Convert Ed25519PrivateKeyParameters to PKCS8

I have generated an Ed25519 key pair using bouncy castle. For my own source of randomness I need to use BC.

Converting it to PKCS8, to be able to use the key without BC, the encoded key spec is not recognized.

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Goal

Ed25519PrivateKeyParameters.getEncoded() to PrivateKey (PKCS8EncodedKeySpec).

Do you know, that format / bin encoding BC use at class Ed25519PrivateKeyParameters?

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Ed25519PrivateKeyParameters

public byte[] getEncoded()

Source

Currently I am failing in converting it to PKCS8.

Not working:

KeySpec keySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
java.security.KeyFactory keyFactory = java.security.KeyFactory.getInstance("Ed25519", "BC");
PrivateKey privKey = keyFactory.generatePrivate(keySpec);

failing with

java.security.spec.InvalidKeySpecException: encoded key spec not recognized: failed to construct sequence from byte[]: long form definite-length more than 31 bits

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Extra

Generate a key

public static Ed25519PrivateKeyParameters createKeyBc() throws Exception {

        Security.addProvider(new BouncyCastleProvider());

        Ed25519KeyPairGenerator generator = new Ed25519KeyPairGenerator();
        Ed25519KeyGenerationParameters spec = new Ed25519KeyGenerationParameters(new SecureRandom());
        generator.init(spec);

        AsymmetricCipherKeyPair kp = generator.generateKeyPair();

        return (Ed25519PrivateKeyParameters) kp.getPrivate();
    }

Convert key

public static PrivateKey convertToJavaPrivateKey(Ed25519PrivateKeyParameters privateKeyParameters)
   throws Exception {

        byte[] privateKeyBytes = privateKeyParameters.getEncoded();

        KeySpec keySpec = new PKCS8EncodedKeySpec(privateKeyBytes);

        java.security.KeyFactory keyFactory = java.security.KeyFactory.getInstance("Ed25519", "BC");

        return keyFactory.generatePrivate(keySpec);
    }

So my goal:

Ed25519PrivateKeyParameters.getEncoded() should be converted to PKCS8.

Thank you for your answer

The following code illustrates one method of doing this conversion. I think the only step you were missing was PrivateKeyInfo privInfo = PrivateKeyInfoFactory.createPrivateKeyInfo(privateKeyParameters);

import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.generators.Ed25519KeyPairGenerator;
import org.bouncycastle.crypto.params.Ed25519KeyGenerationParameters;
import org.bouncycastle.crypto.params.Ed25519PrivateKeyParameters;
import org.bouncycastle.crypto.util.PrivateKeyInfoFactory;

import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.SecureRandom;
import java.security.spec.PKCS8EncodedKeySpec;

public class Ed25519PrivBcToJava1 {
    public static void main(String[] args) throws Exception {
        AsymmetricCipherKeyPair keyPair = generateEd25519KeyPair();
        Ed25519PrivateKeyParameters privateKeyParameters = (Ed25519PrivateKeyParameters) keyPair.getPrivate();
        PrivateKey privKey = convertToJavaPrivateKey(privateKeyParameters);
    }

    private static AsymmetricCipherKeyPair generateEd25519KeyPair() {
        SecureRandom rand = new SecureRandom();
        Ed25519KeyGenerationParameters kpgParams = new Ed25519KeyGenerationParameters(rand);
        Ed25519KeyPairGenerator kpg = new Ed25519KeyPairGenerator();
        kpg.init(kpgParams);
        return kpg.generateKeyPair();
    }

    public static PrivateKey convertToJavaPrivateKey(Ed25519PrivateKeyParameters privateKeyParameters) throws Exception {

        PrivateKeyInfo privInfo = PrivateKeyInfoFactory.createPrivateKeyInfo(privateKeyParameters);
        byte[] pkcs8EncodedBytes = privInfo.getEncoded();
        PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(pkcs8EncodedBytes);
//        System.out.println(Base64.getEncoder().encodeToString(pkcs8EncodedBytes));
        KeyFactory kf = KeyFactory.getInstance("Ed25519");
        return kf.generatePrivate(pkcs8KeySpec);
    }
}