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comparison libtommath/bn_s_mp_invmod_slow.c @ 1692:1051e4eea25a

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Update LibTomMath to 1.2.0 (#84) * update C files * update other files * update headers * update makefiles * remove mp_set/get_double() * use ltm 1.2.0 API * update ltm_desc * use bundled tommath if system-tommath is too old * XMALLOC etc. were changed to MP_MALLOC etc.
author Steffen Jaeckel <s@jaeckel.eu>
date Tue, 26 May 2020 17:36:47 +0200
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1691:2d3745d58843 1692:1051e4eea25a
1 #include "tommath_private.h"
2 #ifdef BN_S_MP_INVMOD_SLOW_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
5
6 /* hac 14.61, pp608 */
7 mp_err s_mp_invmod_slow(const mp_int *a, const mp_int *b, mp_int *c)
8 {
9 mp_int x, y, u, v, A, B, C, D;
10 mp_err err;
11
12 /* b cannot be negative */
13 if ((b->sign == MP_NEG) || MP_IS_ZERO(b)) {
14 return MP_VAL;
15 }
16
17 /* init temps */
18 if ((err = mp_init_multi(&x, &y, &u, &v,
19 &A, &B, &C, &D, NULL)) != MP_OKAY) {
20 return err;
21 }
22
23 /* x = a, y = b */
24 if ((err = mp_mod(a, b, &x)) != MP_OKAY) goto LBL_ERR;
25 if ((err = mp_copy(b, &y)) != MP_OKAY) goto LBL_ERR;
26
27 /* 2. [modified] if x,y are both even then return an error! */
28 if (MP_IS_EVEN(&x) && MP_IS_EVEN(&y)) {
29 err = MP_VAL;
30 goto LBL_ERR;
31 }
32
33 /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
34 if ((err = mp_copy(&x, &u)) != MP_OKAY) goto LBL_ERR;
35 if ((err = mp_copy(&y, &v)) != MP_OKAY) goto LBL_ERR;
36 mp_set(&A, 1uL);
37 mp_set(&D, 1uL);
38
39 top:
40 /* 4. while u is even do */
41 while (MP_IS_EVEN(&u)) {
42 /* 4.1 u = u/2 */
43 if ((err = mp_div_2(&u, &u)) != MP_OKAY) goto LBL_ERR;
44
45 /* 4.2 if A or B is odd then */
46 if (MP_IS_ODD(&A) || MP_IS_ODD(&B)) {
47 /* A = (A+y)/2, B = (B-x)/2 */
48 if ((err = mp_add(&A, &y, &A)) != MP_OKAY) goto LBL_ERR;
49 if ((err = mp_sub(&B, &x, &B)) != MP_OKAY) goto LBL_ERR;
50 }
51 /* A = A/2, B = B/2 */
52 if ((err = mp_div_2(&A, &A)) != MP_OKAY) goto LBL_ERR;
53 if ((err = mp_div_2(&B, &B)) != MP_OKAY) goto LBL_ERR;
54 }
55
56 /* 5. while v is even do */
57 while (MP_IS_EVEN(&v)) {
58 /* 5.1 v = v/2 */
59 if ((err = mp_div_2(&v, &v)) != MP_OKAY) goto LBL_ERR;
60
61 /* 5.2 if C or D is odd then */
62 if (MP_IS_ODD(&C) || MP_IS_ODD(&D)) {
63 /* C = (C+y)/2, D = (D-x)/2 */
64 if ((err = mp_add(&C, &y, &C)) != MP_OKAY) goto LBL_ERR;
65 if ((err = mp_sub(&D, &x, &D)) != MP_OKAY) goto LBL_ERR;
66 }
67 /* C = C/2, D = D/2 */
68 if ((err = mp_div_2(&C, &C)) != MP_OKAY) goto LBL_ERR;
69 if ((err = mp_div_2(&D, &D)) != MP_OKAY) goto LBL_ERR;
70 }
71
72 /* 6. if u >= v then */
73 if (mp_cmp(&u, &v) != MP_LT) {
74 /* u = u - v, A = A - C, B = B - D */
75 if ((err = mp_sub(&u, &v, &u)) != MP_OKAY) goto LBL_ERR;
76
77 if ((err = mp_sub(&A, &C, &A)) != MP_OKAY) goto LBL_ERR;
78
79 if ((err = mp_sub(&B, &D, &B)) != MP_OKAY) goto LBL_ERR;
80 } else {
81 /* v - v - u, C = C - A, D = D - B */
82 if ((err = mp_sub(&v, &u, &v)) != MP_OKAY) goto LBL_ERR;
83
84 if ((err = mp_sub(&C, &A, &C)) != MP_OKAY) goto LBL_ERR;
85
86 if ((err = mp_sub(&D, &B, &D)) != MP_OKAY) goto LBL_ERR;
87 }
88
89 /* if not zero goto step 4 */
90 if (!MP_IS_ZERO(&u)) {
91 goto top;
92 }
93
94 /* now a = C, b = D, gcd == g*v */
95
96 /* if v != 1 then there is no inverse */
97 if (mp_cmp_d(&v, 1uL) != MP_EQ) {
98 err = MP_VAL;
99 goto LBL_ERR;
100 }
101
102 /* if its too low */
103 while (mp_cmp_d(&C, 0uL) == MP_LT) {
104 if ((err = mp_add(&C, b, &C)) != MP_OKAY) goto LBL_ERR;
105 }
106
107 /* too big */
108 while (mp_cmp_mag(&C, b) != MP_LT) {
109 if ((err = mp_sub(&C, b, &C)) != MP_OKAY) goto LBL_ERR;
110 }
111
112 /* C is now the inverse */
113 mp_exch(&C, c);
114 err = MP_OKAY;
115 LBL_ERR:
116 mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
117 return err;
118 }
119 #endif