1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
|
#include "tommath_private.h"
#ifdef BN_MP_REDUCE_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* reduces x mod m, assumes 0 < x < m**2, mu is
* precomputed via mp_reduce_setup.
* From HAC pp.604 Algorithm 14.42
*/
mp_err mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
{
mp_int q;
mp_err err;
int um = m->used;
/* q = x */
if ((err = mp_init_copy(&q, x)) != MP_OKAY) {
return err;
}
/* q1 = x / b**(k-1) */
mp_rshd(&q, um - 1);
/* according to HAC this optimization is ok */
if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
if ((err = mp_mul(&q, mu, &q)) != MP_OKAY) {
goto CLEANUP;
}
} else if (MP_HAS(S_MP_MUL_HIGH_DIGS)) {
if ((err = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
} else if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)) {
if ((err = s_mp_mul_high_digs_fast(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
} else {
err = MP_VAL;
goto CLEANUP;
}
/* q3 = q2 / b**(k+1) */
mp_rshd(&q, um + 1);
/* x = x mod b**(k+1), quick (no division) */
if ((err = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
goto CLEANUP;
}
/* q = q * m mod b**(k+1), quick (no division) */
if ((err = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}
/* x = x - q */
if ((err = mp_sub(x, &q, x)) != MP_OKAY) {
goto CLEANUP;
}
/* If x < 0, add b**(k+1) to it */
if (mp_cmp_d(x, 0uL) == MP_LT) {
mp_set(&q, 1uL);
if ((err = mp_lshd(&q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}
if ((err = mp_add(x, &q, x)) != MP_OKAY) {
goto CLEANUP;
}
}
/* Back off if it's too big */
while (mp_cmp(x, m) != MP_LT) {
if ((err = s_mp_sub(x, m, x)) != MP_OKAY) {
goto CLEANUP;
}
}
CLEANUP:
mp_clear(&q);
return err;
}
#endif
|
