1 /* Copyright (c) 2014 The Chromium OS Authors. All rights reserved.
2  * Use of this source code is governed by a BSD-style license that can be
3  * found in the LICENSE file.
4  */
5 
6 /*
7  * Implementation of RSA signature verification which uses a pre-processed key
8  * for computation. The code extends Android's RSA verification code to support
9  * multiple RSA key lengths and hash digest algorithms.
10  */
11 
12 #include "2sysincludes.h"
13 #include "2common.h"
14 #include "2rsa.h"
15 #include "2sha.h"
16 
17 /**
18  * a[] -= mod
19  */
subM(const struct vb2_public_key * key,uint32_t * a)20 static void subM(const struct vb2_public_key *key, uint32_t *a)
21 {
22 	int64_t A = 0;
23 	uint32_t i;
24 	for (i = 0; i < key->arrsize; ++i) {
25 		A += (uint64_t)a[i] - key->n[i];
26 		a[i] = (uint32_t)A;
27 		A >>= 32;
28 	}
29 }
30 
31 /**
32  * Return a[] >= mod
33  */
vb2_mont_ge(const struct vb2_public_key * key,uint32_t * a)34 int vb2_mont_ge(const struct vb2_public_key *key, uint32_t *a)
35 {
36 	uint32_t i;
37 	for (i = key->arrsize; i;) {
38 		--i;
39 		if (a[i] < key->n[i])
40 			return 0;
41 		if (a[i] > key->n[i])
42 			return 1;
43 	}
44 	return 1;  /* equal */
45 }
46 
47 /**
48  * Montgomery c[] += a * b[] / R % mod
49  */
montMulAdd(const struct vb2_public_key * key,uint32_t * c,const uint32_t a,const uint32_t * b)50 static void montMulAdd(const struct vb2_public_key *key,
51                        uint32_t *c,
52                        const uint32_t a,
53                        const uint32_t *b)
54 {
55 	uint64_t A = (uint64_t)a * b[0] + c[0];
56 	uint32_t d0 = (uint32_t)A * key->n0inv;
57 	uint64_t B = (uint64_t)d0 * key->n[0] + (uint32_t)A;
58 	uint32_t i;
59 
60 	for (i = 1; i < key->arrsize; ++i) {
61 		A = (A >> 32) + (uint64_t)a * b[i] + c[i];
62 		B = (B >> 32) + (uint64_t)d0 * key->n[i] + (uint32_t)A;
63 		c[i - 1] = (uint32_t)B;
64 	}
65 
66 	A = (A >> 32) + (B >> 32);
67 
68 	c[i - 1] = (uint32_t)A;
69 
70 	if (A >> 32) {
71 		subM(key, c);
72 	}
73 }
74 
75 /**
76  * Montgomery c[] = a[] * b[] / R % mod
77  */
montMul(const struct vb2_public_key * key,uint32_t * c,const uint32_t * a,const uint32_t * b)78 static void montMul(const struct vb2_public_key *key,
79                     uint32_t *c,
80                     const uint32_t *a,
81                     const uint32_t *b)
82 {
83 	uint32_t i;
84 	for (i = 0; i < key->arrsize; ++i) {
85 		c[i] = 0;
86 	}
87 	for (i = 0; i < key->arrsize; ++i) {
88 		montMulAdd(key, c, a[i], b);
89 	}
90 }
91 
92 /**
93  * In-place public exponentiation. (65537}
94  *
95  * @param key		Key to use in signing
96  * @param inout		Input and output big-endian byte array
97  * @param workbuf32	Work buffer; caller must verify this is
98  *			(3 * key->arrsize) elements long.
