I’m trying to convert Ed25519 public key to x25519 public key in Java, using the lazysodium package:

import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import com.goterl.lazysodium.LazySodium;
import com.goterl.lazysodium.LazySodiumJava;
import com.goterl.lazysodium.SodiumJava;

public class KeyOps {

    public static PublicKey ed25519ToX25519(PublicKey ed25519PublicKey) throws NoSuchAlgorithmException, NoSuchProviderException, InvalidKeySpecException {
        byte[] x25519PublicKeyBytes = new byte[32];
        LazySodium lazySodium = new LazySodiumJava(new SodiumJava());
        byte[] ed25519PublicKeyBytes = new byte[32];
        System.arraycopy(ed25519PublicKey.getEncoded(), 12, ed25519PublicKeyBytes, 0, 32);
        lazySodium.convertPublicKeyEd25519ToCurve25519(x25519PublicKeyBytes, ed25519PublicKeyBytes);
        return KeyFactory.getInstance("X25519", "BC").generatePublic(new X509EncodedKeySpec(x25519PublicKeyBytes));
    }

}

… where ed25519PublicKey will be generated like:

KeyPair keyPair = KeyPairGenerator.getInstance("Ed25519", "BC").generateKeyPair();
PublicKey ed25519PublicKey = keyPair.getPublic();

However, I’m getting random errors, like one of below:

An exception occured while executing the Java class. encoded key spec not recognized: failed to construct sequence from byte[]: Extra data detected in stream

or

An exception occured while executing the Java class. encoded key spec not recognized: failed to construct sequence from byte[]: corrupted stream - out of bounds length found: 57 >= 54

(57 and 54 can be other values)

May I know what’s wrong here? How do I convert Ed25519 pk to x25519 pk, and convert it back to PublicKey format?