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

I'm using this function:

int CalculateMod(int base, int exp ,int mod){
    int result;
    result = (int) pow(base,exp);
    result = result % mod;
    return result;
}

I parse the variable result because pow give you back a double, and for using mod you need two variables of type int, anyway, in a RSA decryption, you should just use integer numbers.