99  */
modpowF4(const struct vb2_public_key * key,uint8_t * inout,uint32_t * workbuf32)100 static void modpowF4(const struct vb2_public_key *key, uint8_t *inout,
101 		    uint32_t *workbuf32)
102 {
103 	uint32_t *a = workbuf32;
104 	uint32_t *aR = a + key->arrsize;
105 	uint32_t *aaR = aR + key->arrsize;
106 	uint32_t *aaa = aaR;  /* Re-use location. */
107 	int i;
108 
109 	/* Convert from big endian byte array to little endian word array. */
110 	for (i = 0; i < (int)key->arrsize; ++i) {
111 		uint32_t tmp =
112 			(inout[((key->arrsize - 1 - i) * 4) + 0] << 24) |
113 			(inout[((key->arrsize - 1 - i) * 4) + 1] << 16) |
114 			(inout[((key->arrsize - 1 - i) * 4) + 2] << 8) |
115 			(inout[((key->arrsize - 1 - i) * 4) + 3] << 0);
116 		a[i] = tmp;
117 	}
118 
119 	montMul(key, aR, a, key->rr);  /* aR = a * RR / R mod M   */
120 	for (i = 0; i < 16; i+=2) {
121 		montMul(key, aaR, aR, aR);  /* aaR = aR * aR / R mod M */
122 		montMul(key, aR, aaR, aaR);  /* aR = aaR * aaR / R mod M */
123 	}
124 	montMul(key, aaa, aR, a);  /* aaa = aR * a / R mod M */
125 
126 
127 	/* Make sure aaa < mod; aaa is at most 1x mod too large. */
128 	if (vb2_mont_ge(key, aaa)) {
129 		subM(key, aaa);
130 	}
131 
132 	/* Convert to bigendian byte array */
133 	for (i = (int)key->arrsize - 1; i >= 0; --i) {
134 		uint32_t tmp = aaa[i];
135 		*inout++ = (uint8_t)(tmp >> 24);
136 		*inout++ = (uint8_t)(tmp >> 16);
137 		*inout++ = (uint8_t)(tmp >>  8);
138 		*inout++ = (uint8_t)(tmp >>  0);
139 	}
140 }
141 
142 
143 static const uint8_t crypto_to_sig[] = {
144 	VB2_SIG_RSA1024,
145 	VB2_SIG_RSA1024,
146 	VB2_SIG_RSA1024,
147 	VB2_SIG_RSA2048,
148 	VB2_SIG_RSA2048,
149 	VB2_SIG_RSA2048,
150 	VB2_SIG_RSA4096,
151 	VB2_SIG_RSA4096,
152 	VB2_SIG_RSA4096,
153 	VB2_SIG_RSA8192,
154 	VB2_SIG_RSA8192,
155 	VB2_SIG_RSA8192,
156 };
157 
158 /**
159  * Convert vb2_crypto_algorithm to vb2_signature_algorithm.
160  *
161  * @param algorithm	Crypto algorithm (vb2_crypto_algorithm)
162  *
163  * @return The signature algorithm for that crypto algorithm, or
164  * VB2_SIG_INVALID if the crypto algorithm or its corresponding signature
165  * algorithm is invalid or not supported.
166  */
vb2_crypto_to_signature(uint32_t algorithm)167 enum vb2_signature_algorithm vb2_crypto_to_signature(uint32_t algorithm)
168 {
169 	if (algorithm < ARRAY_SIZE(crypto_to_sig))
170 		return crypto_to_sig[algorithm];
171 	else
172 		return VB2_SIG_INVALID;
173 }
174 
vb2_rsa_sig_size(enum vb2_signature_algorithm sig_alg)175 uint32_t vb2_rsa_sig_size(enum vb2_signature_algorithm sig_alg)
176 {
177 	switch (sig_alg) {
178 	case VB2_SIG_RSA1024:
179 		return 1024 / 8;
180 	case VB2_SIG_RSA2048:
181 		return 2048 / 8;
182 	case VB2_SIG_RSA4096:
183 		return 4096 / 8;
184 	case VB2_SIG_RSA8192:
185 		return 8192 / 8;
186 	default:
187 		return 0;
188 	}
189 }
190 
vb2_packed_key_size(enum vb2_signature_algorithm sig_alg)191 uint32_t vb2_packed_key_size(enum vb2_signature_algorithm sig_alg)
192 {
193 	uint32_t sig_size = vb2_rsa_sig_size(sig_alg);
194 
195 	if (!sig_size)
196 		return 0;
197 
198 	/*
199 	 * Total size needed by a RSAPublicKey buffer is =
200 	 *  2 * key_len bytes for the n and rr arrays
201 	 *  + sizeof len + sizeof n0inv.
202 	 */
203 	return 2 * sig_size + 2 * sizeof(uint32_t);
204 }
205 
206 /*
207  * PKCS 1.5 padding (from the RSA PKCS#1 v2.1 standard)
208  *
209  * Depending on the RSA key size and hash function, the padding is calculated
210  * as follows:
211  *
212  * 0x00 || 0x01 || PS || 0x00 || T
213  *
214  * T: DER Encoded DigestInfo value which depends on the hash function used.
215  *
216  * SHA-1:   (0x)30 21 30 09 06 05 2b 0e 03 02 1a 05 00 04 14 || H.
217  * SHA-256: (0x)30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20 || H.
218  * SHA-512: (0x)30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40 || H.
