I need to derive an EC Public Key from an EC private key string without the "help" of any third party library.

The Private key is externally produced and provided and I need to get the Public Key to generate a Bitcoin address. As my project is working "offline" I do not need a library like Bouncy Castle for any other purpose, so I would like to eliminate it.

The following program is fully working and shows the (very short) example when working with Bouncy Castle to get a solution. The second part is the native Java solution with the kindly help from the routines by the so user SkateScout, for details see his answer https://stackoverflow.com/a/42797410/8166854.

Please keep in mind that this solution is working only for the Elliptic curve "secp256k1". You can check my keypair on https://gobittest.appspot.com/Address.

My question: is there any other solution available to avoid the mass of code for scalar operations?

import org.bouncycastle.jce.ECNamedCurveTable;

import org.bouncycastle.jce.spec.ECNamedCurveParameterSpec;

import java.math.BigInteger;

import java.security.*;

import java.security.interfaces.ECPrivateKey;

import java.security.interfaces.ECPublicKey;

import java.security.spec.*;

public class DerivePublicKeyFromPrivateKeyCurveSecp256k1 {

// get bouncycastle here: https://mvnrepository.com/artifact/org.bouncycastle/bcprov-jdk15on/1.65

// tested with version 15 1.65

final static BigInteger FieldP_2 = BigInteger.TWO; // constant for scalar operations

final static BigInteger FieldP_3 = BigInteger.valueOf(3); // constant for scalar operations

public static void main(String[] args) throws GeneralSecurityException {

System.out.println("Generate ECPublicKey from PrivateKey (String) for curve secp256k1");

System.out.println("Check keys with https://gobittest.appspot.com/Address");

// https://gobittest.appspot.com/Address

String privateKey = "D12D2FACA9AD92828D89683778CB8DFCCDBD6C9E92F6AB7D6065E8AACC1FF6D6";

String publicKeyExpected = "04661BA57FED0D115222E30FE7E9509325EE30E7E284D3641E6FB5E67368C2DB185ADA8EFC5DC43AF6BF474A41ED6237573DC4ED693D49102C42FFC88510500799";

System.out.println("\nprivatekey given : " + privateKey);

System.out.println("publicKeyExpected: " + publicKeyExpected);

// routine with bouncy castle

System.out.println("\nGenerate PublicKey from PrivateKey with BouncyCastle");

ECNamedCurveParameterSpec spec = ECNamedCurveTable.getParameterSpec("secp256k1"); // this ec curve is used for bitcoin operations

org.bouncycastle.math.ec.ECPoint pointQ = spec.getG().multiply(new BigInteger(1, hexStringToByteArray(privateKey)));

byte[] publickKeyByte = pointQ.getEncoded(false);

String publicKeyBc = byteArrayToHexString(publickKeyByte);

System.out.println("publicKeyExpected: " + publicKeyExpected);

System.out.println("publicKey BC : " + publicKeyBc);

System.out.println("publicKeys match : " + publicKeyBc.contentEquals(publicKeyExpected));

// regeneration of ECPublicKey with java native starts here

System.out.println("\nGenerate PublicKey from PrivateKey with Java native routines");

// the preset "303E.." only works for elliptic curve secp256k1

// see answer by user dave_thompson_085

// https://stackoverflow.com/questions/48832170/generate-ec-public-key-from-byte-array-private-key-in-native-java-7

String privateKeyFull = "303E020100301006072A8648CE3D020106052B8104000A042730250201010420" +

privateKey;

byte[] privateKeyFullByte = hexStringToByteArray(privateKeyFull);

System.out.println("privateKey full : " + privateKeyFull);

KeyFactory kecFactory = KeyFactory.getInstance("EC");

PrivateKey privateKeyNative = kecFactory.generatePrivate(new PKCS8EncodedKeySpec(privateKeyFullByte));

ECPrivateKey ecPrivateKeyNative = (ECPrivateKey) privateKeyNative;

ECPublicKey ecPublicKeyNative = getPublicKey(ecPrivateKeyNative);

byte[] ecPublicKeyNativeByte = ecPublicKeyNative.getEncoded();

String publicKeyNativeFull = byteArrayToHexString(ecPublicKeyNativeByte);

String publicKeyNativeHeader = publicKeyNativeFull.substring(0, 46);

String publicKeyNativeKey = publicKeyNativeFull.substring(46, 176);

System.out.println("ecPublicKeyFull : " + publicKeyNativeFull);

System.out.println("ecPublicKeyHeader: " + publicKeyNativeHeader);

System.out.println("ecPublicKeyKey : " + publicKeyNativeKey);

System.out.println("publicKeyExpected: " + publicKeyExpected);

System.out.println("publicKeys match : " + publicKeyNativeKey.contentEquals(publicKeyExpected));

}

private static String byteArrayToHexString(byte[] a) {