1*0957b409SSimon J. Gerraty /*
2*0957b409SSimon J. Gerraty * Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
3*0957b409SSimon J. Gerraty *
4*0957b409SSimon J. Gerraty * Permission is hereby granted, free of charge, to any person obtaining
5*0957b409SSimon J. Gerraty * a copy of this software and associated documentation files (the
6*0957b409SSimon J. Gerraty * "Software"), to deal in the Software without restriction, including
7*0957b409SSimon J. Gerraty * without limitation the rights to use, copy, modify, merge, publish,
8*0957b409SSimon J. Gerraty * distribute, sublicense, and/or sell copies of the Software, and to
9*0957b409SSimon J. Gerraty * permit persons to whom the Software is furnished to do so, subject to
10*0957b409SSimon J. Gerraty * the following conditions:
11*0957b409SSimon J. Gerraty *
12*0957b409SSimon J. Gerraty * The above copyright notice and this permission notice shall be
13*0957b409SSimon J. Gerraty * included in all copies or substantial portions of the Software.
14*0957b409SSimon J. Gerraty *
15*0957b409SSimon J. Gerraty * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16*0957b409SSimon J. Gerraty * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17*0957b409SSimon J. Gerraty * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18*0957b409SSimon J. Gerraty * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19*0957b409SSimon J. Gerraty * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20*0957b409SSimon J. Gerraty * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21*0957b409SSimon J. Gerraty * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22*0957b409SSimon J. Gerraty * SOFTWARE.
23*0957b409SSimon J. Gerraty */
24*0957b409SSimon J. Gerraty
25*0957b409SSimon J. Gerraty #include "inner.h"
26*0957b409SSimon J. Gerraty
27*0957b409SSimon J. Gerraty /*
28*0957b409SSimon J. Gerraty * Recompute public exponent, based on factor p and reduced private
29*0957b409SSimon J. Gerraty * exponent dp.
30*0957b409SSimon J. Gerraty */
31*0957b409SSimon J. Gerraty static uint32_t
get_pubexp(const unsigned char * pbuf,size_t plen,const unsigned char * dpbuf,size_t dplen)32*0957b409SSimon J. Gerraty get_pubexp(const unsigned char *pbuf, size_t plen,
33*0957b409SSimon J. Gerraty const unsigned char *dpbuf, size_t dplen)
34*0957b409SSimon J. Gerraty {
35*0957b409SSimon J. Gerraty /*
36*0957b409SSimon J. Gerraty * dp is the inverse of e modulo p-1. If p = 3 mod 4, then
37*0957b409SSimon J. Gerraty * p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
38*0957b409SSimon J. Gerraty * thus, dp must be odd.
39*0957b409SSimon J. Gerraty *
40*0957b409SSimon J. Gerraty * We compute the inverse of dp modulo (p-1)/2. This requires
41*0957b409SSimon J. Gerraty * first reducing dp modulo (p-1)/2 (this can be done with a
42*0957b409SSimon J. Gerraty * conditional subtract, no need to use the generic modular
43*0957b409SSimon J. Gerraty * reduction function); then, we use moddiv.
44*0957b409SSimon J. Gerraty */
45*0957b409SSimon J. Gerraty
46*0957b409SSimon J. Gerraty uint32_t tmp[6 * ((BR_MAX_RSA_FACTOR + 61) / 31)];
47*0957b409SSimon J. Gerraty uint32_t *p, *dp, *x;
48*0957b409SSimon J. Gerraty size_t len;
49*0957b409SSimon J. Gerraty uint32_t e;
50*0957b409SSimon J. Gerraty
51*0957b409SSimon J. Gerraty /*
52*0957b409SSimon J. Gerraty * Compute actual factor length (in bytes) and check that it fits
53*0957b409SSimon J. Gerraty * under our size constraints.
54*0957b409SSimon J. Gerraty */
55*0957b409SSimon J. Gerraty while (plen > 0 && *pbuf == 0) {
56*0957b409SSimon J. Gerraty pbuf ++;
57*0957b409SSimon J. Gerraty plen --;
58*0957b409SSimon J. Gerraty }
59*0957b409SSimon J. Gerraty if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
60*0957b409SSimon J. Gerraty return 0;
61*0957b409SSimon J. Gerraty }
62*0957b409SSimon J. Gerraty
63*0957b409SSimon J. Gerraty /*
64*0957b409SSimon J. Gerraty * Compute actual reduced exponent length (in bytes) and check that
65*0957b409SSimon J. Gerraty * it is not longer than p.
66*0957b409SSimon J. Gerraty */
67*0957b409SSimon J. Gerraty while (dplen > 0 && *dpbuf == 0) {
68*0957b409SSimon J. Gerraty dpbuf ++;
69*0957b409SSimon J. Gerraty dplen --;
70*0957b409SSimon J. Gerraty }
71*0957b409SSimon J. Gerraty if (dplen > plen || dplen == 0
72*0957b409SSimon J. Gerraty || (dplen == plen && dpbuf[0] > pbuf[0]))
73*0957b409SSimon J. Gerraty {
74*0957b409SSimon J. Gerraty return 0;
75*0957b409SSimon J. Gerraty }
76*0957b409SSimon J. Gerraty
77*0957b409SSimon J. Gerraty /*
78*0957b409SSimon J. Gerraty * Verify that p = 3 mod 4 and that dp is odd.
