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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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/*
 * Copyright 2012-2017 Brian Campbell
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.jose4j.keys;

import org.jose4j.base64url.SimplePEMEncoder;
import org.jose4j.lang.JoseException;

import java.security.KeyFactory;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.Security;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;
import java.util.Set;

/**
 */
abstract class KeyPairUtil
{
    private static final String BEGIN_PUBLIC_KEY = "-----BEGIN PUBLIC KEY-----";
    private static final String END_PUBLIC_KEY = "-----END PUBLIC KEY-----";

    protected String provider;
    protected SecureRandom secureRandom;

    protected KeyPairUtil(String provider, SecureRandom secureRandom)
    {
        this.provider = provider;
        this.secureRandom = secureRandom;
    }

    abstract String getAlgorithm();

    protected KeyFactory getKeyFactory() throws JoseException
    {
        String agl = getAlgorithm();
        try
        {
            return provider == null ? KeyFactory.getInstance(agl) : KeyFactory.getInstance(agl, provider);
        }
        catch (NoSuchAlgorithmException e)
        {
            throw new JoseException("Couldn't find " + agl + " KeyFactory! " + e, e);
        }
        catch (NoSuchProviderException e)
        {
            throw new JoseException("Cannot get KeyFactory instance with provider " + provider, e);
        }
    }

    protected KeyPairGenerator getKeyPairGenerator() throws JoseException
    {
        String alg = getAlgorithm();
        try
        {
            return provider == null ? KeyPairGenerator.getInstance(alg) : KeyPairGenerator.getInstance(alg, provider);
        }
        catch (NoSuchAlgorithmException e)
        {
            throw new JoseException("Couldn't find " + alg + " KeyPairGenerator! " + e, e);
        }
        catch (NoSuchProviderException e)
        {
            throw new JoseException("Cannot get KeyPairGenerator instance with provider " + provider, e);
        }
    }

    public PublicKey fromPemEncoded(String pem) throws JoseException, InvalidKeySpecException
    {
        int beginIndex = pem.indexOf(BEGIN_PUBLIC_KEY) + BEGIN_PUBLIC_KEY.length();
        int endIndex = pem.indexOf(END_PUBLIC_KEY);
        String base64 = pem.substring(beginIndex, endIndex).trim();
        byte[] decode = SimplePEMEncoder.decode(base64);
        X509EncodedKeySpec spec = new X509EncodedKeySpec(decode);
        KeyFactory kf = getKeyFactory();
        return kf.generatePublic(spec);
    }

    public static String pemEncode(PublicKey publicKey)
    {
        byte[] encoded = publicKey.getEncoded(); // X509 SPKI
        return BEGIN_PUBLIC_KEY + "\r\n" + SimplePEMEncoder.encode(encoded) + END_PUBLIC_KEY;
    }

    public boolean isAvailable()
    {
        Set keyFactories = Security.getAlgorithms("KeyFactory");
        Set keyPairGenerators = Security.getAlgorithms("KeyPairGenerator");
        String algorithm = getAlgorithm();
        return keyPairGenerators.contains(algorithm) && keyFactories.contains(algorithm);
    }
}