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comparison libtommath/bn_mp_gcd.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_GCD_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 /* Greatest Common Divisor using the binary method */
19 int mp_gcd (mp_int * a, mp_int * b, mp_int * c)
20 {
21 mp_int u, v;
22 int k, u_lsb, v_lsb, res;
23
24 /* either zero than gcd is the largest */
25 if (mp_iszero (a) == 1 && mp_iszero (b) == 0) {
26 return mp_abs (b, c);
27 }
28 if (mp_iszero (a) == 0 && mp_iszero (b) == 1) {
29 return mp_abs (a, c);
30 }
31
32 /* optimized. At this point if a == 0 then
33 * b must equal zero too
34 */
35 if (mp_iszero (a) == 1) {
36 mp_zero(c);
37 return MP_OKAY;
38 }
39
40 /* get copies of a and b we can modify */
41 if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
42 return res;
43 }
44
45 if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
46 goto LBL_U;
47 }
48
49 /* must be positive for the remainder of the algorithm */
50 u.sign = v.sign = MP_ZPOS;
51
52 /* B1. Find the common power of two for u and v */
53 u_lsb = mp_cnt_lsb(&u);
54 v_lsb = mp_cnt_lsb(&v);
55 k = MIN(u_lsb, v_lsb);
56
57 if (k > 0) {
58 /* divide the power of two out */
59 if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
60 goto LBL_V;
61 }
62
63 if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
64 goto LBL_V;
65 }
66 }
67
68 /* divide any remaining factors of two out */
69 if (u_lsb != k) {
70 if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
71 goto LBL_V;
72 }
73 }
74
75 if (v_lsb != k) {
76 if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
77 goto LBL_V;
78 }
79 }
80
81 while (mp_iszero(&v) == 0) {
82 /* make sure v is the largest */
83 if (mp_cmp_mag(&u, &v) == MP_GT) {
84 /* swap u and v to make sure v is >= u */
85 mp_exch(&u, &v);
86 }
87
88 /* subtract smallest from largest */
89 if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
90 goto LBL_V;
91 }
92
93 /* Divide out all factors of two */
94 if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
95 goto LBL_V;
96 }
97 }
98
99 /* multiply by 2**k which we divided out at the beginning */
100 if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
101 goto LBL_V;
102 }
103 c->sign = MP_ZPOS;
104 res = MP_OKAY;
105 LBL_V:mp_clear (&u);
106 LBL_U:mp_clear (&v);
107 return res;
108 }
109 #endif