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fscrypt: clarify what is meant by a per-file key

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Now that there's sometimes a second type of per-file key (the dirhash
key), clarify some function names, macros, and documentation that
specifically deal with per-file *encryption* keys.

Link: https://lore.kernel.org/r/20200120223201.241390-4-ebiggers@kernel.org
Reviewed-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2020-01-20 14:31:58 -08:00
parent aa408f835d
commit f592efe735
4 changed files with 37 additions and 36 deletions
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24
Documentation/filesystems/fscrypt.rst
View File

@ -234,8 +234,8 @@ HKDF is more flexible, is nonreversible, and evenly distributes
entropy from the master key. HKDF is also standardized and widely
used by other software, whereas the AES-128-ECB based KDF is ad-hoc.
Per-file keys
-------------
Per-file encryption keys
------------------------
Since each master key can protect many files, it is necessary to
"tweak" the encryption of each file so that the same plaintext in two
@ -268,9 +268,9 @@ is greater than that of an AES-256-XTS key.
Therefore, to improve performance and save memory, for Adiantum a
"direct key" configuration is supported. When the user has enabled
this by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy,
per-file keys are not used. Instead, whenever any data (contents or
filenames) is encrypted, the file's 16-byte nonce is included in the
IV. Moreover:
per-file encryption keys are not used. Instead, whenever any data
(contents or filenames) is encrypted, the file's 16-byte nonce is
included in the IV. Moreover:
- For v1 encryption policies, the encryption is done directly with the
master key. Because of this, users **must not** use the same master
@ -335,11 +335,11 @@ used.
Adiantum is a (primarily) stream cipher-based mode that is fast even
on CPUs without dedicated crypto instructions. It's also a true
wide-block mode, unlike XTS. It can also eliminate the need to derive
per-file keys. However, it depends on the security of two primitives,
XChaCha12 and AES-256, rather than just one. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. To use
Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast
per-file encryption keys. However, it depends on the security of two
primitives, XChaCha12 and AES-256, rather than just one. See the
paper "Adiantum: length-preserving encryption for entry-level
processors" (https://eprint.iacr.org/2018/720.pdf) for more details.
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.
@ -1149,8 +1149,8 @@ The context structs contain the same information as the corresponding
policy structs (see `Setting an encryption policy`_), except that the
context structs also contain a nonce. The nonce is randomly generated
by the kernel and is used as KDF input or as a tweak to cause
different files to be encrypted differently; see `Per-file keys`_ and
`DIRECT_KEY policies`_.
different files to be encrypted differently; see `Per-file encryption
keys`_ and `DIRECT_KEY policies`_.
Data path changes
-----------------

6
fs/crypto/fscrypt_private.h
View File

@ -269,7 +269,7 @@ extern int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
* output doesn't reveal another.
*/
#define HKDF_CONTEXT_KEY_IDENTIFIER 1
#define HKDF_CONTEXT_PER_FILE_KEY 2
#define HKDF_CONTEXT_PER_FILE_ENC_KEY 2
#define HKDF_CONTEXT_DIRECT_KEY 3
#define HKDF_CONTEXT_IV_INO_LBLK_64_KEY 4
#define HKDF_CONTEXT_DIRHASH_KEY 5
@ -441,8 +441,8 @@ extern struct crypto_skcipher *
fscrypt_allocate_skcipher(struct fscrypt_mode *mode, const u8 *raw_key,
const struct inode *inode);
extern int fscrypt_set_derived_key(struct fscrypt_info *ci,
const u8 *derived_key);
extern int fscrypt_set_per_file_enc_key(struct fscrypt_info *ci,
const u8 *raw_key);
extern int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
const struct fscrypt_master_key *mk);

