Example usage for org.bouncycastle.math.ec ECFieldElement sqrt

Introduction

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

Prototype

public abstract ECFieldElement sqrt();

Source Link

Usage

From source file:com.github.horrorho.inflatabledonkey.crypto.ec.ECPointsCompact.java

License:Open Source License

public static BigInteger y(ECCurve curve, BigInteger x) {
    // Andrey Jivsov https://www.ietf.org/archive/id/draft-jivsov-ecc-compact-05.txt.
    ECFieldElement X = curve.fromBigInteger(x);
    ECFieldElement rhs = X.square().add(curve.getA()).multiply(X).add(curve.getB());

    // y' = sqrt( C(x) ), where y'>0
    ECFieldElement yTilde = rhs.sqrt();

    if (yTilde == null) {
        throw new IllegalArgumentException("invalid point compression");
    }/*from ww  w.  ja  va 2 s.c  o m*/

    // y = min(y',p-y')
    BigInteger yT = yTilde.toBigInteger();
    BigInteger yTn = yTilde.negate().toBigInteger();
    BigInteger y = yT.compareTo(yTn) == -1 ? yT : yTn;

    return y;
}

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From source file:com.github.horrorho.inflatabledonkey.crypto.ec.ECPointsCompact.java

License:Open Source License

@Deprecated
public static ECPoint decompressFPPoint(ECCurve curve, BigInteger X) {
    // See Andrey Jivsov https://www.ietf.org/archive/id/draft-jivsov-ecc-compact-05.txt.
    ECFieldElement x = curve.fromBigInteger(X);
    ECFieldElement rhs = x.square().add(curve.getA()).multiply(x).add(curve.getB());

    // y' = sqrt( C(x) ), where y'>0
    ECFieldElement yTilde = rhs.sqrt();

    if (yTilde == null) {
        throw new IllegalArgumentException("invalid point compression");
    }//  w  ww . ja  va 2  s  .co  m

    // y = min(y',p-y')
    BigInteger yT = yTilde.toBigInteger();
    BigInteger yTn = yTilde.negate().toBigInteger();
    BigInteger y = yT.compareTo(yTn) == -1 ? yT : yTn;

    // Q=(x,y) is the canonical representation of the point
    ECPoint Q = curve.createPoint(X, y);

    return Q;
}

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From source file:org.ScripterRon.BitcoinCore.ECKey.java

License:Apache License

/**
 * Decompress a compressed public key (x coordinate and low-bit of y-coordinate).
 *
 * @param       xBN                 X-coordinate
 * @param       yBit                Sign of Y-coordinate
 * @return                          Uncompressed public key
 *//*from ww w .j  a v  a  2s.  c  o  m*/
private static ECPoint decompressKey(BigInteger xBN, boolean yBit) {
    SecP256K1Curve curve = (SecP256K1Curve) ecParams.getCurve();
    ECFieldElement x = curve.fromBigInteger(xBN);
    ECFieldElement alpha = x.multiply(x.square().add(curve.getA())).add(curve.getB());
    ECFieldElement beta = alpha.sqrt();
    if (beta == null)
        throw new IllegalArgumentException("Invalid point compression");
    ECPoint ecPoint;
    BigInteger nBeta = beta.toBigInteger();
    if (nBeta.testBit(0) == yBit) {
        ecPoint = curve.createPoint(x.toBigInteger(), nBeta);
    } else {
        ECFieldElement y = curve.fromBigInteger(curve.getQ().subtract(nBeta));
        ecPoint = curve.createPoint(x.toBigInteger(), y.toBigInteger());
    }
    return ecPoint;
}

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