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The jose.4.j library is a robust and easy to use open source implementation of JSON Web Token (JWT) and the JOSE specification suite (JWS, JWE, and JWK). It is written in Java and relies solely on the JCA APIs for cryptography. Please see https://bitbucket.org/b_c/jose4j/wiki/Home for more info, examples, etc..

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package org.jose4j.keys;

import org.jose4j.lang.ByteUtil;
import org.jose4j.lang.JoseException;

import java.math.BigInteger;
import java.security.Key;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.interfaces.EdECPrivateKey;
import java.security.interfaces.EdECPublicKey;
import java.security.spec.EdECPoint;
import java.security.spec.EdECPrivateKeySpec;
import java.security.spec.EdECPublicKeySpec;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.NamedParameterSpec;
import java.util.Arrays;
import java.util.Optional;

public class EdDsaKeyUtil extends OctetKeyPairUtil
{
    public static final String ED25519 = "Ed25519";
    public static final String ED448 = "Ed448";

    private static final byte BYTE_01111111 = (byte)127;
    private static final byte BYTE_10000000 = (byte)-128;
    private static final byte BYTE_00000000 = (byte)0;

    public EdDsaKeyUtil()
    {
        this(null, null);
    }

    public EdDsaKeyUtil(String provider, SecureRandom secureRandom)
    {
        super(provider, secureRandom);
    }

    @Override
    String getAlgorithm()
    {
        return "EDDSA";
    }

    public byte[] rawPublicKey(Key key)
    {
        EdECPublicKey edECPublicKey = (EdECPublicKey) key;
        EdECPoint edEdPoint = edECPublicKey.getPoint();
        BigInteger y = edEdPoint.getY();
        byte[] yBytes = y.toByteArray();
        byte[] yReversedBytes = ByteUtil.reverse(yBytes);
        int byteLen = edECPublicKey.getParams().getName().equals(ED25519) ? 32 : 57; // Ed25519 / Ed448
        if (yReversedBytes.length != byteLen)
        {
            yReversedBytes = Arrays.copyOf(yReversedBytes, byteLen);
        }
        byte byteToOrWith = (edEdPoint.isXOdd() ? BYTE_10000000 : BYTE_00000000);
        yReversedBytes[yReversedBytes.length - 1] |= byteToOrWith;
        return yReversedBytes;
    }

    public byte[] rawPrivateKey(PrivateKey privateKey)
    {
        EdECPrivateKey edECPrivateKey = (EdECPrivateKey) privateKey;
        Optional optionalBytes = edECPrivateKey.getBytes();
        return optionalBytes.orElse(ByteUtil.EMPTY_BYTES);
    }

    @Override
    public EdECPublicKey publicKey(byte[] publicKeyBytes, String name) throws JoseException
    {
        publicKeyBytes = publicKeyBytes.clone();
        byte rightByte = publicKeyBytes[publicKeyBytes.length - 1];
        publicKeyBytes[publicKeyBytes.length - 1] &= BYTE_01111111;
        boolean xIsOdd = (rightByte & BYTE_10000000) != 0;
        publicKeyBytes = ByteUtil.reverse(publicKeyBytes);
        BigInteger y = BigEndianBigInteger.fromBytes(publicKeyBytes);

        NamedParameterSpec paramSpec = getNamedParameterSpec(name);
        EdECPoint ep = new EdECPoint(xIsOdd, y);
        EdECPublicKeySpec keySpec = new EdECPublicKeySpec(paramSpec, ep);

        try
        {
            PublicKey publicKey = getKeyFactory().generatePublic(keySpec);
            return (EdECPublicKey) publicKey;
        }
        catch (InvalidKeySpecException e)
        {
            throw new JoseException("Invalid key spec: " + e, e);
        }
    }

    @Override
    public EdECPrivateKey privateKey(byte[] privateKeyBytes, String name) throws JoseException
    {
        NamedParameterSpec paramSpec = getNamedParameterSpec(name);
        EdECPrivateKeySpec privateKeySpec = new EdECPrivateKeySpec(paramSpec, privateKeyBytes);
        try
        {
            PrivateKey privateKey = getKeyFactory().generatePrivate(privateKeySpec);
            return (EdECPrivateKey) privateKey;
        }
        catch (InvalidKeySpecException e)
        {
            throw new JoseException("Invalid key spec: " + e, e);
        }
    }

    static public boolean isEdECPublicKey(Key key)
    {
        try
        {
            return key instanceof EdECPublicKey;
        }
        catch (NoClassDefFoundError e)
        {
            return false;
        }
    }


    static public boolean isEdECPrivateKey(Key key)
    {
        try
        {
            return key instanceof EdECPrivateKey;
        }
        catch (NoClassDefFoundError e)
        {
            return false;
        }
    }
}