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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 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.jcajce;

import java.security.Key;
import java.security.PrivateKey;
import java.security.Provider;
import java.util.HashMap;
import java.util.Map;

import javax.crypto.Cipher;

import org.bouncycastle.asn1.cms.GenericHybridParameters;
import org.bouncycastle.asn1.cms.RsaKemParameters;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.crypto.util.DEROtherInfo;
import org.bouncycastle.jcajce.spec.KTSParameterSpec;
import org.bouncycastle.jcajce.util.DefaultJcaJceHelper;
import org.bouncycastle.jcajce.util.NamedJcaJceHelper;
import org.bouncycastle.jcajce.util.ProviderJcaJceHelper;
import org.bouncycastle.operator.AsymmetricKeyUnwrapper;
import org.bouncycastle.operator.GenericKey;
import org.bouncycastle.operator.OperatorException;
import org.bouncycastle.util.Arrays;

public class JceKTSKeyUnwrapper
    extends AsymmetricKeyUnwrapper
{
    private OperatorHelper helper = new OperatorHelper(new DefaultJcaJceHelper());
    private Map extraMappings = new HashMap();
    private PrivateKey privKey;
    private byte[] partyUInfo;
    private byte[] partyVInfo;

    public JceKTSKeyUnwrapper(AlgorithmIdentifier algorithmIdentifier, PrivateKey privKey, byte[] partyUInfo, byte[] partyVInfo)
    {
        super(algorithmIdentifier);

        this.privKey = privKey;
        this.partyUInfo = Arrays.clone(partyUInfo);
        this.partyVInfo = Arrays.clone(partyVInfo);
    }

    public JceKTSKeyUnwrapper setProvider(Provider provider)
    {
        this.helper = new OperatorHelper(new ProviderJcaJceHelper(provider));

        return this;
    }

    public JceKTSKeyUnwrapper setProvider(String providerName)
    {
        this.helper = new OperatorHelper(new NamedJcaJceHelper(providerName));

        return this;
    }

    public GenericKey generateUnwrappedKey(AlgorithmIdentifier encryptedKeyAlgorithm, byte[] encryptedKey)
        throws OperatorException
    {
        GenericHybridParameters params = GenericHybridParameters.getInstance(this.getAlgorithmIdentifier().getParameters());
        Cipher keyCipher = helper.createAsymmetricWrapper(this.getAlgorithmIdentifier().getAlgorithm(), extraMappings);
        String symmetricWrappingAlg = helper.getWrappingAlgorithmName(params.getDem().getAlgorithm());
        RsaKemParameters kemParameters = RsaKemParameters.getInstance(params.getKem().getParameters());
        int keySizeInBits = kemParameters.getKeyLength().intValue() * 8;
        Key sKey;

        try
        {
            DEROtherInfo otherInfo = new DEROtherInfo.Builder(params.getDem(), partyUInfo, partyVInfo).build();
            KTSParameterSpec ktsSpec = new KTSParameterSpec.Builder(symmetricWrappingAlg, keySizeInBits, otherInfo.getEncoded()).withKdfAlgorithm(kemParameters.getKeyDerivationFunction()).build();

            keyCipher.init(Cipher.UNWRAP_MODE, privKey, ktsSpec);

            sKey = keyCipher.unwrap(encryptedKey, helper.getKeyAlgorithmName(encryptedKeyAlgorithm.getAlgorithm()), Cipher.SECRET_KEY);
        }
        catch (Exception e)
        {
            throw new OperatorException("Unable to unwrap contents key: " + e.getMessage(), e);
        }

        return new JceGenericKey(encryptedKeyAlgorithm, sKey);
    }
}