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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.cms;

import org.spongycastle.asn1.DEROctetString;
import org.spongycastle.asn1.cms.IssuerAndSerialNumber;
import org.spongycastle.asn1.cms.KeyTransRecipientInfo;
import org.spongycastle.asn1.cms.RecipientIdentifier;
import org.spongycastle.asn1.cms.RecipientInfo;
import org.spongycastle.operator.AsymmetricKeyWrapper;
import org.spongycastle.operator.GenericKey;
import org.spongycastle.operator.OperatorException;

public abstract class KeyTransRecipientInfoGenerator
    implements RecipientInfoGenerator
{
    protected final AsymmetricKeyWrapper wrapper;

    private IssuerAndSerialNumber issuerAndSerial;
    private byte[] subjectKeyIdentifier;

    protected KeyTransRecipientInfoGenerator(IssuerAndSerialNumber issuerAndSerial, AsymmetricKeyWrapper wrapper)
    {
        this.issuerAndSerial = issuerAndSerial;
        this.wrapper = wrapper;
    }

    protected KeyTransRecipientInfoGenerator(byte[] subjectKeyIdentifier, AsymmetricKeyWrapper wrapper)
    {
        this.subjectKeyIdentifier = subjectKeyIdentifier;
        this.wrapper = wrapper;
    }

    public final RecipientInfo generate(GenericKey contentEncryptionKey)
        throws CMSException
    {
        byte[] encryptedKeyBytes;
        try
        {
            encryptedKeyBytes = wrapper.generateWrappedKey(contentEncryptionKey);
        }
        catch (OperatorException e)
        {
            throw new CMSException("exception wrapping content key: " + e.getMessage(), e);
        }

        RecipientIdentifier recipId;
        if (issuerAndSerial != null)
        {
            recipId = new RecipientIdentifier(issuerAndSerial);
        }
        else
        {
            recipId = new RecipientIdentifier(new DEROctetString(subjectKeyIdentifier));
        }

        return new RecipientInfo(new KeyTransRecipientInfo(recipId, wrapper.getAlgorithmIdentifier(),
            new DEROctetString(encryptedKeyBytes)));
    }
}