Feige Fiat Shamir Scheme not working - c

Question

I'm trying to implement the Feige Fiat Shamir Identification Scheme in C (Arduino) and it works, but only when e = 0. When e = 1 it doesn't work.
How can I make it work?
#include <Wire.h>
int getGCD(int a, int b)
{
int c;
while (a != 0)
{
c = a;
a = b % a;
b = c;
}
return b;
}
int getCoprime(int n)
{
int coprime;
do
{
coprime = random(1, n);
}
while (getGCD(n, coprime) != 1);
return coprime;
}
//Preparation
int n = 7 * 3;
int s = getCoprime(n);
int v = (s * s) % n;
void loop ()
{
e = random(0, 2);
r = random(1, n);
int y = (r * (int)pow(s, e)) % n;
int x = (r * r) % n;
int ysqmodn = y * y % n;
int test = (x * (int)pow(v, e)) % n;
if(ysqmodn == test)
{
Serial.print("The current ICC matches. \n");
}
else
{
Serial.print(String(e));
Serial.print("\n");
}
delay(500);
}

Answer 1

It does work when e==1. When e==0 the computation is trivial, since s and v fall out due to power of 0 always being 1. This is the code copied and altered only enough to get it to compile.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <math.h>
int random (int low, int high) {
return low + rand() % (high - low);
}
int getGCD(int a, int b) {
int c;
while (a != 0)
{
c = a;
a = b % a;
b = c;
}
return b;
}
int getCoprime(int n) {
int coprime;
do
{
coprime = random(1, n);
}
while (getGCD(n, coprime) != 1);
return coprime;
}
int main(void) {
int e, x, y, r, n, s, v, test, ysqmodn;
srand((unsigned)time(NULL));
n = 7 * 3;
s = getCoprime(n);
v = (s * s) % n;
e = random(0, 2);
r = random(1, n);
printf("n=%d, s=%d, e=%d, r=%d\n", n,s,e,r);
y = (r * (int)pow(s, e)) % n;
x = (r * r) % n;
ysqmodn = y * y % n;
test = (x * (int)pow(v, e)) % n;
if(ysqmodn == test)
printf("The current ICC matches. \n");
else
printf("%d\n", e);
return 0;
}
Sample results:
n=21, s=2, e=1, r=2
The current ICC matches.
n=21, s=11, e=0, r=12
The current ICC matches.
n=21, s=8, e=1, r=14
The current ICC matches.
n=21, s=17, e=1, r=13
The current ICC matches.
n=21, s=1, e=0, r=9
The current ICC matches.
n=21, s=4, e=0, r=13
The current ICC matches.