Example usage for org.bouncycastle.math.ec ECFieldElement.Fp toBigInteger

Introduction

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

Prototype

public abstract BigInteger toBigInteger();

Source Link

Usage

From source file:edu.biu.scapi.primitives.dlog.ECFpUtility.java

License:Open Source License

/**
 * This function finds the y coordinate of a point in the curve for a given x, if it exists.
 * @param params the parameters of the group
 * @param x/*from  w  ww. j  ava 2 s  .c  om*/
 * @return the y coordinate of point in the curve for a given x, if it exists
 *          else, null
 */
public BigInteger findYInCurveEquationForX(ECFpGroupParams params, BigInteger x) {

    /* get a, b, p from group params */
    BigInteger a = params.getA();
    BigInteger b = params.getB();
    BigInteger p = params.getP();

    // compute x^3
    BigInteger x3 = x.modPow(new BigInteger("3"), p);
    // compute x^3+ax+b
    BigInteger rightSide = x3.add(a.multiply(x)).add(b).mod(p);
    //try to compute y = square_root(x^3+ax+b)
    //If it exists return it
    //else, return null
    //We compute the square root via the ECFieldElement.Fp of Bouncy Castle, since BigInteger does not implement this function.
    //ECFieldElement.Fp ySquare = new ECFieldElement.Fp(params.getQ(), rightSide);
    ECFieldElement.Fp ySquare = new ECFieldElement.Fp(params.getP(), rightSide);

    //TODO I am not sure which one of the square roots it returns (if they exist). We need to check this!! (Yael)
    ECFieldElement.Fp y = (Fp) ySquare.sqrt();
    if (y != null) {
        return y.toBigInteger().mod(p);
    } else {
        return null;
    }
}

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