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comparison bn_mp_invmod.c @ 1:22d5cf7d4b1a libtommath

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Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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children d29b64170cf0
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-1:000000000000 1:22d5cf7d4b1a
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is a library that provides multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library was designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
14 */
15 #include <tommath.h>
16
17 /* hac 14.61, pp608 */
18 int mp_invmod (mp_int * a, mp_int * b, mp_int * c)
19 {
20 mp_int x, y, u, v, A, B, C, D;
21 int res;
22
23 /* b cannot be negative */
24 if (b->sign == MP_NEG || mp_iszero(b) == 1) {
25 return MP_VAL;
26 }
27
28 /* if the modulus is odd we can use a faster routine instead */
29 if (mp_isodd (b) == 1) {
30 return fast_mp_invmod (a, b, c);
31 }
32
33 /* init temps */
34 if ((res = mp_init_multi(&x, &y, &u, &v,
35 &A, &B, &C, &D, NULL)) != MP_OKAY) {
36 return res;
37 }
38
39 /* x = a, y = b */
40 if ((res = mp_copy (a, &x)) != MP_OKAY) {
41 goto __ERR;
42 }
43 if ((res = mp_copy (b, &y)) != MP_OKAY) {
44 goto __ERR;
45 }
46
47 /* 2. [modified] if x,y are both even then return an error! */
48 if (mp_iseven (&x) == 1 && mp_iseven (&y) == 1) {
49 res = MP_VAL;
50 goto __ERR;
51 }
52
53 /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
54 if ((res = mp_copy (&x, &u)) != MP_OKAY) {
55 goto __ERR;
56 }
57 if ((res = mp_copy (&y, &v)) != MP_OKAY) {
58 goto __ERR;
59 }
60 mp_set (&A, 1);
61 mp_set (&D, 1);
62
63 top:
64 /* 4. while u is even do */
65 while (mp_iseven (&u) == 1) {
66 /* 4.1 u = u/2 */
67 if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
68 goto __ERR;
69 }
70 /* 4.2 if A or B is odd then */
71 if (mp_isodd (&A) == 1 || mp_isodd (&B) == 1) {
72 /* A = (A+y)/2, B = (B-x)/2 */
73 if ((res = mp_add (&A, &y, &A)) != MP_OKAY) {
74 goto __ERR;
75 }
76 if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
77 goto __ERR;
78 }
79 }
80 /* A = A/2, B = B/2 */
81 if ((res = mp_div_2 (&A, &A)) != MP_OKAY) {
82 goto __ERR;
83 }
84 if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
85 goto __ERR;
86 }
87 }
88
89 /* 5. while v is even do */
90 while (mp_iseven (&v) == 1) {
91 /* 5.1 v = v/2 */
92 if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
93 goto __ERR;
94 }
95 /* 5.2 if C or D is odd then */
96 if (mp_isodd (&C) == 1 || mp_isodd (&D) == 1) {
97 /* C = (C+y)/2, D = (D-x)/2 */
98 if ((res = mp_add (&C, &y, &C)) != MP_OKAY) {
99 goto __ERR;
100 }
101 if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
102 goto __ERR;
103 }
104 }
105 /* C = C/2, D = D/2 */
106 if ((res = mp_div_2 (&C, &C)) != MP_OKAY) {
107 goto __ERR;
108 }
109 if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
110 goto __ERR;
111 }
112 }
113
114 /* 6. if u >= v then */
115 if (mp_cmp (&u, &v) != MP_LT) {
116 /* u = u - v, A = A - C, B = B - D */
117 if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
118 goto __ERR;
119 }
120
121 if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) {
122 goto __ERR;
123 }
124
125 if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
126 goto __ERR;
127 }
128 } else {
129 /* v - v - u, C = C - A, D = D - B */
130 if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
131 goto __ERR;
132 }
133
134 if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) {
135 goto __ERR;
136 }
137
138 if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
139 goto __ERR;
140 }
141 }
142
143 /* if not zero goto step 4 */
144 if (mp_iszero (&u) == 0)
145 goto top;
146
147 /* now a = C, b = D, gcd == g*v */
148
149 /* if v != 1 then there is no inverse */
150 if (mp_cmp_d (&v, 1) != MP_EQ) {
151 res = MP_VAL;
152 goto __ERR;
153 }
154
155 /* if its too low */
156 while (mp_cmp_d(&C, 0) == MP_LT) {
157 if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
158 goto __ERR;
159 }
160 }
161
162 /* too big */
163 while (mp_cmp_mag(&C, b) != MP_LT) {
164 if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
165 goto __ERR;
166 }
167 }
168
169 /* C is now the inverse */
170 mp_exch (&C, c);
171 res = MP_OKAY;
172 __ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL);
173 return res;
174 }