1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
4 * Function"), aka RFC 5869. See also the original paper (Krawczyk 2010):
5 * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
6 *
7 * This is used to derive keys from the fscrypt master keys.
8 *
9 * Copyright 2019 Google LLC
10 */
11
12 #include <crypto/hash.h>
13 #include <crypto/sha2.h>
14
15 #include "fscrypt_private.h"
16
17 /*
18 * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
19 * SHA-512 because it is well-established, secure, and reasonably efficient.
20 *
21 * HKDF-SHA256 was also considered, as its 256-bit security strength would be
22 * sufficient here. A 512-bit security strength is "nice to have", though.
23 * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the
24 * common case of deriving an AES-256-XTS key (512 bits), that can result in
25 * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
26 * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
27 */
28 #define HKDF_HMAC_ALG "hmac(sha512)"
29 #define HKDF_HASHLEN SHA512_DIGEST_SIZE
30
31 /*
32 * HKDF consists of two steps:
33 *
34 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
35 * the input keying material and optional salt.
36 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
37 * any length, parameterized by an application-specific info string.
38 *
39 * HKDF-Extract can be skipped if the input is already a pseudorandom key of
40 * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take
41 * shorter keys, and we don't want to force users of those modes to provide
42 * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
43 * salt is used, since fscrypt master keys should already be pseudorandom and
44 * there's no way to persist a random salt per master key from kernel mode.
45 */
46
47 /* HKDF-Extract (RFC 5869 section 2.2), unsalted */
48 static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
49 unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
50 {
51 static const u8 default_salt[HKDF_HASHLEN];
52 int err;
53
54 err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
55 if (err)
56 return err;
57
58 return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
59 }
60
61 /*
62 * Compute HKDF-Extract using the given master key as the input keying material,
63 * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
64 *
65 * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
66 * times without having to recompute HKDF-Extract each time.
67 */
68 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
69 unsigned int master_key_size)
70 {
71 struct crypto_shash *hmac_tfm;
72 u8 prk[HKDF_HASHLEN];
73 int err;
74
75 hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
76 if (IS_ERR(hmac_tfm)) {
77 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
78 PTR_ERR(hmac_tfm));
79 return PTR_ERR(hmac_tfm);
80 }
81
82 if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
83 err = -EINVAL;
84 goto err_free_tfm;
85 }
86
87 err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
88 if (err)
89 goto err_free_tfm;
90
91 err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
92 if (err)
93 goto err_free_tfm;
94
95 hkdf->hmac_tfm = hmac_tfm;
96 goto out;
97
98 err_free_tfm:
99 crypto_free_shash(hmac_tfm);
100 out:
101 memzero_explicit(prk, sizeof(prk));
102 return err;
103 }
104
105 /*
106 * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
107 * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
108 * bytes of output keying material parameterized by the application-specific
109 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
110 * byte. This is thread-safe and may be called by multiple threads in parallel.
111 *
112 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
113 * adds to its application-specific info strings to guarantee that it doesn't
114 * accidentally repeat an info string when using HKDF for different purposes.)
115 */
116 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
117 const u8 *info, unsigned int infolen,
118 u8 *okm, unsigned int okmlen)
119 {
120 SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
121 u8 prefix[9];
122 unsigned int i;
123 int err;
124 const u8 *prev = NULL;
125 u8 counter = 1;
126 u8 tmp[HKDF_HASHLEN];
127
128 if (WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN))
129 return -EINVAL;
130
131 desc->tfm = hkdf->hmac_tfm;
132
133 memcpy(prefix, "fscrypt\0", 8);
134 prefix[8] = context;
135
136 for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
137
138 err = crypto_shash_init(desc);
139 if (err)
140 goto out;
141
142 if (prev) {
143 err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
144 if (err)
145 goto out;
146 }
147
148 err = crypto_shash_update(desc, prefix, sizeof(prefix));
149 if (err)
150 goto out;
151
152 err = crypto_shash_update(desc, info, infolen);
153 if (err)
154 goto out;
155
156 BUILD_BUG_ON(sizeof(counter) != 1);
157 if (okmlen - i < HKDF_HASHLEN) {
158 err = crypto_shash_finup(desc, &counter, 1, tmp);
159 if (err)
160 goto out;
161 memcpy(&okm[i], tmp, okmlen - i);
162 memzero_explicit(tmp, sizeof(tmp));
163 } else {
164 err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
165 if (err)
166 goto out;
167 }
168 counter++;
169 prev = &okm[i];
170 }
171 err = 0;
172 out:
173 if (unlikely(err))
174 memzero_explicit(okm, okmlen); /* so caller doesn't need to */
175 shash_desc_zero(desc);
176 return err;
177 }
178
179 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
180 {
181 crypto_free_shash(hkdf->hmac_tfm);
182 }
183
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