Example usage for org.bouncycastle.math.ec ECFieldElement.F2m add

Introduction

In this page you can find the example usage for org.bouncycastle.math.ec ECFieldElement.F2m add.

Prototype

public abstract ECFieldElement add(ECFieldElement b);

Source Link

Usage

From source file:edu.biu.scapi.primitives.dlog.bc.BcDlogECF2m.java

License:Open Source License

/**
 * Checks if the given x and y represent a valid point on the given curve, 
 * i.e. if the point (x, y) is a solution of the curves equation.
 * @param params elliptic curve over F2m parameters
 * @param x coefficient of the point/*  ww w  .j a  v a 2s .  co  m*/
 * @param y coefficient of the point
 * @return true if the given x and y represented a valid point on the given curve
 */
boolean checkCurveMembership(ECF2mGroupParams params, BigInteger x, BigInteger y) {

    int m = params.getM(); // get the field size

    // get curve basis
    int[] k = new int[3];

    if (params instanceof ECF2mKoblitz) {
        getBasis(((ECF2mKoblitz) params).getCurve(), k);
    } else
        getBasis(params, k);

    // construct ECFieldElements from a,b,x,y. 
    // Elements in the binary field are polynomials so we can't treat them as regular BigInteger. 
    // We use BC library to create and deal with such field element.
    ECFieldElement.F2m xElement = new ECFieldElement.F2m(m, k[0], k[1], k[2], x);
    ECFieldElement.F2m yElement = new ECFieldElement.F2m(m, k[0], k[1], k[2], y);
    ECFieldElement.F2m a = new ECFieldElement.F2m(m, k[0], k[1], k[2], params.getA());
    ECFieldElement.F2m b = new ECFieldElement.F2m(m, k[0], k[1], k[2], params.getB());

    // Calculates the curve equation with the given x,y.

    // compute x^3
    ECFieldElement.F2m xPow2 = (F2m) xElement.square();
    ECFieldElement.F2m xPow3 = (F2m) xPow2.multiply(xElement);
    // compute ax^2
    ECFieldElement.F2m axPow2 = (F2m) a.multiply(xPow2);
    // compute x^3+ax^2+b
    ECFieldElement.F2m addition = (F2m) xPow3.add(axPow2);
    ECFieldElement.F2m rightSide = (F2m) addition.add(b);

    // compute xy
    ECFieldElement.F2m xy = (F2m) yElement.multiply(xElement);
    // compute y^2+xy
    ECFieldElement.F2m yPow2 = (F2m) yElement.square();
    ECFieldElement.F2m leftSide = (F2m) yPow2.add(xy);

    //if the the equation is solved - the point is in the elliptic curve and return true
    if (leftSide.equals(rightSide))
        return true;
    else
        return false;
}

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