_private.h */

/*
* Legacy policy version; ad-hoc KDF and no key verification.
--
2.35.1.1178.g4f1659d476-goog