Rebuild of ED25519 keys with Bouncy Castle (Java)

Question

The latest (beta) version of Bouncy Castle (bcprov-jdk15on-161b20.jar) supports ED25519 and ED448 EC cryptography for signing purposes. I setup this full working example and it works as expected.

My question: did I rebuild the private and public keys correctly as I didn't found any example in the bc-tests ? I expected that I have to use some Spec-functionality like "X509EncodedKeySpec pubKeySpec = new X509EncodedKeySpec(content)" for RSA-key rebuilding but my code is working.

package bc;
// original source: https://github.com/bcgit/bc-java/blob/master/core/src/test/java/org/bouncycastle/crypto/test/Ed25519Test.java
// needs bouncy castle beta: bcprov-jdk15on-161b20.jar (version 1.605)
// tested with Java 8 Build 191 x64
// this is a full working example for generating, signing, verififying with ed25519 keys
// code: https://github.com/java-crypto/Bouncy-Castle
import java.security.Provider;
import java.security.SecureRandom;
import java.security.Security;
import java.util.Arrays;
import java.io.UnsupportedEncodingException;
import javax.xml.bind.DatatypeConverter;
import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.CryptoException;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.Signer;
import org.bouncycastle.crypto.generators.Ed25519KeyPairGenerator;
import org.bouncycastle.crypto.params.Ed25519KeyGenerationParameters;
import org.bouncycastle.crypto.params.Ed25519PrivateKeyParameters;
import org.bouncycastle.crypto.params.Ed25519PublicKeyParameters;
import org.bouncycastle.crypto.signers.Ed25519Signer;
import org.bouncycastle.jce.provider.BouncyCastleProvider;

public class Ed25519Bc {

    public static void main(String[] args) throws DataLengthException, CryptoException, UnsupportedEncodingException {
        System.out.println("ED25519 with BC");
        Security.addProvider(new BouncyCastleProvider());
        Provider provider = Security.getProvider("BC");
        System.out.println("Provider          :" + provider.getName() + " Version: " + provider.getVersion());
        // generate ed25519 keys
        SecureRandom RANDOM = new SecureRandom();
        Ed25519KeyPairGenerator keyPairGenerator = new Ed25519KeyPairGenerator();
        keyPairGenerator.init(new Ed25519KeyGenerationParameters(RANDOM));
        AsymmetricCipherKeyPair asymmetricCipherKeyPair = keyPairGenerator.generateKeyPair();
        Ed25519PrivateKeyParameters privateKey = (Ed25519PrivateKeyParameters) asymmetricCipherKeyPair.getPrivate();
        Ed25519PublicKeyParameters publicKey = (Ed25519PublicKeyParameters) asymmetricCipherKeyPair.getPublic();
        // the message
        byte[] message = "Message to sign".getBytes("utf-8");
        // create the signature
        Signer signer = new Ed25519Signer();
        signer.init(true, privateKey);
        signer.update(message, 0, message.length);
        byte[] signature = signer.generateSignature();
        // verify the signature
        Signer verifier = new Ed25519Signer();
        verifier.init(false, publicKey);
        verifier.update(message, 0, message.length);
        boolean shouldVerify = verifier.verifySignature(signature);
        // output
        byte[] privateKeyEncoded = privateKey.getEncoded();
        byte[] publicKeyEncoded = publicKey.getEncoded();
        System.out.println("privateKey Length :" + privateKeyEncoded.length + " Data:"
                + DatatypeConverter.printHexBinary(privateKeyEncoded));
        System.out.println("publicKey Length  :" + publicKeyEncoded.length + " Data:"
                + DatatypeConverter.printHexBinary(publicKeyEncoded));
        System.out.println(
                "signature Length  :" + signature.length + " Data:" + DatatypeConverter.printHexBinary(signature));
        System.out.println("signature correct :" + shouldVerify);
        // rebuild the keys
        System.out.println("Rebuild the keys and verify the signature with rebuild public key");
        Ed25519PrivateKeyParameters privateKeyRebuild = new Ed25519PrivateKeyParameters(privateKeyEncoded, 0);
        Ed25519PublicKeyParameters publicKeyRebuild = new Ed25519PublicKeyParameters(publicKeyEncoded, 0);
        byte[] privateKeyRebuildEncoded = privateKeyRebuild.getEncoded();
        System.out.println("privateKey Length :" + privateKeyRebuild.getEncoded().length + " Data:"
                + DatatypeConverter.printHexBinary(privateKeyRebuild.getEncoded()));
        byte[] publicKeyRebuildEncoded = publicKeyRebuild.getEncoded();
        System.out.println("publicKey Length  :" + publicKeyRebuild.getEncoded().length + " Data:"
                + DatatypeConverter.printHexBinary(publicKeyRebuild.getEncoded()));
        // compare the keys
        System.out.println("private Keys Equal:" + Arrays.equals(privateKeyEncoded, privateKeyRebuildEncoded));
        System.out.println("public Keys Equal :" + Arrays.equals(publicKeyEncoded, publicKeyRebuildEncoded));
        // verify the signature with rebuild public key
        Signer verifierRebuild = new Ed25519Signer();
        verifierRebuild.init(false, publicKeyRebuild);
        verifierRebuild.update(message, 0, message.length);
        boolean shouldVerifyRebuild = verifierRebuild.verifySignature(signature);
        System.out.println("signature correct :" + shouldVerifyRebuild + " with rebuild public key");
    }
}

