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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.jwx;

import org.jose4j.keys.AesKey;
import org.jose4j.lang.ByteUtil;
import org.jose4j.lang.InvalidKeyException;

import java.security.Key;
import java.security.interfaces.RSAKey;

/**
 */
public class KeyValidationSupport
{
    public static final int MIN_RSA_KEY_LENGTH = 2048;

    public static void checkRsaKeySize(Key key) throws InvalidKeyException
    {
        if (key == null)
        {
            throw new InvalidKeyException("The RSA key must not be null.");
        }

        // some keys are valid for RSA operations with the provider
        // (like with Sun PKCS11 provider https://bitbucket.org/b_c/jose4j/issues/77 )
        // but aren't actually an instance of RSAKey so only check the length when we can
        if (key instanceof RSAKey)
        {
            RSAKey rsaKey = (RSAKey) key;
            int size = rsaKey.getModulus().bitLength();
            if  (size < MIN_RSA_KEY_LENGTH)
            {
                throw new InvalidKeyException("An RSA key of size "+MIN_RSA_KEY_LENGTH+
                        " bits or larger MUST be used with the all JOSE RSA algorithms (given key was only "+size+ " bits).");
            }
        }
    }

    public static  K castKey(Key key, Class type) throws InvalidKeyException
    {
        notNull(key);

        try
        {
            return type.cast(key);
        }
        catch (ClassCastException e)
        {
            throw new InvalidKeyException("Invalid key " + e);
        }
    }

    public static void notNull(Key key) throws InvalidKeyException
    {
        if (key == null)
        {
            throw new InvalidKeyException("The key must not be null.");
        }
    }

    public static void cekNotAllowed(byte[] cekOverride, String alg) throws InvalidKeyException
    {
        if (cekOverride != null)
        {
            throw new InvalidKeyException("An explicit content encryption key cannot be used with " + alg);
        }
    }

    public static void validateAesWrappingKey(Key managementKey, String joseAlg, int expectedKeyByteLength) throws InvalidKeyException
    {
        KeyValidationSupport.notNull(managementKey);

        String alg = managementKey.getAlgorithm();

        if (!AesKey.ALGORITHM.equals(alg))
        {
            throw new InvalidKeyException("Invalid key for JWE " + joseAlg + ", expected an "
                    + AesKey.ALGORITHM+ " key but an " + alg + " key was provided.");
        }

        if (managementKey.getEncoded() != null)
        {
            int managementKeyByteLength = managementKey.getEncoded().length;
            if (managementKeyByteLength != expectedKeyByteLength)
            {
                throw new InvalidKeyException("Invalid key for JWE " + joseAlg + ", expected a "
                        + ByteUtil.bitLength(expectedKeyByteLength)+ " bit key but a "
                        + ByteUtil.bitLength(managementKeyByteLength) + " bit key was provided.");
            }
        }
    }
}