Calculating pow(a,b) mod n

Question

I want to calculate a^b mod n for use in RSA decryption. My code (below) returns incorrect answers. What is wrong with it?

unsigned long i         


        

Answer 1

For my code a^k mod n in php:

function pmod(a, k, n)
{
    if (n==1) return 0;
    power = 1;
    for(i=1; i<=k; $i++)
    {
        power = (power*a) % n;
    }
    return power;
}

Answer 2

#include <cmath>
...
static_cast<int>(std::pow(a,b))%n

but my best bet is you are overflowing int (IE: the number is two large for the int) on the power I had the same problem creating the exact same function.