summaryrefslogtreecommitdiffstats
path: root/fs/crypto/hkdf.c
blob: 5a384dad2c72f36af28657b5983b45bf9f70207a (plain)
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
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
// SPDX-License-Identifier: GPL-2.0
/*
 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
 * Function"), aka RFC 5869.  See also the original paper (Krawczyk 2010):
 * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
 *
 * This is used to derive keys from the fscrypt master keys.
 *
 * Copyright 2019 Google LLC
 */

#include <crypto/hash.h>
#include <crypto/sha2.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.
 */

/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
			unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
{
	static const u8 default_salt[HKDF_HASHLEN];
	int err;

	err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
	if (err)
		return err;

	return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
}

/*
 * 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;
	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, 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 prefix[9];
	unsigned int i;
	int err;
	const u8 *prev = NULL;
	u8 counter = 1;
	u8 tmp[HKDF_HASHLEN];

	if (WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN))
		return -EINVAL;

	desc->tfm = hkdf->hmac_tfm;

	memcpy(prefix, "fscrypt\0", 8);
	prefix[8] = context;

	for (i = 0; i < okmlen; i += HKDF_HASHLEN) {

		err = crypto_shash_init(desc);
		if (err)
			goto out;

		if (prev) {
			err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
			if (err)
				goto out;
		}

		err = crypto_shash_update(desc, prefix, sizeof(prefix));
		if (err)
			goto out;

		err = crypto_shash_update(desc, info, infolen);
		if (err)
			goto out;

		BUILD_BUG_ON(sizeof(counter) != 1);
		if (okmlen - i < HKDF_HASHLEN) {
			err = crypto_shash_finup(desc, &counter, 1, tmp);
			if (err)
				goto out;
			memcpy(&okm[i], tmp, okmlen - i);
			memzero_explicit(tmp, sizeof(tmp));
		} else {
			err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
			if (err)
				goto out;
		}
		counter++;
		prev = &okm[i];
	}
	err = 0;
out:
	if (unlikely(err))
		memzero_explicit(okm, okmlen); /* so caller doesn't need to */
	shash_desc_zero(desc);
	return err;
}

void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
{
	crypto_free_shash(hkdf->hmac_tfm);
}