File: bn_mp_reduce.c

package info (click to toggle)
libtommath 1.1.0-3
  • links: PTS, VCS
  • area: main
  • in suites: buster
  • size: 5,268 kB
  • sloc: ansic: 17,976; perl: 699; makefile: 329; sh: 219; asm: 30
file content (97 lines) | stat: -rw-r--r-- 2,418 bytes parent folder | download
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
84
85
86
87
88
89
90
91
92
93
94
95
96
97
#include "tommath_private.h"
#ifdef BN_MP_REDUCE_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
 *
 * LibTomMath is a library that provides multiple-precision
 * integer arithmetic as well as number theoretic functionality.
 *
 * The library was designed directly after the MPI library by
 * Michael Fromberger but has been written from scratch with
 * additional optimizations in place.
 *
 * 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
 */
int mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
{
   mp_int  q;
   int     res, um = m->used;

   /* q = x */
   if ((res = mp_init_copy(&q, x)) != MP_OKAY) {
      return res;
   }

   /* q1 = x / b**(k-1)  */
   mp_rshd(&q, um - 1);

   /* according to HAC this optimization is ok */
   if ((mp_digit)um > ((mp_digit)1 << (DIGIT_BIT - 1))) {
      if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) {
         goto CLEANUP;
      }
   } else {
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
      if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
         goto CLEANUP;
      }
#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
      if ((res = fast_s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
         goto CLEANUP;
      }
#else
      {
         res = MP_VAL;
         goto CLEANUP;
      }
#endif
   }

   /* q3 = q2 / b**(k+1) */
   mp_rshd(&q, um + 1);

   /* x = x mod b**(k+1), quick (no division) */
   if ((res = mp_mod_2d(x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
      goto CLEANUP;
   }

   /* q = q * m mod b**(k+1), quick (no division) */
   if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
      goto CLEANUP;
   }

   /* x = x - q */
   if ((res = 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 ((res = mp_lshd(&q, um + 1)) != MP_OKAY)
         goto CLEANUP;
      if ((res = mp_add(x, &q, x)) != MP_OKAY)
         goto CLEANUP;
   }

   /* Back off if it's too big */
   while (mp_cmp(x, m) != MP_LT) {
      if ((res = s_mp_sub(x, m, x)) != MP_OKAY) {
         goto CLEANUP;
      }
   }

CLEANUP:
   mp_clear(&q);

   return res;
}
#endif

/* ref:         HEAD -> master, tag: v1.1.0 */
/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
/* commit time: 2019-01-28 20:32:32 +0100 */