1 // SPDX-License-Identifier: GPL-2.0 1 // SPDX-License-Identifier: GPL-2.0
2 /* 2 /*
3 * Implementation of HKDF ("HMAC-based Extract 3 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
4 * Function"), aka RFC 5869. See also the ori 4 * Function"), aka RFC 5869. See also the original paper (Krawczyk 2010):
5 * "Cryptographic Extraction and Key Derivatio 5 * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
6 * 6 *
7 * This is used to derive keys from the fscryp 7 * This is used to derive keys from the fscrypt master keys.
8 * 8 *
9 * Copyright 2019 Google LLC 9 * Copyright 2019 Google LLC
10 */ 10 */
11 11
12 #include <crypto/hash.h> 12 #include <crypto/hash.h>
13 #include <crypto/sha2.h> 13 #include <crypto/sha2.h>
14 14
15 #include "fscrypt_private.h" 15 #include "fscrypt_private.h"
16 16
17 /* 17 /*
18 * HKDF supports any unkeyed cryptographic has 18 * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
19 * SHA-512 because it is well-established, sec !! 19 * SHA-512 because it is reasonably secure and efficient; and since it produces
20 * !! 20 * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of
21 * HKDF-SHA256 was also considered, as its 256 !! 21 * entropy from the master key and requires only one iteration of HKDF-Expand.
22 * sufficient here. A 512-bit security streng <<
23 * Also, on 64-bit CPUs, SHA-512 is usually ju <<
24 * common case of deriving an AES-256-XTS key <<
25 * HKDF-SHA512 being much faster than HKDF-SHA <<
26 * SHA-512 causes HKDF-Expand to only need to <<
27 */ 22 */
28 #define HKDF_HMAC_ALG "hmac(sha512)" 23 #define HKDF_HMAC_ALG "hmac(sha512)"
29 #define HKDF_HASHLEN SHA512_DIGEST_ 24 #define HKDF_HASHLEN SHA512_DIGEST_SIZE
30 25
31 /* 26 /*
32 * HKDF consists of two steps: 27 * HKDF consists of two steps:
33 * 28 *
34 * 1. HKDF-Extract: extract a pseudorandom key 29 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
35 * the input keying material and optional s 30 * the input keying material and optional salt.
36 * 2. HKDF-Expand: expand the pseudorandom key 31 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
37 * any length, parameterized by an applicat 32 * any length, parameterized by an application-specific info string.
38 * 33 *
39 * HKDF-Extract can be skipped if the input is 34 * HKDF-Extract can be skipped if the input is already a pseudorandom key of
40 * length HKDF_HASHLEN bytes. However, cipher 35 * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take
41 * shorter keys, and we don't want to force us 36 * shorter keys, and we don't want to force users of those modes to provide
42 * unnecessarily long master keys. Thus fscry 37 * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
43 * salt is used, since fscrypt master keys sho 38 * salt is used, since fscrypt master keys should already be pseudorandom and
44 * there's no way to persist a random salt per 39 * there's no way to persist a random salt per master key from kernel mode.
45 */ 40 */
46 41
47 /* HKDF-Extract (RFC 5869 section 2.2), unsalt 42 /* HKDF-Extract (RFC 5869 section 2.2), unsalted */
48 static int hkdf_extract(struct crypto_shash *h 43 static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
49 unsigned int ikmlen, u 44 unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
50 { 45 {
51 static const u8 default_salt[HKDF_HASH 46 static const u8 default_salt[HKDF_HASHLEN];
52 int err; 47 int err;
53 48
54 err = crypto_shash_setkey(hmac_tfm, de 49 err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
55 if (err) 50 if (err)
56 return err; 51 return err;
57 52
58 return crypto_shash_tfm_digest(hmac_tf 53 return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
59 } 54 }
60 55
61 /* 56 /*
62 * Compute HKDF-Extract using the given master 57 * Compute HKDF-Extract using the given master key as the input keying material,
63 * and prepare an HMAC transform object keyed 58 * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
64 * 59 *
65 * Afterwards, the keyed HMAC transform object 60 * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
66 * times without having to recompute HKDF-Extr 61 * times without having to recompute HKDF-Extract each time.
