1/*
2 * Copyright 1995-2022 The OpenSSL Project Authors. All Rights Reserved.
3 *
4 * Licensed under the Apache License 2.0 (the "License").  You may not use
5 * this file except in compliance with the License.  You can obtain a copy
6 * in the file LICENSE in the source distribution or at
7 * https://www.openssl.org/source/license.html
8 */
9
10#include <stdio.h>
11#include "internal/cryptlib.h"
12#include "bn_local.h"
13
14#define TABLE_SIZE      32
15
16int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
17                     const BIGNUM *a2, const BIGNUM *p2, const BIGNUM *m,
18                     BN_CTX *ctx, BN_MONT_CTX *in_mont)
19{
20    int i, j, bits, b, bits1, bits2, ret =
21        0, wpos1, wpos2, window1, window2, wvalue1, wvalue2;
22    int r_is_one = 1;
23    BIGNUM *d, *r;
24    const BIGNUM *a_mod_m;
25    /* Tables of variables obtained from 'ctx' */
26    BIGNUM *val1[TABLE_SIZE], *val2[TABLE_SIZE];
27    BN_MONT_CTX *mont = NULL;
28
29    bn_check_top(a1);
30    bn_check_top(p1);
31    bn_check_top(a2);
32    bn_check_top(p2);
33    bn_check_top(m);
34
35    if (!BN_is_odd(m)) {
36        ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
37        return 0;
38    }
39    bits1 = BN_num_bits(p1);
40    bits2 = BN_num_bits(p2);
41    if ((bits1 == 0) && (bits2 == 0)) {
42        ret = BN_one(rr);
43        return ret;
44    }
45
46    bits = (bits1 > bits2) ? bits1 : bits2;
47
48    BN_CTX_start(ctx);
49    d = BN_CTX_get(ctx);
50    r = BN_CTX_get(ctx);
51    val1[0] = BN_CTX_get(ctx);
52    val2[0] = BN_CTX_get(ctx);
53    if (val2[0] == NULL)
54        goto err;
55
56    if (in_mont != NULL)
57        mont = in_mont;
58    else {
59        if ((mont = BN_MONT_CTX_new()) == NULL)
60            goto err;
61        if (!BN_MONT_CTX_set(mont, m, ctx))
62            goto err;
63    }
64
65    window1 = BN_window_bits_for_exponent_size(bits1);
66    window2 = BN_window_bits_for_exponent_size(bits2);
67
68    /*
69     * Build table for a1:   val1[i] := a1^(2*i + 1) mod m  for i = 0 .. 2^(window1-1)
70     */
71    if (a1->neg || BN_ucmp(a1, m) >= 0) {
72        if (!BN_mod(val1[0], a1, m, ctx))
73            goto err;
74        a_mod_m = val1[0];
75    } else
76        a_mod_m = a1;
77    if (BN_is_zero(a_mod_m)) {
78        BN_zero(rr);
79        ret = 1;
80        goto err;
81    }
82
83    if (!BN_to_montgomery(val1[0], a_mod_m, mont, ctx))
84        goto err;
85    if (window1 > 1) {
86        if (!BN_mod_mul_montgomery(d, val1[0], val1[0], mont, ctx))
87            goto err;
88
89        j = 1 << (window1 - 1);
90        for (i = 1; i < j; i++) {
91            if (((val1[i] = BN_CTX_get(ctx)) == NULL) ||
92                !BN_mod_mul_montgomery(val1[i], val1[i - 1], d, mont, ctx))
93                goto err;
94        }
95    }
96
97    /*
98     * Build table for a2:   val2[i] := a2^(2*i + 1) mod m  for i = 0 .. 2^(window2-1)
99     */
100    if (a2->neg || BN_ucmp(a2, m) >= 0) {
101        if (!BN_mod(val2[0], a2, m, ctx))
102            goto err;
103        a_mod_m = val2[0];
104    } else
105        a_mod_m = a2;
106    if (BN_is_zero(a_mod_m)) {
107        BN_zero(rr);
108        ret = 1;
109        goto err;
110    }
111    if (!BN_to_montgomery(val2[0], a_mod_m, mont, ctx))
112        goto err;
113    if (window2 > 1) {
114        if (!BN_mod_mul_montgomery(d, val2[0], val2[0], mont, ctx))
115            goto err;
116
117        j = 1 << (window2 - 1);
118        for (i = 1; i < j; i++) {
119            if (((val2[i] = BN_CTX_get(ctx)) == NULL) ||
120                !BN_mod_mul_montgomery(val2[i], val2[i - 1], d, mont, ctx))
121                goto err;
122        }
123    }
124
125    /* Now compute the power product, using independent windows. */
126    r_is_one = 1;
127    wvalue1 = 0;                /* The 'value' of the first window */
128    wvalue2 = 0;                /* The 'value' of the second window */
129    wpos1 = 0;                  /* If wvalue1 > 0, the bottom bit of the
130                                 * first window */
131    wpos2 = 0;                  /* If wvalue2 > 0, the bottom bit of the
132                                 * second window */
133
134    if (!BN_to_montgomery(r, BN_value_one(), mont, ctx))
135        goto err;
136    for (b = bits - 1; b >= 0; b--) {
137        if (!r_is_one) {
138            if (!BN_mod_mul_montgomery(r, r, r, mont, ctx))
139                goto err;
140        }
141
142        if (!wvalue1)
143            if (BN_is_bit_set(p1, b)) {
144                /*
145                 * consider bits b-window1+1 .. b for this window
146                 */
147                i = b - window1 + 1;
148                while (!BN_is_bit_set(p1, i)) /* works for i<0 */
149                    i++;
150                wpos1 = i;
151                wvalue1 = 1;
152                for (i = b - 1; i >= wpos1; i--) {
153                    wvalue1 <<= 1;
154                    if (BN_is_bit_set(p1, i))
155                        wvalue1++;
156                }
157            }
158
159        if (!wvalue2)
160            if (BN_is_bit_set(p2, b)) {
161                /*
162                 * consider bits b-window2+1 .. b for this window
163                 */
164                i = b - window2 + 1;
165                while (!BN_is_bit_set(p2, i))
166                    i++;
167                wpos2 = i;
168                wvalue2 = 1;
169                for (i = b - 1; i >= wpos2; i--) {
170                    wvalue2 <<= 1;
171                    if (BN_is_bit_set(p2, i))
172                        wvalue2++;
173                }
174            }
175
176        if (wvalue1 && b == wpos1) {
177            /* wvalue1 is odd and < 2^window1 */
178            if (!BN_mod_mul_montgomery(r, r, val1[wvalue1 >> 1], mont, ctx))
179                goto err;
180            wvalue1 = 0;
181            r_is_one = 0;
182        }
183
184        if (wvalue2 && b == wpos2) {
185            /* wvalue2 is odd and < 2^window2 */
186            if (!BN_mod_mul_montgomery(r, r, val2[wvalue2 >> 1], mont, ctx))
187                goto err;
188            wvalue2 = 0;
189            r_is_one = 0;
190        }
191    }
192    if (!BN_from_montgomery(rr, r, mont, ctx))
193        goto err;
194    ret = 1;
195 err:
196    if (in_mont == NULL)
197        BN_MONT_CTX_free(mont);
198    BN_CTX_end(ctx);
199    bn_check_top(rr);
200    return ret;
201}
202