79*0957b409SSimon J. Gerraty */
80*0957b409SSimon J. Gerraty if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
81*0957b409SSimon J. Gerraty return 0;
82*0957b409SSimon J. Gerraty }
83*0957b409SSimon J. Gerraty
84*0957b409SSimon J. Gerraty /*
85*0957b409SSimon J. Gerraty * Decode p and compute (p-1)/2.
86*0957b409SSimon J. Gerraty */
87*0957b409SSimon J. Gerraty p = tmp;
88*0957b409SSimon J. Gerraty br_i31_decode(p, pbuf, plen);
89*0957b409SSimon J. Gerraty len = (p[0] + 63) >> 5;
90*0957b409SSimon J. Gerraty br_i31_rshift(p, 1);
91*0957b409SSimon J. Gerraty
92*0957b409SSimon J. Gerraty /*
93*0957b409SSimon J. Gerraty * Decode dp and make sure its announced bit length matches that of
94*0957b409SSimon J. Gerraty * p (we already know that the size of dp, in bits, does not exceed
95*0957b409SSimon J. Gerraty * the size of p, so we just have to copy the header word).
96*0957b409SSimon J. Gerraty */
97*0957b409SSimon J. Gerraty dp = p + len;
98*0957b409SSimon J. Gerraty memset(dp, 0, len * sizeof *dp);
99*0957b409SSimon J. Gerraty br_i31_decode(dp, dpbuf, dplen);
100*0957b409SSimon J. Gerraty dp[0] = p[0];
101*0957b409SSimon J. Gerraty
102*0957b409SSimon J. Gerraty /*
103*0957b409SSimon J. Gerraty * Subtract (p-1)/2 from dp if necessary.
104*0957b409SSimon J. Gerraty */
105*0957b409SSimon J. Gerraty br_i31_sub(dp, p, NOT(br_i31_sub(dp, p, 0)));
106*0957b409SSimon J. Gerraty
107*0957b409SSimon J. Gerraty /*
108*0957b409SSimon J. Gerraty * If another subtraction is needed, then this means that the
109*0957b409SSimon J. Gerraty * value was invalid. We don't care to leak information about
110*0957b409SSimon J. Gerraty * invalid keys.
111*0957b409SSimon J. Gerraty */
112*0957b409SSimon J. Gerraty if (br_i31_sub(dp, p, 0) == 0) {
113*0957b409SSimon J. Gerraty return 0;
114*0957b409SSimon J. Gerraty }
115*0957b409SSimon J. Gerraty
116*0957b409SSimon J. Gerraty /*
117*0957b409SSimon J. Gerraty * Invert dp modulo (p-1)/2. If the inversion fails, then the
118*0957b409SSimon J. Gerraty * key value was invalid.
119*0957b409SSimon J. Gerraty */
120*0957b409SSimon J. Gerraty x = dp + len;
121*0957b409SSimon J. Gerraty br_i31_zero(x, p[0]);
122*0957b409SSimon J. Gerraty x[1] = 1;
123*0957b409SSimon J. Gerraty if (br_i31_moddiv(x, dp, p, br_i31_ninv31(p[1]), x + len) == 0) {
124*0957b409SSimon J. Gerraty return 0;
125*0957b409SSimon J. Gerraty }
126*0957b409SSimon J. Gerraty
127*0957b409SSimon J. Gerraty /*
128*0957b409SSimon J. Gerraty * We now have an inverse. We must set it to zero (error) if its
129*0957b409SSimon J. Gerraty * length is greater than 32 bits and/or if it is an even integer.
130*0957b409SSimon J. Gerraty * Take care that the bit_length function returns an encoded
131*0957b409SSimon J. Gerraty * bit length.
132*0957b409SSimon J. Gerraty */
133*0957b409SSimon J. Gerraty e = (uint32_t)x[1] | ((uint32_t)x[2] << 31);
134*0957b409SSimon J. Gerraty e &= -LT(br_i31_bit_length(x + 1, len - 1), 34);
135*0957b409SSimon J. Gerraty e &= -(e & 1);
136*0957b409SSimon J. Gerraty return e;
137*0957b409SSimon J. Gerraty }
138*0957b409SSimon J. Gerraty
139*0957b409SSimon J. Gerraty /* see bearssl_rsa.h */
140*0957b409SSimon J. Gerraty uint32_t
br_rsa_i31_compute_pubexp(const br_rsa_private_key * sk)141*0957b409SSimon J. Gerraty br_rsa_i31_compute_pubexp(const br_rsa_private_key *sk)
142*0957b409SSimon J. Gerraty {
143*0957b409SSimon J. Gerraty /*
144*0957b409SSimon J. Gerraty * Get the public exponent from both p and q. This is the right
145*0957b409SSimon J. Gerraty * exponent if we get twice the same value.
146*0957b409SSimon J. Gerraty */
147*0957b409SSimon J. Gerraty uint32_t ep, eq;
148*0957b409SSimon J. Gerraty
149*0957b409SSimon J. Gerraty ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
150*0957b409SSimon J. Gerraty eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
151*0957b409SSimon J. Gerraty return ep & -EQ(ep, eq);
152*0957b409SSimon J. Gerraty }
153