67 */ 62 */
68 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkd 63 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
69 unsigned int master_key_ 64 unsigned int master_key_size)
70 { 65 {
71 struct crypto_shash *hmac_tfm; 66 struct crypto_shash *hmac_tfm;
72 u8 prk[HKDF_HASHLEN]; 67 u8 prk[HKDF_HASHLEN];
73 int err; 68 int err;
74 69
75 hmac_tfm = crypto_alloc_shash(HKDF_HMA 70 hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
76 if (IS_ERR(hmac_tfm)) { 71 if (IS_ERR(hmac_tfm)) {
77 fscrypt_err(NULL, "Error alloc 72 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
78 PTR_ERR(hmac_tfm)) 73 PTR_ERR(hmac_tfm));
79 return PTR_ERR(hmac_tfm); 74 return PTR_ERR(hmac_tfm);
80 } 75 }
81 76
82 if (WARN_ON_ONCE(crypto_shash_digestsi !! 77 if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
83 err = -EINVAL; 78 err = -EINVAL;
84 goto err_free_tfm; 79 goto err_free_tfm;
85 } 80 }
86 81
87 err = hkdf_extract(hmac_tfm, master_ke 82 err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
88 if (err) 83 if (err)
89 goto err_free_tfm; 84 goto err_free_tfm;
90 85
91 err = crypto_shash_setkey(hmac_tfm, pr 86 err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
92 if (err) 87 if (err)
93 goto err_free_tfm; 88 goto err_free_tfm;
94 89
95 hkdf->hmac_tfm = hmac_tfm; 90 hkdf->hmac_tfm = hmac_tfm;
96 goto out; 91 goto out;
97 92
98 err_free_tfm: 93 err_free_tfm:
99 crypto_free_shash(hmac_tfm); 94 crypto_free_shash(hmac_tfm);
100 out: 95 out:
101 memzero_explicit(prk, sizeof(prk)); 96 memzero_explicit(prk, sizeof(prk));
102 return err; 97 return err;
103 } 98 }
104 99
105 /* 100 /*
106 * HKDF-Expand (RFC 5869 section 2.3). This e 101 * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
107 * was already keyed into 'hkdf->hmac_tfm' by 102 * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
108 * bytes of output keying material parameteriz 103 * bytes of output keying material parameterized by the application-specific
109 * 'info' of length 'infolen' bytes, prefixed 104 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
110 * byte. This is thread-safe and may be calle 105 * byte. This is thread-safe and may be called by multiple threads in parallel.
111 * 106 *
112 * ('context' isn't part of the HKDF specifica 107 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
113 * adds to its application-specific info strin 108 * adds to its application-specific info strings to guarantee that it doesn't
114 * accidentally repeat an info string when usi 109 * accidentally repeat an info string when using HKDF for different purposes.)
115 */ 110 */
116 int fscrypt_hkdf_expand(const struct fscrypt_h 111 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
117 const u8 *info, unsign 112 const u8 *info, unsigned int infolen,
118 u8 *okm, unsigned int 113 u8 *okm, unsigned int okmlen)
119 { 114 {
120 SHASH_DESC_ON_STACK(desc, hkdf->hmac_t 115 SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
121 u8 prefix[9]; 116 u8 prefix[9];
122 unsigned int i; 117 unsigned int i;
123 int err; 118 int err;
124 const u8 *prev = NULL; 119 const u8 *prev = NULL;
125 u8 counter = 1; 120 u8 counter = 1;
126 u8 tmp[HKDF_HASHLEN]; 121 u8 tmp[HKDF_HASHLEN];
127 122
128 if (WARN_ON_ONCE(okmlen > 255 * HKDF_H !! 123 if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
129 return -EINVAL; 124 return -EINVAL;
130 125
131 desc->tfm = hkdf->hmac_tfm; 126 desc->tfm = hkdf->hmac_tfm;
132 127
133 memcpy(prefix, "fscrypt\0", 8); 128 memcpy(prefix, "fscrypt\0", 8);
134 prefix[8] = context; 129 prefix[8] = context;
135 130
136 for (i = 0; i < okmlen; i += HKDF_HASH 131 for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
137 132
138 err = crypto_shash_init(desc); 133 err = crypto_shash_init(desc);
139 if (err) 134 if (err)
140 goto out; 135 goto out;
141 136
142 if (prev) { 137 if (prev) {
143 err = crypto_shash_upd 138 err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
144 if (err) 139 if (err)
145 goto out; 140 goto out;
146 } 141 }
147 142
148 err = crypto_shash_update(desc 143 err = crypto_shash_update(desc, prefix, sizeof(prefix));
149 if (err) 144 if (err)
150 goto out; 145 goto out;
151 146
152 err = crypto_shash_update(desc 147 err = crypto_shash_update(desc, info, infolen);
153 if (err) 148 if (err)
154 goto out; 149 goto out;
155 150
156 BUILD_BUG_ON(sizeof(counter) ! 151 BUILD_BUG_ON(sizeof(counter) != 1);
157 if (okmlen - i < HKDF_HASHLEN) 152 if (okmlen - i < HKDF_HASHLEN) {
158 err = crypto_shash_fin 153 err = crypto_shash_finup(desc, &counter, 1, tmp);
159 if (err) 154 if (err)
160 goto out; 155 goto out;
161 memcpy(&okm[i], tmp, o 156 memcpy(&okm[i], tmp, okmlen - i);
162 memzero_explicit(tmp, 157 memzero_explicit(tmp, sizeof(tmp));
163 } else { 158 } else {
164 err = crypto_shash_fin 159 err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
165 if (err) 160 if (err)
166 goto out; 161 goto out;
167 } 162 }
168 counter++; 163 counter++;
169 prev = &okm[i]; 164 prev = &okm[i];
170 } 165 }
171 err = 0; 166 err = 0;
172 out: 167 out:
173 if (unlikely(err)) 168 if (unlikely(err))
174 memzero_explicit(okm, okmlen); 169 memzero_explicit(okm, okmlen); /* so caller doesn't need to */
175 shash_desc_zero(desc); 170 shash_desc_zero(desc);
176 return err; 171 return err;
177 } 172 }
178 173
179 void fscrypt_destroy_hkdf(struct fscrypt_hkdf 174 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
180 { 175 {
181 crypto_free_shash(hkdf->hmac_tfm); 176 crypto_free_shash(hkdf->hmac_tfm);
182 } 177 }
183 178
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