2 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
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.
12 * The library is free for all purposes without any express
15 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
18 /* this is a modified version of fast_s_mul_digs that only produces
19 * output digits *above* digs. See the comments for fast_s_mul_digs
20 * to see how it works.
22 * This is used in the Barrett reduction since for one of the multiplications
23 * only the higher digits were needed. This essentially halves the work.
25 * Based on Algorithm 14.12 on pp.595 of HAC.
27 int fast_s_mp_mul_high_digs (mp_int
* a
, mp_int
* b
, mp_int
* c
, int digs
)
29 int olduse
, res
, pa
, ix
, iz
;
30 mp_digit W
[MP_WARRAY
];
33 /* grow the destination as required */
34 pa
= a
->used
+ b
->used
;
36 if ((res
= mp_grow (c
, pa
)) != MP_OKAY
) {
41 /* number of output digits to produce */
42 pa
= a
->used
+ b
->used
;
44 for (ix
= digs
; ix
< pa
; ix
++) {
46 mp_digit
*tmpx
, *tmpy
;
48 /* get offsets into the two bignums */
49 ty
= MIN(b
->used
-1, ix
);
52 /* setup temp aliases */
56 /* this is the number of times the loop will iterrate, essentially its
57 while (tx++ < a->used && ty-- >= 0) { ... }
59 iy
= MIN(a
->used
-tx
, ty
+1);
62 for (iz
= 0; iz
< iy
; iz
++) {
63 _W
+= ((mp_word
)*tmpx
++)*((mp_word
)*tmpy
--);
67 W
[ix
] = ((mp_digit
)_W
) & MP_MASK
;
70 _W
= _W
>> ((mp_word
)DIGIT_BIT
);
78 register mp_digit
*tmpc
;
81 for (ix
= digs
; ix
< pa
; ix
++) {
82 /* now extract the previous digit [below the carry] */
86 /* clear unused digits [that existed in the old copy of c] */
87 for (; ix
< olduse
; ix
++) {
96 /* $Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_high_digs.c,v $ */
97 /* $Revision: 1.5 $ */
98 /* $Date: 2006/11/14 03:46:25 $ */