1 /*
2 * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22 * SOFTWARE.
23 */
24
25 #include "inner.h"
26
27 #if BR_INT128 || BR_UMUL128
28
29 #define U (2 + ((BR_MAX_RSA_FACTOR + 30) / 31))
30 #define TLEN (4 * U) /* TLEN is counted in 64-bit words */
31
32 /* see bearssl_rsa.h */
33 uint32_t
br_rsa_i62_private(unsigned char * x,const br_rsa_private_key * sk)34 br_rsa_i62_private(unsigned char *x, const br_rsa_private_key *sk)
35 {
36 const unsigned char *p, *q;
37 size_t plen, qlen;
38 size_t fwlen;
39 uint32_t p0i, q0i;
40 size_t xlen, u;
41 uint64_t tmp[TLEN];
42 long z;
43 uint32_t *mp, *mq, *s1, *s2, *t1, *t2, *t3;
44 uint32_t r;
45
46 /*
47 * Compute the actual lengths of p and q, in bytes.
48 * These lengths are not considered secret (we cannot really hide
49 * them anyway in constant-time code).
50 */
51 p = sk->p;
52 plen = sk->plen;
53 while (plen > 0 && *p == 0) {
54 p ++;
55 plen --;
56 }
57 q = sk->q;
58 qlen = sk->qlen;
59 while (qlen > 0 && *q == 0) {
60 q ++;
61 qlen --;
62 }
63
64 /*
65 * Compute the maximum factor length, in words.
66 */
67 z = (long)(plen > qlen ? plen : qlen) << 3;
68 fwlen = 1;
69 while (z > 0) {
70 z -= 31;
71 fwlen ++;
72 }
73
74 /*
75 * Convert size to 62-bit words.
76 */
77 fwlen = (fwlen + 1) >> 1;
78
79 /*
80 * We need to fit at least 6 values in the stack buffer.
81 */
82 if (6 * fwlen > TLEN) {
83 return 0;
84 }
85
86 /*
87 * Compute signature length (in bytes).
88 */
89 xlen = (sk->n_bitlen + 7) >> 3;
90
91 /*
92 * Decode q.
93 */
94 mq = (uint32_t *)tmp;
95 br_i31_decode(mq, q, qlen);
96
97 /*
98 * Decode p.
99 */
100 t1 = (uint32_t *)(tmp + fwlen);
101 br_i31_decode(t1, p, plen);
102
103 /*
104 * Compute the modulus (product of the two factors), to compare
105 * it with the source value. We use br_i31_mulacc(), since it's
106 * already used later on.
107 */
108 t2 = (uint32_t *)(tmp + 2 * fwlen);
109 br_i31_zero(t2, mq[0]);
110 br_i31_mulacc(t2, mq, t1);
111
112 /*
113 * We encode the modulus into bytes, to perform the comparison
114 * with bytes. We know that the product length, in bytes, is
115 * exactly xlen.
116 * The comparison actually computes the carry when subtracting
117 * the modulus from the source value; that carry must be 1 for
118 * a value in the correct range. We keep it in r, which is our
119 * accumulator for the error code.
120 */
121 t3 = (uint32_t *)(tmp + 4 * fwlen);
122 br_i31_encode(t3, xlen, t2);
123 u = xlen;
124 r = 0;
125 while (u > 0) {
126 uint32_t wn, wx;
127
128 u --;
129 wn = ((unsigned char *)t3)[u];
130 wx = x[u];
131 r = ((wx - (wn + r)) >> 8) & 1;
132 }
133
134 /*
135 * Move the decoded p to another temporary buffer.
136 */
137 mp = (uint32_t *)(tmp + 2 * fwlen);
138 memmove(mp, t1, 2 * fwlen * sizeof *t1);
139
140 /*
141 * Compute s2 = x^dq mod q.
142 */
143 q0i = br_i31_ninv31(mq[1]);
144 s2 = (uint32_t *)(tmp + fwlen);
145 br_i31_decode_reduce(s2, x, xlen, mq);
146 r &= br_i62_modpow_opt(s2, sk->dq, sk->dqlen, mq, q0i,
147 tmp + 3 * fwlen, TLEN - 3 * fwlen);
148
149 /*
150 * Compute s1 = x^dp mod p.
151 */
152 p0i = br_i31_ninv31(mp[1]);
153 s1 = (uint32_t *)(tmp + 3 * fwlen);
154 br_i31_decode_reduce(s1, x, xlen, mp);
155 r &= br_i62_modpow_opt(s1, sk->dp, sk->dplen, mp, p0i,
156 tmp + 4 * fwlen, TLEN - 4 * fwlen);
157
158 /*
159 * Compute:
160 * h = (s1 - s2)*(1/q) mod p
161 * s1 is an integer modulo p, but s2 is modulo q. PKCS#1 is
162 * unclear about whether p may be lower than q (some existing,
163 * widely deployed implementations of RSA don't tolerate p < q),
164 * but we want to support that occurrence, so we need to use the
165 * reduction function.
166 *
167 * Since we use br_i31_decode_reduce() for iq (purportedly, the
168 * inverse of q modulo p), we also tolerate improperly large
169 * values for this parameter.
170 */
171 t1 = (uint32_t *)(tmp + 4 * fwlen);
172 t2 = (uint32_t *)(tmp + 5 * fwlen);
173 br_i31_reduce(t2, s2, mp);
174 br_i31_add(s1, mp, br_i31_sub(s1, t2, 1));
175 br_i31_to_monty(s1, mp);
176 br_i31_decode_reduce(t1, sk->iq, sk->iqlen, mp);
177 br_i31_montymul(t2, s1, t1, mp, p0i);
178
179 /*
180 * h is now in t2. We compute the final result:
181 * s = s2 + q*h
182 * All these operations are non-modular.
183 *
184 * We need mq, s2 and t2. We use the t3 buffer as destination.
185 * The buffers mp, s1 and t1 are no longer needed, so we can
186 * reuse them for t3. Moreover, the first step of the computation
187 * is to copy s2 into t3, after which s2 is not needed. Right
188 * now, mq is in slot 0, s2 is in slot 1, and t2 is in slot 5.
189 * Therefore, we have ample room for t3 by simply using s2.
190 */
191 t3 = s2;
192 br_i31_mulacc(t3, mq, t2);
193
194 /*
195 * Encode the result. Since we already checked the value of xlen,
196 * we can just use it right away.
197 */
198 br_i31_encode(x, xlen, t3);
199
200 /*
201 * The only error conditions remaining at that point are invalid
202 * values for p and q (even integers).
203 */
204 return p0i & q0i & r;
205 }
206
207 /* see bearssl_rsa.h */
208 br_rsa_private
br_rsa_i62_private_get(void)209 br_rsa_i62_private_get(void)
210 {
211 return &br_rsa_i62_private;
212 }
213
214 #else
215
216 /* see bearssl_rsa.h */
217 br_rsa_private
br_rsa_i62_private_get(void)218 br_rsa_i62_private_get(void)
219 {
220 return 0;
221 }
222
223 #endif
224