This is the console output that shows the properly rebuild keys:

ED25519 with BC
Provider          :BC Version: 1.605
privateKey Length :32 Data:F6A1F3A0B8F44EE64ACE636AFCA262F656160A728C042E3F98F9A0FD45717DE7
publicKey Length  :32 Data:858C2D6D5910B8AA7B52F7DF8E5806DAD3A7E43DC19C5A548F241BD8B82510FE
signature Length  :64 Data:4D402B0095F6692742DCACB0C2C39BFB70A5687F162DFAB3721A660D2259C96B972DF41B97502347E534FAD8D59496811CDFFFA831264ECBB1429439CF350E08
signature correct :true
Rebuild the keys and verify the signature with rebuild public key
privateKey Length :32 Data:F6A1F3A0B8F44EE64ACE636AFCA262F656160A728C042E3F98F9A0FD45717DE7
publicKey Length  :32 Data:858C2D6D5910B8AA7B52F7DF8E5806DAD3A7E43DC19C5A548F241BD8B82510FE
private Keys Equal:true
public Keys Equal :true
signature correct :true with rebuild public key

Answer 1

I'm also doing some research into Ed25519 so I've been looking at the BouncyCastle implementation along with Tink and libsodium; I certainly can't see anything wrong with how you're rebuilding the key pair, it seems be consistent with how BouncyCastle is using it in other places: OpenSSHPrivateKeyUtil and Ed25519Test.

Pure BouncyCastle

I re-wrote your test using the key pair and example signature from [RFC8037][3], which works fine as follows (Java 11):

import org.bouncycastle.crypto.CryptoException;
import org.bouncycastle.crypto.Signer;
import org.bouncycastle.crypto.params.Ed25519PrivateKeyParameters;
import org.bouncycastle.crypto.params.Ed25519PublicKeyParameters;
import org.bouncycastle.crypto.signers.Ed25519Signer;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.nio.charset.StandardCharsets;
import java.util.Base64;

import static org.junit.Assert.assertEquals;

public class BouncyCastleTest {

    private static final Logger LOG = LoggerFactory.getLogger(BouncyCastleTest.class);

