Current File : //data/down/eBOOKS/Handbook of Applied Cryptography/swindow.c
/*
Author: Pate Williams (c) 1997
Sliding window exponentiation. See "Handboook of
Applied Cryptography" by Alfred J. Menezes et al
14.6.1 Section pages 614 - 620.
*/
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "lip.h"
#define DEBUG
void long_to_binary(char *e, long a, long *t)
{
long i;
*t = z2logs(a);
for (i = 0; i < *t; i++) {
e[i] = (char) (a & 1);
a >>= 1;
}
}
long binary_to_long(char *e, long i, long l)
{
long a, t;
a = e[i];
for (t = i - 1; t >= l; t--)
a = (a << 1) + e[t];
return a;
}
void sliding_window(long exp, long k,
verylong zg, verylong *zA)
{
char e[32];
long i, l, length, limit, m, t;
verylong za = 0, zb = 0;
verylong zg1 = 0, zg2 = 0, *zg3;
limit = pow(2, k - 1) - 1;
length = 2 * limit + 2;
zg3 = calloc(length, sizeof(verylong));
long_to_binary(e, exp, &t);
#ifdef DEBUG
for (i = t - 1; i >= 0; i--)
printf("%d", e[i]);
printf("\n");
#endif
zcopy(zg, &zg1);
zsq(zg, &zg2);
for (i = 1; i <= limit; i++)
zmul(zg2, zg3[2 * i - 1], &zg3[2 * i + 1]);
zone(zA);
i = t - 1;
while (i >= 0) {
if (e[i] == 0) {
zsq(*zA, &za);
zcopy(za, zA);
i--;
}
else {
l = i - 1;
while (i - l + 1 <= k && l >= 0) {
if (e[l] == 1) m = l;
l--;
}
if (i - l + 1 > k) {
zmul(*zA, *zA, &za);
zmul(za, zg, zA);
i = i - 1;
}
else {
l = m;
zsexp(*zA, pow(2, i - l + 1), &za);
#ifdef DEBUG
printf("%ld %ld %ld\n", i, l, binary_to_long(e, i, l));
#endif
zsexp(zg, binary_to_long(e, i, l), &zb);
zmul(za, zb, zA);
i = l - 1;
}
}
}
zfree(&za);
zfree(&zb);
zfree(&zg1);
zfree(&zg2);
for (i = 0; i < length; i++) zfree(&zg3[i]);
}
int main(void)
{
long exp = 39l;
verylong zA = 0, zg = 0;
zintoz(2l, &zg);
sliding_window(exp, 3l, zg, &zA);
zwriteln(zA);
zsexp(zg, exp, &zA);
zwriteln(zA);
zfree(&zA);
zfree(&zg);
return 0;
}
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