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comparison libtommath/bn_mp_rshd.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_MP_RSHD_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 /* shift right a certain amount of digits */
19 void mp_rshd (mp_int * a, int b)
20 {
21 int x;
22
23 /* if b <= 0 then ignore it */
24 if (b <= 0) {
25 return;
26 }
27
28 /* if b > used then simply zero it and return */
29 if (a->used <= b) {
30 mp_zero (a);
31 return;
32 }
33
34 {
35 register mp_digit *bottom, *top;
36
37 /* shift the digits down */
38
39 /* bottom */
40 bottom = a->dp;
41
42 /* top [offset into digits] */
43 top = a->dp + b;
44
45 /* this is implemented as a sliding window where
46 * the window is b-digits long and digits from
47 * the top of the window are copied to the bottom
48 *
49 * e.g.
50
51 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
52 /\ | ---->
53 \-------------------/ ---->
54 */
55 for (x = 0; x < (a->used - b); x++) {
56 *bottom++ = *top++;
57 }
58
59 /* zero the top digits */
60 for (; x < a->used; x++) {
61 *bottom++ = 0;
62 }
63 }
64
65 /* remove excess digits */
66 a->used -= b;
67 }
68 #endif