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comparison libtommath/bn_mp_invmod_slow.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
parents
children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
1 #include <tommath.h>
2 #ifdef BN_MP_INVMOD_SLOW_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
16 */
17
18 /* hac 14.61, pp608 */
19 int mp_invmod_slow (mp_int * a, mp_int * b, mp_int * c)
20 {
21 mp_int x, y, u, v, A, B, C, D;
22 int res;
23
24 /* b cannot be negative */
25 if (b->sign == MP_NEG || mp_iszero(b) == 1) {
26 return MP_VAL;
27 }
28
29 /* init temps */
30 if ((res = mp_init_multi(&x, &y, &u, &v,
31 &A, &B, &C, &D, NULL)) != MP_OKAY) {
32 return res;
33 }
34
35 /* x = a, y = b */
36 if ((res = mp_mod(a, b, &x)) != MP_OKAY) {
37 goto LBL_ERR;
38 }
39 if ((res = mp_copy (b, &y)) != MP_OKAY) {
40 goto LBL_ERR;
41 }
42
43 /* 2. [modified] if x,y are both even then return an error! */
44 if (mp_iseven (&x) == 1 && mp_iseven (&y) == 1) {
45 res = MP_VAL;
46 goto LBL_ERR;
47 }
48
49 /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
50 if ((res = mp_copy (&x, &u)) != MP_OKAY) {
51 goto LBL_ERR;
52 }
53 if ((res = mp_copy (&y, &v)) != MP_OKAY) {
54 goto LBL_ERR;
55 }
56 mp_set (&A, 1);
57 mp_set (&D, 1);
58
59 top:
60 /* 4. while u is even do */
61 while (mp_iseven (&u) == 1) {
62 /* 4.1 u = u/2 */
63 if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
64 goto LBL_ERR;
65 }
66 /* 4.2 if A or B is odd then */
67 if (mp_isodd (&A) == 1 || mp_isodd (&B) == 1) {
68 /* A = (A+y)/2, B = (B-x)/2 */
69 if ((res = mp_add (&A, &y, &A)) != MP_OKAY) {
70 goto LBL_ERR;
71 }
72 if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
73 goto LBL_ERR;
74 }
75 }
76 /* A = A/2, B = B/2 */
77 if ((res = mp_div_2 (&A, &A)) != MP_OKAY) {
78 goto LBL_ERR;
79 }
80 if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
81 goto LBL_ERR;
82 }
83 }
84
85 /* 5. while v is even do */
86 while (mp_iseven (&v) == 1) {
87 /* 5.1 v = v/2 */
88 if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
89 goto LBL_ERR;
90 }
91 /* 5.2 if C or D is odd then */
92 if (mp_isodd (&C) == 1 || mp_isodd (&D) == 1) {
93 /* C = (C+y)/2, D = (D-x)/2 */
94 if ((res = mp_add (&C, &y, &C)) != MP_OKAY) {
95 goto LBL_ERR;
96 }
97 if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
98 goto LBL_ERR;
99 }
100 }
101 /* C = C/2, D = D/2 */
102 if ((res = mp_div_2 (&C, &C)) != MP_OKAY) {
103 goto LBL_ERR;
104 }
105 if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
106 goto LBL_ERR;
107 }
108 }
109
110 /* 6. if u >= v then */
111 if (mp_cmp (&u, &v) != MP_LT) {
112 /* u = u - v, A = A - C, B = B - D */
113 if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
114 goto LBL_ERR;
115 }
116
117 if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) {
118 goto LBL_ERR;
119 }
120
121 if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
122 goto LBL_ERR;
123 }
124 } else {
125 /* v - v - u, C = C - A, D = D - B */
126 if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
127 goto LBL_ERR;
128 }
129
130 if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) {
131 goto LBL_ERR;
132 }
133
134 if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
135 goto LBL_ERR;
136 }
137 }
138
139 /* if not zero goto step 4 */
140 if (mp_iszero (&u) == 0)
141 goto top;
142
143 /* now a = C, b = D, gcd == g*v */
144
145 /* if v != 1 then there is no inverse */
146 if (mp_cmp_d (&v, 1) != MP_EQ) {
147 res = MP_VAL;
148 goto LBL_ERR;
149 }
150
151 /* if its too low */
152 while (mp_cmp_d(&C, 0) == MP_LT) {
153 if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
154 goto LBL_ERR;
155 }
156 }
157
158 /* too big */
159 while (mp_cmp_mag(&C, b) != MP_LT) {
160 if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
161 goto LBL_ERR;
162 }
163 }
164
165 /* C is now the inverse */
166 mp_exch (&C, c);
167 res = MP_OKAY;
168 LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL);
169 return res;
170 }
171 #endif