1 // SPDX-License-Identifier: MIT OR BSD-3-Clause
2 /*
3 * Copyright (C) 2016 The Android Open Source Project
4 */
5
6 /* Implementation of RSA signature verification which uses a pre-processed
7 * key for computation. The code extends libmincrypt RSA verification code to
8 * support multiple RSA key lengths and hash digest algorithms.
9 */
10
11 #include "avb_rsa.h"
12 #include "avb_sha.h"
13 #include "avb_util.h"
14 #include "avb_vbmeta_image.h"
15
16 typedef struct IAvbKey {
17 unsigned int len; /* Length of n[] in number of uint32_t */
18 uint32_t n0inv; /* -1 / n[0] mod 2^32 */
19 uint32_t* n; /* modulus as array (host-byte order) */
20 uint32_t* rr; /* R^2 as array (host-byte order) */
21 } IAvbKey;
22
iavb_parse_key_data(const uint8_t * data,size_t length)23 static IAvbKey* iavb_parse_key_data(const uint8_t* data, size_t length) {
24 AvbRSAPublicKeyHeader h;
25 IAvbKey* key = NULL;
26 size_t expected_length;
27 unsigned int i;
28 const uint8_t* n;
29 const uint8_t* rr;
30
31 if (!avb_rsa_public_key_header_validate_and_byteswap(
32 (const AvbRSAPublicKeyHeader*)data, &h)) {
33 avb_error("Invalid key.\n");
34 goto fail;
35 }
36
37 if (!(h.key_num_bits == 2048 || h.key_num_bits == 4096 ||
38 h.key_num_bits == 8192)) {
39 avb_error("Unexpected key length.\n");
40 goto fail;
41 }
42
43 expected_length = sizeof(AvbRSAPublicKeyHeader) + 2 * h.key_num_bits / 8;
44 if (length != expected_length) {
45 avb_error("Key does not match expected length.\n");
46 goto fail;
47 }
48
49 n = data + sizeof(AvbRSAPublicKeyHeader);
50 rr = data + sizeof(AvbRSAPublicKeyHeader) + h.key_num_bits / 8;
51
52 /* Store n and rr following the key header so we only have to do one
53 * allocation.
54 */
55 key = (IAvbKey*)(avb_malloc(sizeof(IAvbKey) + 2 * h.key_num_bits / 8));
56 if (key == NULL) {
57 goto fail;
58 }
59
60 key->len = h.key_num_bits / 32;
61 key->n0inv = h.n0inv;
62 key->n = (uint32_t*)(key + 1); /* Skip ahead sizeof(IAvbKey) bytes. */
63 key->rr = key->n + key->len;
64
65 /* Crypto-code below (modpowF4() and friends) expects the key in
66 * little-endian format (rather than the format we're storing the
67 * key in), so convert it.
68 */
69 for (i = 0; i < key->len; i++) {
70 key->n[i] = avb_be32toh(((uint32_t*)n)[key->len - i - 1]);
71 key->rr[i] = avb_be32toh(((uint32_t*)rr)[key->len - i - 1]);
72 }
73 return key;
74
75 fail:
76 if (key != NULL) {
77 avb_free(key);
78 }
79 return NULL;
80 }
81
iavb_free_parsed_key(IAvbKey * key)82 static void iavb_free_parsed_key(IAvbKey* key) {
83 avb_free(key);
84 }
85
86 /* a[] -= mod */
subM(const IAvbKey * key,uint32_t * a)87 static void subM(const IAvbKey* key, uint32_t* a) {
88 int64_t A = 0;
89 uint32_t i;
90 for (i = 0; i < key->len; ++i) {
91 A += (uint64_t)a[i] - key->n[i];
92 a[i] = (uint32_t)A;
93 A >>= 32;
94 }
95 }
96
97 /* return a[] >= mod */
geM(const IAvbKey * key,uint32_t * a)98 static int geM(const IAvbKey* key, uint32_t* a) {
99 uint32_t i;
100 for (i = key->len; i;) {
101 --i;
102 if (a[i] < key->n[i]) {
103 return 0;
104 }
105 if (a[i] > key->n[i]) {
106 return 1;
107 }
108 }
109 return 1; /* equal */
110 }
111
112 /* montgomery c[] += a * b[] / R % mod */
montMulAdd(const IAvbKey * key,uint32_t * c,const uint32_t a,const uint32_t * b)113 static void montMulAdd(const IAvbKey* key,
114 uint32_t* c,
115 const uint32_t a,
116 const uint32_t* b) {
117 uint64_t A = (uint64_t)a * b[0] + c[0];
118 uint32_t d0 = (uint32_t)A * key->n0inv;
119 uint64_t B = (uint64_t)d0 * key->n[0] + (uint32_t)A;
120 uint32_t i;
121
122 for (i = 1; i < key->len; ++i) {
123 A = (A >> 32) + (uint64_t)a * b[i] + c[i];
124 B = (B >> 32) + (uint64_t)d0 * key->n[i] + (uint32_t)A;
125 c[i - 1] = (uint32_t)B;
126 }
127
128 A = (A >> 32) + (B >> 32);
129
130 c[i - 1] = (uint32_t)A;
131
132 if (A >> 32) {
133 subM(key, c);
134 }
135 }
136
137 /* montgomery c[] = a[] * b[] / R % mod */
montMul(const IAvbKey * key,uint32_t * c,uint32_t * a,uint32_t * b)138 static void montMul(const IAvbKey* key, uint32_t* c, uint32_t* a, uint32_t* b) {
139 uint32_t i;
140 for (i = 0; i < key->len; ++i) {
141 c[i] = 0;
142 }
143 for (i = 0; i < key->len; ++i) {
144 montMulAdd(key, c, a[i], b);
145 }
146 }
147
148 /* In-place public exponentiation. (65537}
149 * Input and output big-endian byte array in inout.
