Java Crypto Libraries

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Cryptography in Java

• JCA, Bouncy Castle and Web3j:Hashes, ECC and ECDSA

JCA, Bouncy Castle and Web3j

• Cryptography in Java is based on the Java Cryptography Architecture (JCA)
  • Typical Java style: lot of boilerplate code
• Bouncy Castle is the leading Java cryptography library
  • Docs: https://www.bouncycastle.org/documentation.html
• Web3j – a simplified library for Ethereum and secp256k1
  • Web3j – https://github.com/web3j
  • The cryptographic functionality is in web3j/crypto

ECDSA in Java: Install the Crypto Libraries

• This Maven dependency will install the following libraries:
  • org.web3j.crypto– Ethereum style secp256k1 EC cryptography
  • org.bouncycastle– BouncyCastle crypto provider for Java

<dependency>
  <groupId>org.web3j</groupId>
  <artifactId>crypto</artifactId>
  <version>3.3.1</version>
</dependency>

ECDSA in Java: Initialize the Application

import org.bouncycastle.util.encoders.Hex;
import org.web3j.crypto.*;
import java.math.BigInteger;

ECDSA in Java: Generate / Load Keys

// Generate random private key
// BigInteger privKey = Keys.createEcKeyPair().getPrivateKey(); 

BigInteger privKey = new BigInteger(
 "97ddae0f3a25b92268175400149d65d6887b9cefaf28ea2c078e05cdc15a3c0a", 16);
BigInteger pubKey = Sign.publicKeyFromPrivate(privKey);
ECKeyPair keyPair = new ECKeyPair(privKey, pubKey);

System.out.println("Private key: " + privKey.toString(16));
System.out.println("Public key: " + pubKey.toString(16));
System.out.println("Public key (compressed): " +
  compressPubKey(pubKey));

ECDSA in Java: Sign Message

String msg = "Message for signing";
byte[] msgHash = Hash.sha3(msg.getBytes());
Sign.SignatureData signature =
  Sign.signMessage(msgHash, keyPair, false);

System.out.println("Msg: " + msg);
System.out.println("Msg hash: " + Hex.toHexString(msgHash));
System.out.printf(
  "Signature: [v = %d, r = %s, s = %s]\n",
  signature.getV() - 27,
  Hex.toHexString(signature.getR()),
  Hex.toHexString(signature.getS()));

ECDSA in Java: Verify Signature

BigInteger pubKeyRecovered =
  Sign.signedMessageToKey(msg.getBytes(), signature);
System.out.println("Recovered public key: " +
  pubKeyRecovered.toString(16));

boolean validSig = pubKey.equals(pubKeyRecovered);
System.out.println("Signature valid? " + validSig);