Koblitz method in Java

Question

I'm trying to use Koblitz's method to convert String to ECPoint, Eclipse says that there are no errors in my code, but when I try to run my application, it gets an error everytime the code gets to the Koblitz method.

Sorry, I know this kind of post is not educative, but I can't seem to find the problem in my source code.

public static ECPoint ConvertToPoint(String S){
    BigInteger neg = new BigInteger("-1");
    BigInteger one = BigInteger.ONE;
    BigInteger adder = BigInteger.ONE;
    BigInteger two = new BigInteger("2");
    BigInteger M,MK,Y,s = null, n = BigInteger.ONE,x,x2,x3 = null,B,B2,B3,g,g2,g3;
    BigInteger X = null;
    BigInteger k = new BigInteger("20");
    BigInteger P  = new BigInteger("751"); //Prime number (P)
    BigInteger a  = new BigInteger("-1"); //Curve Parameter a
    BigInteger b  = new BigInteger("188"); //Curve Parameter b
    int e = 1;
    int m = 0;
    int r;

    if (S=="B"){
        M = new BigInteger("11");
        MK = M.multiply(k);
        X = MK.add(adder); // x = m.k + i
        Y = (X.multiply(X).multiply(X)).add(a.multiply(X)).add(b); // y = (x^3 + ax + b)

        BigInteger C = (P.subtract(one)).divide(two);

        while((Y.mod(one) != BigInteger.ZERO) && (P.gcd(Y) != one) && Y.modPow(C, P) != one){
            adder = adder.add(one);
            X = MK.add(adder);   
            Y = (X.multiply(X).multiply(X)).add(a.multiply(X)).add(b);
        }   
        s = (P.subtract(one)).divide(two.pow(e));
        while (s.mod(two)== BigInteger.ZERO){
            e = e+1;
            s = (P.subtract(one)).divide(two.pow(e));
        }
        while (((n.modPow(C, P)) != neg.mod(P)) || (sqrt(n).mod(one)== BigInteger.ZERO)){
            n = n.add(one);
        }
        x = Y.modPow((s.add(one).divide(two)), P);
        B = Y.modPow(s, P);
        g = n.modPow(s, P);
        r = e;

        while (B.modPow(two.pow(m), P) != BigInteger.ONE){
            m = m+1;
        }
        if (m>0){
            x2 = x.multiply(g.pow(2^(r-m-1)));
            x3 = x2.mod(P);
            B2 = B.multiply(g.pow(2^(r-m)));
            B3 = B2.mod(P);
            g2 = g.pow(2^(r-m));
            g3 = g.mod(P);
            r = m;
        }
        else if (m==0){
            x3 = x;
            B3 = B;
            g3 = g;
            r = e;
        }
    }
       //This is when the program crashes
   ECPoint T = new ECPoint(X,x3); 
   return T;

Answer 1

There are two problems that I see with your code, although I don't know if this is causing your error:

• Again, as stated in my comment, don't compare Strings using ==. Use the equals(...) or the equalsIgnoreCase(...) method instead. Understand that == checks if the two objects are the same which is not what you're interested in. The methods on the other hand check if the two Strings have the same characters in the same order, and that's what matters here.
• The same goes for BigIntegers. You should either use the equals(...) method or the compareTo(...) method if you want to test for equality/inequality or > or <.
• It's a truism that is usually correct, that all reference types should be checked for equality using their equals(...) method. This is not so for primitives which have no methods. Also enums can be tested for equality via == and != although equals(...) will work fine for them.

---

As an aside, you will want to learn and use Java naming conventions. Variable names should all begin with a lower letter while class names with an upper case letter. Also you should avoid using trivial variable names such as b or s unless they are being used for trivial purposes such as the index of a for loop. Instead use names that have some meaning so that your code becomes self-commenting.

Following these suggestions as well as following good code formatting practices will allow others (such as us!) to better understand your code, and more importantly, will allow your future self to better understand just what you were thinking 6 months ago when you wrote the code.