150 */
modpowF4(const IAvbKey * key,uint8_t * inout)151 static void modpowF4(const IAvbKey* key, uint8_t* inout) {
152 uint32_t* a = (uint32_t*)avb_malloc(key->len * sizeof(uint32_t));
153 uint32_t* aR = (uint32_t*)avb_malloc(key->len * sizeof(uint32_t));
154 uint32_t* aaR = (uint32_t*)avb_malloc(key->len * sizeof(uint32_t));
155 if (a == NULL || aR == NULL || aaR == NULL) {
156 goto out;
157 }
158
159 uint32_t* aaa = aaR; /* Re-use location. */
160 int i;
161
162 /* Convert from big endian byte array to little endian word array. */
163 for (i = 0; i < (int)key->len; ++i) {
164 uint32_t tmp = (inout[((key->len - 1 - i) * 4) + 0] << 24) |
165 (inout[((key->len - 1 - i) * 4) + 1] << 16) |
166 (inout[((key->len - 1 - i) * 4) + 2] << 8) |
167 (inout[((key->len - 1 - i) * 4) + 3] << 0);
168 a[i] = tmp;
169 }
170
171 montMul(key, aR, a, key->rr); /* aR = a * RR / R mod M */
172 for (i = 0; i < 16; i += 2) {
173 montMul(key, aaR, aR, aR); /* aaR = aR * aR / R mod M */
174 montMul(key, aR, aaR, aaR); /* aR = aaR * aaR / R mod M */
175 }
176 montMul(key, aaa, aR, a); /* aaa = aR * a / R mod M */
177
178 /* Make sure aaa < mod; aaa is at most 1x mod too large. */
179 if (geM(key, aaa)) {
180 subM(key, aaa);
181 }
182
183 /* Convert to bigendian byte array */
184 for (i = (int)key->len - 1; i >= 0; --i) {
185 uint32_t tmp = aaa[i];
186 *inout++ = (uint8_t)(tmp >> 24);
187 *inout++ = (uint8_t)(tmp >> 16);
188 *inout++ = (uint8_t)(tmp >> 8);
189 *inout++ = (uint8_t)(tmp >> 0);
190 }
191
192 out:
193 if (a != NULL) {
194 avb_free(a);
195 }
196 if (aR != NULL) {
197 avb_free(aR);
198 }
199 if (aaR != NULL) {
200 avb_free(aaR);
201 }
202 }
203
204 /* Verify a RSA PKCS1.5 signature against an expected hash.
205 * Returns false on failure, true on success.
206 */
avb_rsa_verify(const uint8_t * key,size_t key_num_bytes,const uint8_t * sig,size_t sig_num_bytes,const uint8_t * hash,size_t hash_num_bytes,const uint8_t * padding,size_t padding_num_bytes)207 bool avb_rsa_verify(const uint8_t* key,
208 size_t key_num_bytes,
209 const uint8_t* sig,
210 size_t sig_num_bytes,
211 const uint8_t* hash,
212 size_t hash_num_bytes,
213 const uint8_t* padding,
214 size_t padding_num_bytes) {
215 uint8_t* buf = NULL;
216 IAvbKey* parsed_key = NULL;
217 bool success = false;
218
219 if (key == NULL || sig == NULL || hash == NULL || padding == NULL) {
220 avb_error("Invalid input.\n");
221 goto out;
222 }
223
224 parsed_key = iavb_parse_key_data(key, key_num_bytes);
225 if (parsed_key == NULL) {
226 avb_error("Error parsing key.\n");
227 goto out;
228 }
229
230 if (sig_num_bytes != (parsed_key->len * sizeof(uint32_t))) {
231 avb_error("Signature length does not match key length.\n");
232 goto out;
233 }
234
235 if (padding_num_bytes != sig_num_bytes - hash_num_bytes) {
236 avb_error("Padding length does not match hash and signature lengths.\n");
237 goto out;
238 }
239
240 buf = (uint8_t*)avb_malloc(sig_num_bytes);
241 if (buf == NULL) {
242 avb_error("Error allocating memory.\n");
243 goto out;
244 }
245 avb_memcpy(buf, sig, sig_num_bytes);
246
247 modpowF4(parsed_key, buf);
248
249 /* Check padding bytes.
250 *
251 * Even though there are probably no timing issues here, we use
252 * avb_safe_memcmp() just to be on the safe side.
253 */
254 if (avb_safe_memcmp(buf, padding, padding_num_bytes)) {
255 avb_error("Padding check failed.\n");
256 goto out;
257 }
258
259 /* Check hash. */
260 if (avb_safe_memcmp(buf + padding_num_bytes, hash, hash_num_bytes)) {
261 avb_error("Hash check failed.\n");
262 goto out;
263 }
264
265 success = true;
266
267 out:
268 if (parsed_key != NULL) {
269 iavb_free_parsed_key(parsed_key);
270 }
271 if (buf != NULL) {
272 avb_free(buf);
273 }
274 return success;
275 }
276