|
282
|
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 * |
|
386
|
15 * Tom St Denis, [email protected], http://math.libtomcrypt.com |
|
282
|
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 |
|
386
|
86 |
|
|
87 /* $Source: /cvs/libtom/libtommath/bn_s_mp_sub.c,v $ */ |
|
|
88 /* $Revision: 1.3 $ */ |
|
|
89 /* $Date: 2006/03/31 14:18:44 $ */ |
