Question

I am playing around with simple encryption using RSA algorithms and found a strange bug.

private static Integer testEnc(Integer value){
    Integer val = (int)Math.pow(value, 37);
    return val % 437; 
}

private static Integer testDec(Integer value){
    Integer val = new Integer((int)Math.pow(value, 289));
    return val % 437;
}

public static void main(String[] args) {
    System.out.print("Encode 55  = ");
    Integer encoded = testEnc(2);
    System.out.println(encoded + "\n");

    System.out.print(encoded + " decoded = ");
    Integer decoded = testDec(3977645);
    System.out.println(decoded + "n");
}

Both of the following functions return 97 regardless of input. If I comment out the modulus and just return val, the returned value is 2147483647.

Type casting double to int seems to be the issue but I am not sure why this is.
These methods are static only because I was calling them from a main method.

Answer 1

2147483647 is the int max value, aka 2^31-1.
For any value >=2 you get an overflowm because 2^37 > 2^31-1

In order to get the power modulo 437 you should get the modulo at every step. E.g. like:

private static int myPow(int a, int b, int mod){
    int ret = 1;
    for(int i = 0; i < b; i++)
    {
        ret = (ret * a) % mod;
    }
    return ret;
}