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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 1.8 and up. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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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);
}