code about to generate bitcoin address

GenerateKeypair.java

package org.example.keypair;

import org.bitcoinj.core.Base58;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.encoders.Hex;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;
import org.bouncycastle.util.io.pem.PemWriter;

import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.UnsupportedEncodingException;
import java.security.*;
import java.security.interfaces.ECPrivateKey;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECGenParameterSpec;
import java.security.spec.ECPoint;
import java.security.spec.ECPublicKeySpec;
import java.security.spec.PKCS8EncodedKeySpec;

/**
 * https://www.novixys.com/blog/generate-bitcoin-addresses-java/
 * https://academy.bit2me.com/pt/o-que-%C3%A9-curva-el%C3%ADptica-ecdsa/
 * https://crypto.stackexchange.com/questions/12823/ecdsa-vs-ecies-vs-ecdh
 */
public class GenerateKeypair {

    public static void main( String[] args ) throws Exception {
        Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
        KeyPair kp = null;
        PrivateKey pk = null;
        ECPrivateKey epvt = null;
        ECPublicKey epub = null;

        File walletkey = new File(".", "walletkey.pem");

        if(!walletkey.exists()) { // if there's no wallet, generate one
            kp = generate("ECDSA");
            epvt = (ECPrivateKey) kp.getPrivate();
            epub = (ECPublicKey) kp.getPublic();

            PemObject obj = new PemObject("PRIVATE KEY", kp.getPrivate().getEncoded());
            PemWriter writer = new PemWriter(new FileWriter(walletkey));
            writer.writeObject(obj);
            writer.close();
        } else { // otherwise, get file with privatekey and get the public key to generate address
            PemReader pr = new PemReader(new FileReader(walletkey));
            PemObject obj = pr.readPemObject();
            KeyFactory kf = KeyFactory.getInstance("ECDSA");

            PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(obj.getContent());
            epvt = (ECPrivateKey) kf.generatePrivate(spec);

            ECPublicKeySpec publicKeySpec = new ECPublicKeySpec(epvt.getParams().getGenerator(), epvt.getParams());
            epub = (ECPublicKey) kf.generatePublic(publicKeySpec);
        }

        // 3. The ECDSA Private Key
        String sepvt = adjustTo64(epvt.getS().toString(16)).toUpperCase();
        System.out.println("s[" + sepvt.length() + "]: " + sepvt);

        // 4. The ECDSA Public Key

        ECPoint pt = epub.getW();
        String sx = adjustTo64(pt.getAffineX().toString(16)).toUpperCase();
        String sy = adjustTo64(pt.getAffineY().toString(16)).toUpperCase();
        String bcPub = "04" + sx + sy;
        System.out.println("bcPub: " + bcPub);
        performHash(bcPub);
    }

    public static void performHash(String bcPub) throws NoSuchAlgorithmException, UnsupportedEncodingException, NoSuchProviderException {
        // SHA-256
        MessageDigest sha = MessageDigest.getInstance("SHA-256");
        byte[] s1 = sha.digest(bcPub.getBytes("UTF-8"));
        System.out.println("  sha: " + Hex.toHexString(s1).toUpperCase());

        // RipeMD160
        MessageDigest rmd = MessageDigest.getInstance("RipeMD160", BouncyCastleProvider.PROVIDER_NAME);
        byte[] r1 = rmd.digest(s1);

        // add 0x00
        byte[] r2 = new byte[r1.length + 1];
        r2[0] = 0;
        for (int i = 0 ; i < r1.length ; i++) r2[i+1] = r1[i];

        System.out.println("  rmd1: " + Hex.toHexString(r1).toUpperCase());
        System.out.println("  rmd2: " + Hex.toHexString(r2).toUpperCase());

        // 6. Repeat the SHA-256 Hashing Twice
        byte[] s2 = sha.digest(r2);
        System.out.println("  sha s2: " + Hex.toHexString(s2).toUpperCase());
        byte[] s3 = sha.digest(s2);
        System.out.println("  sha s3: " + Hex.toHexString(s3).toUpperCase());

        byte[] a1 = new byte[25];
        for (int i = 0 ; i < r2.length ; i++) a1[i] = r2[i];
        for (int i = 0 ; i < 5 ; i++) a1[20 + i] = s3[i];

        // 7. Encode the Address Using Base58
        System.out.println("  final adr: " + Base58.encode(a1));
    }

    public static KeyPair generate(String alg) throws NoSuchAlgorithmException, NoSuchProviderException, InvalidAlgorithmParameterException {
        ECGenParameterSpec ecSpec = new ECGenParameterSpec("secp256k1");
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(alg, BouncyCastleProvider.PROVIDER_NAME);
        keyPairGenerator.initialize(ecSpec, new SecureRandom());
        return keyPairGenerator.generateKeyPair();
    }

    static private String adjustTo64(String s) {
        switch(s.length()) {
            case 62: return "00" + s;
            case 63: return "0" + s;
            case 64: return s;
            default:
                throw new IllegalArgumentException("not a valid key: " + s);
        }
    }
}

outpu example

s[64]: 12BB8F00B59B95ED377E9D359CBE3989B69735E7D5157561E0F7A79C2BB1D5F9
bcPub: 0479BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8
  sha: 024A8E4F2343424FBF9CDCC22198477813DED7E2F94B04D6081274EA4FE3926B
  rmd1: 64BC8F970EF0D40E1F0C89B6471209A6E94E2CEA
  rmd2: 0064BC8F970EF0D40E1F0C89B6471209A6E94E2CEA
  sha s2: 22A7CB65FCE60BA391D92D51FE75B9E3E295BCAF5CBBD5A0C0DE0FE88544716B
  sha s3: 00DB0584FF5D443B11D4458F57B156A93FF3C24AF57B679D783FD9E6F59AC9C7
  final adr: 1ABeTdC2kg7YGXRH1UdMyfxUzwi9aArFav

Process finished with exit code 0

pom.xml

  <dependencies>
    <dependency>
      <groupId>org.bouncycastle</groupId>
      <artifactId>bcprov-jdk15on</artifactId>
      <version>1.68</version>
    </dependency>
    <dependency>
      <groupId>org.bitcoinj</groupId>
      <artifactId>bitcoinj-core</artifactId>
      <version>0.16.1</version>
    </dependency>
  </dependencies>

walletkey.pem

-----BEGIN PRIVATE KEY-----
MIGNAgEAMBAGByqGSM49AgEGBSuBBAAKBHYwdAIBAQQgEruPALWble03fp01nL45
ibaXNefVFXVh4PennCux1fmgBwYFK4EEAAqhRANCAASnSF+TXum0ZI6I4W8afjtB
6qIRbuHhXHgnTM2j9CZcroSvR5JZXXDI02+IdEaT8LkZ5UMb4sxLvWy1B5rXAZ5n
-----END PRIVATE KEY-----