219  *
220  * Length(T) = 35 octets for SHA-1
221  * Length(T) = 51 octets for SHA-256
222  * Length(T) = 83 octets for SHA-512
223  *
224  * PS: octet string consisting of {Length(RSA Key) - Length(T) - 3} 0xFF
225  */
226 static const uint8_t sha1_tail[] = {
227 	0x00,0x30,0x21,0x30,0x09,0x06,0x05,0x2b,
228 	0x0e,0x03,0x02,0x1a,0x05,0x00,0x04,0x14
229 };
230 
231 static const uint8_t sha256_tail[] = {
232 	0x00,0x30,0x31,0x30,0x0d,0x06,0x09,0x60,
233 	0x86,0x48,0x01,0x65,0x03,0x04,0x02,0x01,
234 	0x05,0x00,0x04,0x20
235 };
236 
237 static const uint8_t sha512_tail[] = {
238 	0x00,0x30,0x51,0x30,0x0d,0x06,0x09,0x60,
239 	0x86,0x48,0x01,0x65,0x03,0x04,0x02,0x03,
240 	0x05,0x00,0x04,0x40
241 };
242 
vb2_check_padding(const uint8_t * sig,const struct vb2_public_key * key)243 int vb2_check_padding(const uint8_t *sig, const struct vb2_public_key *key)
244 {
245 	/* Determine padding to use depending on the signature type */
246 	uint32_t sig_size = vb2_rsa_sig_size(key->sig_alg);
247 	uint32_t hash_size = vb2_digest_size(key->hash_alg);
248 	uint32_t pad_size = sig_size - hash_size;
249 	const uint8_t *tail;
250 	uint32_t tail_size;
251 	int result = 0;
252 	int i;
253 
254 	if (!sig_size || !hash_size || hash_size > sig_size)
255 		return VB2_ERROR_RSA_PADDING_SIZE;
256 
257 	switch (key->hash_alg) {
258 	case VB2_HASH_SHA1:
259 		tail = sha1_tail;
260 		tail_size = sizeof(sha1_tail);
261 		break;
262 	case VB2_HASH_SHA256:
263 		tail = sha256_tail;
264 		tail_size = sizeof(sha256_tail);
265 		break;
266 	case VB2_HASH_SHA512:
267 		tail = sha512_tail;
268 		tail_size = sizeof(sha512_tail);
269 		break;
270 	default:
271 		return VB2_ERROR_RSA_PADDING_ALGORITHM;
272 	}
273 
274 	/* First 2 bytes are always 0x00 0x01 */
275 	result |= *sig++ ^ 0x00;
276 	result |= *sig++ ^ 0x01;
277 
278 	/* Then 0xff bytes until the tail */
279 	for (i = 0; i < pad_size - tail_size - 2; i++)
280 		result |= *sig++ ^ 0xff;
281 
282 	/*
283 	 * Then the tail.  Even though there are probably no timing issues
284 	 * here, we use vb2_safe_memcmp() just to be on the safe side.
285 	 */
286 	result |= vb2_safe_memcmp(sig, tail, tail_size);
287 
288 	return result ? VB2_ERROR_RSA_PADDING : VB2_SUCCESS;
289 }
290 
vb2_rsa_verify_digest(const struct vb2_public_key * key,uint8_t * sig,const uint8_t * digest,const struct vb2_workbuf * wb)291 int vb2_rsa_verify_digest(const struct vb2_public_key *key,
292 			  uint8_t *sig,
293 			  const uint8_t *digest,
294 			  const struct vb2_workbuf *wb)
295 {
296 	struct vb2_workbuf wblocal = *wb;
297 	uint32_t *workbuf32;
298 	uint32_t key_bytes;
299 	int sig_size;
300 	int pad_size;
301 	int rv;
302 
303 	if (!key || !sig || !digest)
304 		return VB2_ERROR_RSA_VERIFY_PARAM;
305 
306 	sig_size = vb2_rsa_sig_size(key->sig_alg);
307 	if (!sig_size) {
308 		VB2_DEBUG("Invalid signature type!\n");
309 		return VB2_ERROR_RSA_VERIFY_ALGORITHM;
310 	}
311 
312 	/* Signature length should be same as key length */
313 	key_bytes = key->arrsize * sizeof(uint32_t);
314 	if (key_bytes != sig_size) {
315 		VB2_DEBUG("Signature is of incorrect length!\n");
316 		return VB2_ERROR_RSA_VERIFY_SIG_LEN;
317 	}
318 
319 	workbuf32 = vb2_workbuf_alloc(&wblocal, 3 * key_bytes);
320 	if (!workbuf32)
321 		return VB2_ERROR_RSA_VERIFY_WORKBUF;
322 
323 	modpowF4(key, sig, workbuf32);
324 
325 	vb2_workbuf_free(&wblocal, 3 * key_bytes);
326 
327 	/*
328 	 * Check padding.  Only fail immediately if the padding size is bad.
329 	 * Otherwise, continue on to check the digest to reduce the risk of
330 	 * timing based attacks.
331 	 */
332 	rv = vb2_check_padding(sig, key);
333 	if (rv == VB2_ERROR_RSA_PADDING_SIZE)
334 		return rv;
335 
336 	/*
337 	 * Check digest.  Even though there are probably no timing issues here,
338 	 * use vb2_safe_memcmp() just to be on the safe side.  (That's also why
339 	 * we don't return before this check if the padding check failed.)
340 	 */
341 	pad_size = sig_size - vb2_digest_size(key->hash_alg);
342 	if (vb2_safe_memcmp(sig + pad_size, digest, key_bytes - pad_size)) {
343 		VB2_DEBUG("Digest check failed!\n");
344 		if (!rv)
345 			rv = VB2_ERROR_RSA_VERIFY_DIGEST;
346 	}
347 
348 	return rv;
349 }
350