Current File : //data/down/eBOOKS//Handbook of Applied Cryptography/fermat.c
/*
Author: Pate Williams (c) 1997
4.9 Algorithm Fermat primality test.
See "Handbook of Applied Cryptography"
by Alfred J. Menezes et al page 136.
*/
#include <ctype.h>
#include <stdio.h>
#include "lip.h"
int Fermat(long t, verylong zn)
/* returns 0 if composite 1 if probably prime */
{
int flag = 1;
long i;
verylong za = 0, zn1 = 0, zr = 0;
if (zodd(zn)) {
if (zscompare(zn, 3l) > 0) {
zsadd(zn, - 1l, &zn1);
for (i = 1; flag && i <= t; i++) {
do zrandomb(zn1, &za); while (zscompare(za, 2l) < 0);
zexpmod(za, zn1, zn, &zr);
if (zscompare(zr, 1l) != 0) flag = 0;
}
}
}
else flag = zscompare(zn, 2l) == 0;
zfree(&za);
zfree(&zn1);
zfree(&zr);
return flag;
}
int main(void)
{
char answer[256];
long t;
verylong zn = 0;
do {
do {
printf("enter the security parameter ( >= 1): ");
scanf("%d", &t);
} while (t < 1);
printf("enter the number to be tested below:\n");
zread(&zn);
if (Fermat(t, zn))
printf("number is probably prime\n");
else
printf("number is composite\n");
printf("another number (n or y)? ");
scanf("%s", answer);
} while (tolower(answer[0]) == 'y');
zfree(&zn);
return 0;
}
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