    @Test
    public void testBouncyCastle() throws CryptoException  {
        // Test case defined in https://www.rfc-editor.org/rfc/rfc8037
        var msg = "eyJhbGciOiJFZERTQSJ9.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc".getBytes(StandardCharsets.UTF_8);
        var expectedSig = "hgyY0il_MGCjP0JzlnLWG1PPOt7-09PGcvMg3AIbQR6dWbhijcNR4ki4iylGjg5BhVsPt9g7sVvpAr_MuM0KAg";

        var privateKeyBytes = Base64.getUrlDecoder().decode("nWGxne_9WmC6hEr0kuwsxERJxWl7MmkZcDusAxyuf2A");
        var publicKeyBytes = Base64.getUrlDecoder().decode("11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo");

        var privateKey = new Ed25519PrivateKeyParameters(privateKeyBytes, 0);
        var publicKey = new Ed25519PublicKeyParameters(publicKeyBytes, 0);

        // Generate new signature
        Signer signer = new Ed25519Signer();
        signer.init(true, privateKey);
        signer.update(msg, 0, msg.length);
        byte[] signature = signer.generateSignature();
        var actualSignature = Base64.getUrlEncoder().encodeToString(signature).replace("=", "");

        LOG.info("Expected signature: {}", expectedSig);
        LOG.info("Actual signature  : {}", actualSignature);

        assertEquals(expectedSig, actualSignature);
    }
}

JCA

You can also achieve the same thing using JCA, in this example my keypair are in the 'raw' format (i.e. the X and D coordinates):

import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.edec.EdECObjectIdentifiers;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.nio.charset.StandardCharsets;
import java.security.KeyFactory;
import java.security.Security;
import java.security.Signature;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

import static org.junit.Assert.assertEquals;

public class Ed25519JCA {

    private static final Logger LOG = LoggerFactory.getLogger(Ed25519JCA.class);

    @Test
    public void testEd25519WithJCA() throws Exception {
        Security.addProvider(new BouncyCastleProvider());

        // Test case defined in https://www.rfc-editor.org/rfc/rfc8037
        var msg = "eyJhbGciOiJFZERTQSJ9.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc".getBytes(StandardCharsets.UTF_8);
        var expectedSig = "hgyY0il_MGCjP0JzlnLWG1PPOt7-09PGcvMg3AIbQR6dWbhijcNR4ki4iylGjg5BhVsPt9g7sVvpAr_MuM0KAg";

        // Both formatted as 32bit raw key values (x and d)
        var privateKeyBytes = Base64.getUrlDecoder().decode("nWGxne_9WmC6hEr0kuwsxERJxWl7MmkZcDusAxyuf2A");
        var publicKeyBytes = Base64.getUrlDecoder().decode("11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo");

        var keyFactory = KeyFactory.getInstance("Ed25519");

        // Wrap public key in ASN.1 format so we can use X509EncodedKeySpec to read it
        var pubKeyInfo = new SubjectPublicKeyInfo(new AlgorithmIdentifier(EdECObjectIdentifiers.id_Ed25519), publicKeyBytes);
        var x509KeySpec = new X509EncodedKeySpec(pubKeyInfo.getEncoded());

        var jcaPublicKey = keyFactory.generatePublic(x509KeySpec);

        // Wrap private key in ASN.1 format so we can use
        var privKeyInfo = new PrivateKeyInfo(new AlgorithmIdentifier(EdECObjectIdentifiers.id_Ed25519), new DEROctetString(privateKeyBytes));
        var pkcs8KeySpec = new PKCS8EncodedKeySpec(privKeyInfo.getEncoded());

        var jcaPrivateKey = keyFactory.generatePrivate(pkcs8KeySpec);

        // Generate new signature
        var dsa = Signature.getInstance("EdDSA"); // Edwards digital signature algorithm
        dsa.initSign(jcaPrivateKey);
        dsa.update(msg, 0, msg.length);
        byte[] signature = dsa.sign();
        var actualSignature = Base64.getUrlEncoder().encodeToString(signature).replace("=", "");

        LOG.info("Expected signature: {}", expectedSig);
        LOG.info("Actual signature  : {}", actualSignature);

        assertEquals(expectedSig, actualSignature);
    }

}

Just for completeness, you can also generate the keypair using JCA in the first place, this avoids lots of format conversion:

    Security.addProvider(new BouncyCastleProvider());
    var keyPair = KeyPairGenerator.getInstance("Ed25519").generateKeyPair();