1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
|
// SPDX-License-Identifier: GPL-2.0
/*
* This is used to derive keys from the fscrypt master keys.
*
* Copyright 2019 Google LLC
*/
#include <crypto/hash.h>
#include <crypto/sha2.h>
#include <crypto/hkdf.h>
#include "fscrypt_private.h"
/*
* HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
* SHA-512 because it is well-established, secure, and reasonably efficient.
*
* HKDF-SHA256 was also considered, as its 256-bit security strength would be
* sufficient here. A 512-bit security strength is "nice to have", though.
* Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the
* common case of deriving an AES-256-XTS key (512 bits), that can result in
* HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
* SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
*/
#define HKDF_HMAC_ALG "hmac(sha512)"
#define HKDF_HASHLEN SHA512_DIGEST_SIZE
/*
* HKDF consists of two steps:
*
* 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
* the input keying material and optional salt.
* 2. HKDF-Expand: expand the pseudorandom key into output keying material of
* any length, parameterized by an application-specific info string.
*
* HKDF-Extract can be skipped if the input is already a pseudorandom key of
* length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take
* shorter keys, and we don't want to force users of those modes to provide
* unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
* salt is used, since fscrypt master keys should already be pseudorandom and
* there's no way to persist a random salt per master key from kernel mode.
*/
/*
* Compute HKDF-Extract using the given master key as the input keying material,
* and prepare an HMAC transform object keyed by the resulting pseudorandom key.
*
* Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
* times without having to recompute HKDF-Extract each time.
*/
int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
unsigned int master_key_size)
{
struct crypto_shash *hmac_tfm;
static const u8 default_salt[HKDF_HASHLEN];
u8 prk[HKDF_HASHLEN];
int err;
hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
if (IS_ERR(hmac_tfm)) {
fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
PTR_ERR(hmac_tfm));
return PTR_ERR(hmac_tfm);
}
if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
err = -EINVAL;
goto err_free_tfm;
}
err = hkdf_extract(hmac_tfm, master_key, master_key_size,
default_salt, HKDF_HASHLEN, prk);
if (err)
goto err_free_tfm;
err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
if (err)
goto err_free_tfm;
hkdf->hmac_tfm = hmac_tfm;
goto out;
err_free_tfm:
crypto_free_shash(hmac_tfm);
out:
memzero_explicit(prk, sizeof(prk));
return err;
}
/*
* HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
* was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
* bytes of output keying material parameterized by the application-specific
* 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
* byte. This is thread-safe and may be called by multiple threads in parallel.
*
* ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
* adds to its application-specific info strings to guarantee that it doesn't
* accidentally repeat an info string when using HKDF for different purposes.)
*/
int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen)
{
SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
u8 *full_info;
int err;
full_info = kzalloc(infolen + 9, GFP_KERNEL);
if (!full_info)
return -ENOMEM;
desc->tfm = hkdf->hmac_tfm;
memcpy(full_info, "fscrypt\0", 8);
full_info[8] = context;
memcpy(full_info + 9, info, infolen);
err = hkdf_expand(hkdf->hmac_tfm, full_info, infolen + 9,
okm, okmlen);
kfree_sensitive(full_info);
return err;
}
void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
{
crypto_free_shash(hkdf->hmac_tfm);
}
|
