Codes to generate a public key in an elliptic curve algorithm using a given private key

Question

I need to implement ECC (Elliptic Curve Cryptography) algorithm using jdk 1.7. I tried using bouncy castle, sunEC, but all of them gave errors and errors. My target is to ge

Answer 1

i use this method to recover ECPublicKey from ECPrivateKey. At the end the main Routine prove that it works. It is plain java without any external library.

public final class MULT {
private static ECPoint doublePoint(final BigInteger p, final BigInteger a, final ECPoint R) {
    if (R.equals(ECPoint.POINT_INFINITY)) return R;
    BigInteger slope = (R.getAffineX().pow(2)).multiply(FieldP._3);
    slope = slope.add(a);
    slope = slope.multiply((R.getAffineY().multiply(FieldP._2)).modInverse(p));
    final BigInteger Xout = slope.pow(2).subtract(R.getAffineX().multiply(FieldP._2)).mod(p);
    final BigInteger Yout = (R.getAffineY().negate()).add(slope.multiply(R.getAffineX().subtract(Xout))).mod(p);
    return new ECPoint(Xout, Yout);
}

private static ECPoint addPoint   (final BigInteger p, final BigInteger a, final ECPoint r, final ECPoint g) {
    if (r.equals(ECPoint.POINT_INFINITY)) return g;
    if (g.equals(ECPoint.POINT_INFINITY)) return r;
    if (r==g || r.equals(g)) return doublePoint(p, a, r);
    final BigInteger gX    = g.getAffineX();
    final BigInteger sY    = g.getAffineY();
    final BigInteger rX    = r.getAffineX();
    final BigInteger rY    = r.getAffineY();
    final BigInteger slope = (rY.subtract(sY)).multiply(rX.subtract(gX).modInverse(p)).mod(p);
    final BigInteger Xout  = (slope.modPow(FieldP._2, p).subtract(rX)).subtract(gX).mod(p);
    BigInteger Yout =   sY.negate().mod(p);
    Yout = Yout.add(slope.multiply(gX.subtract(Xout))).mod(p);
    return new ECPoint(Xout, Yout);
}

public static ECPoint scalmult   (final EllipticCurve curve, final ECPoint g, final BigInteger kin) {
    final ECField         field    = curve.getField();
    if(!(field instanceof ECFieldFp)) throw new UnsupportedOperationException(field.getClass().getCanonicalName());
    final BigInteger p = ((ECFieldFp)field).getP();
    final BigInteger a = curve.getA();
    ECPoint R = ECPoint.POINT_INFINITY;
    BigInteger k = kin.mod(p);
    final int length = k.bitLength();
    final byte[] binarray = new byte[length];
    for(int i=0;i<=length-1;i++){
        binarray[i] = k.mod(FieldP._2).byteValue();
        k = k.shiftRight(1);
    }
    for(int i = length-1;i >= 0;i--){
        R = doublePoint(p, a, R);
        if(binarray[i]== 1) R = addPoint(p, a, R, g);
    }
    return R;
}

public static ECPublicKey getPublicKey(final ECPrivateKey pk) throws GeneralSecurityException {
    final ECParameterSpec params = pk.getParams();
    final ECPoint w = scalmult(params.getCurve(), pk.getParams().getGenerator(), pk.getS());
    final KeyFactory kg = KeyFactory.getInstance("EC");
    return (ECPublicKey)kg.generatePublic (new ECPublicKeySpec (w, params));
}

}