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

Some more improvement to the answer of @markw on Generate ECPublicKey from ECPrivateKey, extended with curve name detection by @bas-goossen from here: How to find the matching curve name from an ECPublicKey

public static ECPublicKey publicFromPrivate(final ECPrivateKey privateKey) throws Exception {
    ECParameterSpec params = privateKey.getParams();
    org.bouncycastle.jce.spec.ECParameterSpec bcSpec = org.bouncycastle.jcajce.provider.asymmetric.util.EC5Util
        .convertSpec(params, false);
    org.bouncycastle.math.ec.ECPoint q = bcSpec.getG().multiply(privateKey.getS());
    org.bouncycastle.math.ec.ECPoint bcW = bcSpec.getCurve().decodePoint(q.getEncoded(false));
    ECPoint w = new ECPoint(
        bcW.getAffineXCoord().toBigInteger(),
        bcW.getAffineYCoord().toBigInteger());
    ECPublicKeySpec keySpec = new ECPublicKeySpec(w, tryFindNamedCurveSpec(params));
    return (ECPublicKey) KeyFactory
        .getInstance("EC", org.bouncycastle.jce.provider.BouncyCastleProvider.PROVIDER_NAME)
        .generatePublic(keySpec);
}

@SuppressWarnings("unchecked")
public static ECParameterSpec tryFindNamedCurveSpec(ECParameterSpec params) {
    org.bouncycastle.jce.spec.ECParameterSpec bcSpec
        = org.bouncycastle.jcajce.provider.asymmetric.util.EC5Util.convertSpec(params, false);
    for (Object name : Collections.list(org.bouncycastle.jce.ECNamedCurveTable.getNames())) {
        org.bouncycastle.jce.spec.ECNamedCurveParameterSpec bcNamedSpec
            = org.bouncycastle.jce.ECNamedCurveTable.getParameterSpec((String) name);
        if (bcNamedSpec.getN().equals(bcSpec.getN())
            && bcNamedSpec.getH().equals(bcSpec.getH())
            && bcNamedSpec.getCurve().equals(bcSpec.getCurve())
            && bcNamedSpec.getG().equals(bcSpec.getG())) {
            return new org.bouncycastle.jce.spec.ECNamedCurveSpec(
                bcNamedSpec.getName(),
                bcNamedSpec.getCurve(),
                bcNamedSpec.getG(),
                bcNamedSpec.getN(),
                bcNamedSpec.getH(),
                bcNamedSpec.getSeed());
        }
    }
    return params;
}