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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.operator.bc;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.operator.AsymmetricKeyUnwrapper;
import org.bouncycastle.operator.GenericKey;
import org.bouncycastle.operator.OperatorException;

public abstract class BcAsymmetricKeyUnwrapper
    extends AsymmetricKeyUnwrapper
{
    private AsymmetricKeyParameter privateKey;

    public BcAsymmetricKeyUnwrapper(AlgorithmIdentifier encAlgId, AsymmetricKeyParameter privateKey)
    {
        super(encAlgId);

        this.privateKey = privateKey;
    }

    public GenericKey generateUnwrappedKey(AlgorithmIdentifier encryptedKeyAlgorithm, byte[] encryptedKey)
        throws OperatorException
    {
        AsymmetricBlockCipher keyCipher = createAsymmetricUnwrapper(this.getAlgorithmIdentifier().getAlgorithm());

        keyCipher.init(false, privateKey);
        try
        {
            byte[] key = keyCipher.processBlock(encryptedKey, 0, encryptedKey.length);

            if (encryptedKeyAlgorithm.getAlgorithm().equals(PKCSObjectIdentifiers.des_EDE3_CBC))
            {
                return new GenericKey(encryptedKeyAlgorithm, key);
            }
            else
            {
                return new GenericKey(encryptedKeyAlgorithm, key);
            }
        }
        catch (InvalidCipherTextException e)
        {
            throw new OperatorException("unable to recover secret key: " + e.getMessage(), e);
        }
    }

    protected abstract AsymmetricBlockCipher createAsymmetricUnwrapper(ASN1ObjectIdentifier algorithm);
}