1 /* $NetBSD: bn_mp_invmod_slow.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $ */
2
3 #include <tommath.h>
4 #ifdef BN_MP_INVMOD_SLOW_C
5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
6 *
7 * LibTomMath is a library that provides multiple-precision
8 * integer arithmetic as well as number theoretic functionality.
9 *
10 * The library was designed directly after the MPI library by
11 * Michael Fromberger but has been written from scratch with
12 * additional optimizations in place.
13 *
14 * The library is free for all purposes without any express
15 * guarantee it works.
16 *
17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18 */
19
20 /* hac 14.61, pp608 */
mp_invmod_slow(mp_int * a,mp_int * b,mp_int * c)21 int mp_invmod_slow (mp_int * a, mp_int * b, mp_int * c)
22 {
23 mp_int x, y, u, v, A, B, C, D;
24 int res;
25
26 /* b cannot be negative */
27 if (b->sign == MP_NEG || mp_iszero(b) == 1) {
28 return MP_VAL;
29 }
30
31 /* init temps */
32 if ((res = mp_init_multi(&x, &y, &u, &v,
33 &A, &B, &C, &D, NULL)) != MP_OKAY) {
34 return res;
35 }
36
37 /* x = a, y = b */
38 if ((res = mp_mod(a, b, &x)) != MP_OKAY) {
39 goto LBL_ERR;
40 }
41 if ((res = mp_copy (b, &y)) != MP_OKAY) {
42 goto LBL_ERR;
43 }
44
45 /* 2. [modified] if x,y are both even then return an error! */
46 if (mp_iseven (&x) == 1 && mp_iseven (&y) == 1) {
47 res = MP_VAL;
48 goto LBL_ERR;
49 }
50
51 /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
52 if ((res = mp_copy (&x, &u)) != MP_OKAY) {
53 goto LBL_ERR;
54 }
55 if ((res = mp_copy (&y, &v)) != MP_OKAY) {
56 goto LBL_ERR;
57 }
58 mp_set (&A, 1);
59 mp_set (&D, 1);
60
61 top:
62 /* 4. while u is even do */
63 while (mp_iseven (&u) == 1) {
64 /* 4.1 u = u/2 */
65 if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
66 goto LBL_ERR;
67 }
68 /* 4.2 if A or B is odd then */
69 if (mp_isodd (&A) == 1 || mp_isodd (&B) == 1) {
70 /* A = (A+y)/2, B = (B-x)/2 */
71 if ((res = mp_add (&A, &y, &A)) != MP_OKAY) {
72 goto LBL_ERR;
73 }
74 if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
75 goto LBL_ERR;
76 }
77 }
78 /* A = A/2, B = B/2 */
79 if ((res = mp_div_2 (&A, &A)) != MP_OKAY) {
80 goto LBL_ERR;
81 }
82 if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
83 goto LBL_ERR;
84 }
85 }
86
87 /* 5. while v is even do */
88 while (mp_iseven (&v) == 1) {
89 /* 5.1 v = v/2 */
90 if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
91 goto LBL_ERR;
92 }
93 /* 5.2 if C or D is odd then */
94 if (mp_isodd (&C) == 1 || mp_isodd (&D) == 1) {
95 /* C = (C+y)/2, D = (D-x)/2 */
96 if ((res = mp_add (&C, &y, &C)) != MP_OKAY) {
97 goto LBL_ERR;
98 }
99 if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
100 goto LBL_ERR;
101 }
102 }
103 /* C = C/2, D = D/2 */
104 if ((res = mp_div_2 (&C, &C)) != MP_OKAY) {
105 goto LBL_ERR;
106 }
107 if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
108 goto LBL_ERR;
109 }
110 }
111
112 /* 6. if u >= v then */
113 if (mp_cmp (&u, &v) != MP_LT) {
114 /* u = u - v, A = A - C, B = B - D */
115 if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
116 goto LBL_ERR;
117 }
118
119 if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) {
120 goto LBL_ERR;
121 }
122
123 if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
124 goto LBL_ERR;
125 }
126 } else {
127 /* v - v - u, C = C - A, D = D - B */
128 if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
129 goto LBL_ERR;
130 }
131
132 if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) {
133 goto LBL_ERR;
134 }
135
136 if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
137 goto LBL_ERR;
138 }
139 }
140
141 /* if not zero goto step 4 */
142 if (mp_iszero (&u) == 0)
143 goto top;
144
145 /* now a = C, b = D, gcd == g*v */
146
147 /* if v != 1 then there is no inverse */
148 if (mp_cmp_d (&v, 1) != MP_EQ) {
149 res = MP_VAL;
150 goto LBL_ERR;
151 }
152
153 /* if its too low */
154 while (mp_cmp_d(&C, 0) == MP_LT) {
155 if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
156 goto LBL_ERR;
157 }
158 }
159
160 /* too big */
161 while (mp_cmp_mag(&C, b) != MP_LT) {
162 if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
163 goto LBL_ERR;
164 }
165 }
166
167 /* C is now the inverse */
168 mp_exch (&C, c);
169 res = MP_OKAY;
170 LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL);
171 return res;
172 }
173 #endif
174
175 /* Source: /cvs/libtom/libtommath/bn_mp_invmod_slow.c,v */
176 /* Revision: 1.4 */
177 /* Date: 2006/12/28 01:25:13 */
178