31
fs/crypto/keysetup.c
View File

@ -107,12 +107,12 @@ err_free_tfm:
return ERR_PTR(err);
}
/* Given the per-file key, set up the file's crypto transform object */
int fscrypt_set_derived_key(struct fscrypt_info *ci, const u8 *derived_key)
/* Given a per-file encryption key, set up the file's crypto transform object */
int fscrypt_set_per_file_enc_key(struct fscrypt_info *ci, const u8 *raw_key)
{
struct crypto_skcipher *tfm;
tfm = fscrypt_allocate_skcipher(ci->ci_mode, derived_key, ci->ci_inode);
tfm = fscrypt_allocate_skcipher(ci->ci_mode, raw_key, ci->ci_inode);
if (IS_ERR(tfm))
return PTR_ERR(tfm);
@ -121,7 +121,7 @@ int fscrypt_set_derived_key(struct fscrypt_info *ci, const u8 *derived_key)
return 0;
}
static int setup_per_mode_key(struct fscrypt_info *ci,
static int setup_per_mode_enc_key(struct fscrypt_info *ci,
struct fscrypt_master_key *mk,
struct crypto_skcipher **tfms,
u8 hkdf_context, bool include_fs_uuid)
@ -196,14 +196,14 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
if (ci->ci_policy.v2.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) {
/*
* DIRECT_KEY: instead of deriving per-file keys, the per-file
* nonce will be included in all the IVs. But unlike v1
* policies, for v2 policies in this case we don't encrypt with
* the master key directly but rather derive a per-mode key.
* This ensures that the master key is consistently used only
* for HKDF, avoiding key reuse issues.
* DIRECT_KEY: instead of deriving per-file encryption keys, the
* per-file nonce will be included in all the IVs. But unlike
* v1 policies, for v2 policies in this case we don't encrypt
* with the master key directly but rather derive a per-mode
* encryption key. This ensures that the master key is
* consistently used only for HKDF, avoiding key reuse issues.
*/
err = setup_per_mode_key(ci, mk, mk->mk_direct_tfms,
err = setup_per_mode_enc_key(ci, mk, mk->mk_direct_tfms,
HKDF_CONTEXT_DIRECT_KEY, false);
} else if (ci->ci_policy.v2.flags &
FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) {
@ -213,20 +213,21 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
* the IVs. This format is optimized for use with inline
* encryption hardware compliant with the UFS or eMMC standards.
*/
err = setup_per_mode_key(ci, mk, mk->mk_iv_ino_lblk_64_tfms,
HKDF_CONTEXT_IV_INO_LBLK_64_KEY, true);
err = setup_per_mode_enc_key(ci, mk, mk->mk_iv_ino_lblk_64_tfms,
HKDF_CONTEXT_IV_INO_LBLK_64_KEY,
true);
} else {
u8 derived_key[FSCRYPT_MAX_KEY_SIZE];
err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
HKDF_CONTEXT_PER_FILE_KEY,
HKDF_CONTEXT_PER_FILE_ENC_KEY,
ci->ci_nonce,
FS_KEY_DERIVATION_NONCE_SIZE,
derived_key, ci->ci_mode->keysize);
if (err)
return err;
err = fscrypt_set_derived_key(ci, derived_key);
err = fscrypt_set_per_file_enc_key(ci, derived_key);
memzero_explicit(derived_key, ci->ci_mode->keysize);
}
if (err)

4
fs/crypto/keysetup_v1.c
View File

@ -9,7 +9,7 @@
* This file implements compatibility functions for the original encryption
* policy version ("v1"), including:
*
* - Deriving per-file keys using the AES-128-ECB based KDF
* - Deriving per-file encryption keys using the AES-128-ECB based KDF
* (rather than the new method of using HKDF-SHA512)
*
* - Retrieving fscrypt master keys from process-subscribed keyrings
@ -283,7 +283,7 @@ static int setup_v1_file_key_derived(struct fscrypt_info *ci,
if (err)
goto out;
err = fscrypt_set_derived_key(ci, derived_key);
err = fscrypt_set_per_file_enc_key(ci, derived_key);
out:
kzfree(derived_key);
return err;