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comparison libtommath/bn_s_mp_sub.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
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children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
1 #include <tommath.h>
2 #ifdef BN_S_MP_SUB_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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
19 int
20 s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
21 {
22 int olduse, res, min, max;
23
24 /* find sizes */
25 min = b->used;
26 max = a->used;
27
28 /* init result */
29 if (c->alloc < max) {
30 if ((res = mp_grow (c, max)) != MP_OKAY) {
31 return res;
32 }
33 }
34 olduse = c->used;
35 c->used = max;
36
37 {
38 register mp_digit u, *tmpa, *tmpb, *tmpc;
39 register int i;
40
41 /* alias for digit pointers */
42 tmpa = a->dp;
43 tmpb = b->dp;
44 tmpc = c->dp;
45
46 /* set carry to zero */
47 u = 0;
48 for (i = 0; i < min; i++) {
49 /* T[i] = A[i] - B[i] - U */
50 *tmpc = *tmpa++ - *tmpb++ - u;
51
52 /* U = carry bit of T[i]
53 * Note this saves performing an AND operation since
54 * if a carry does occur it will propagate all the way to the
55 * MSB. As a result a single shift is enough to get the carry
56 */
57 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
58
59 /* Clear carry from T[i] */
60 *tmpc++ &= MP_MASK;
61 }
62
63 /* now copy higher words if any, e.g. if A has more digits than B */
64 for (; i < max; i++) {
65 /* T[i] = A[i] - U */
66 *tmpc = *tmpa++ - u;
67
68 /* U = carry bit of T[i] */
69 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
70
71 /* Clear carry from T[i] */
72 *tmpc++ &= MP_MASK;
73 }
74
75 /* clear digits above used (since we may not have grown result above) */
76 for (i = c->used; i < olduse; i++) {
77 *tmpc++ = 0;
78 }
79 }
80
81 mp_clamp (c);
82 return MP_OKAY;
83 }
84
85 #endif