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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.jwe;

import org.jose4j.jca.ProviderContext;
import org.jose4j.jwa.AlgorithmInfo;
import org.jose4j.jwk.OctetSequenceJsonWebKey;
import org.jose4j.jwx.Headers;
import org.jose4j.jwx.KeyValidationSupport;
import org.jose4j.keys.KeyPersuasion;
import org.jose4j.lang.ByteUtil;
import org.jose4j.lang.InvalidKeyException;
import org.jose4j.lang.JoseException;

import java.security.Key;

/**
 */
public class DirectKeyManagementAlgorithm extends AlgorithmInfo implements KeyManagementAlgorithm
{
    public DirectKeyManagementAlgorithm()
    {
        setAlgorithmIdentifier(KeyManagementAlgorithmIdentifiers.DIRECT);
        setKeyPersuasion(KeyPersuasion.SYMMETRIC);
        setKeyType(OctetSequenceJsonWebKey.KEY_TYPE);
    }

    public ContentEncryptionKeys manageForEncrypt(Key managementKey, ContentEncryptionKeyDescriptor cekDesc, Headers headers, byte[] cekOverride, ProviderContext providerContext) throws JoseException
    {
        KeyValidationSupport.cekNotAllowed(cekOverride, getAlgorithmIdentifier());
        byte[] cekBytes = managementKey.getEncoded();
        return new ContentEncryptionKeys(cekBytes, ByteUtil.EMPTY_BYTES);
    }

    public Key manageForDecrypt(Key managementKey, byte[] encryptedKey, ContentEncryptionKeyDescriptor cekDesc, Headers headers, ProviderContext providerContext) throws JoseException
    {
        if (encryptedKey.length != 0)
        {
            throw new InvalidKeyException("An empty octet sequence is to be used as the JWE Encrypted Key value when utilizing " +
                    "direct encryption but this JWE has " + encryptedKey.length + " octets in the encrypted key part.");
        }
        return managementKey;
    }

    @Override
    public void validateEncryptionKey(Key managementKey, ContentEncryptionAlgorithm contentEncryptionAlg) throws InvalidKeyException
    {
        validateKey(managementKey, contentEncryptionAlg);
    }

    private void validateKey(Key managementKey, ContentEncryptionAlgorithm contentEncryptionAlg) throws InvalidKeyException
    {
        KeyValidationSupport.notNull(managementKey);

        if (managementKey.getEncoded() != null)
        {
            int managementKeyByteLength = managementKey.getEncoded().length;
            int expectedByteLength = contentEncryptionAlg.getContentEncryptionKeyDescriptor().getContentEncryptionKeyByteLength();
            if (expectedByteLength != managementKeyByteLength)
            {
                throw new InvalidKeyException("Invalid key for " + getAlgorithmIdentifier() + " with "
                                  + contentEncryptionAlg.getAlgorithmIdentifier() +", expected a "
                                  + ByteUtil.bitLength(expectedByteLength)+ " bit key but a "
                                  + ByteUtil.bitLength(managementKeyByteLength) + " bit key was provided.");
            }
        }
    }

    @Override
    public void validateDecryptionKey(Key managementKey, ContentEncryptionAlgorithm contentEncryptionAlg) throws InvalidKeyException
    {
        validateKey(managementKey, contentEncryptionAlg);
    }

    @Override
    public boolean isAvailable()
    {
        return true;
    }
}