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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 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.cms;

import java.io.ByteArrayInputStream;
import java.io.IOException;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1Encoding;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.util.io.Streams;

public abstract class RecipientInformation
{
    protected RecipientId rid;
    protected AlgorithmIdentifier   keyEncAlg;
    protected AlgorithmIdentifier messageAlgorithm;
    protected CMSSecureReadable     secureReadable;

    private AuthAttributesProvider additionalData;

    private byte[] resultMac;
    private RecipientOperator     operator;

    RecipientInformation(
        AlgorithmIdentifier keyEncAlg,
        AlgorithmIdentifier messageAlgorithm,
        CMSSecureReadable secureReadable,
        AuthAttributesProvider additionalData)
    {
        this.keyEncAlg = keyEncAlg;
        this.messageAlgorithm = messageAlgorithm;
        this.secureReadable = secureReadable;
        this.additionalData = additionalData;
    }

    public RecipientId getRID()
    {
        return rid;
    }

    private byte[] encodeObj(
        ASN1Encodable obj)
        throws IOException
    {
        if (obj != null)
        {
            return obj.toASN1Primitive().getEncoded();
        }

        return null;
    }

    /**
     * Return the key encryption algorithm details for the key in this recipient.
     *
     * @return AlgorithmIdentifier representing the key encryption algorithm.
     */
    public AlgorithmIdentifier getKeyEncryptionAlgorithm()
    {
        return keyEncAlg;
    }

    /**
     * return the object identifier for the key encryption algorithm.
     *
     * @return OID for key encryption algorithm.
     */
    public String getKeyEncryptionAlgOID()
    {
        return keyEncAlg.getAlgorithm().getId();
    }

    /**
     * return the ASN.1 encoded key encryption algorithm parameters, or null if
     * there aren't any.
     *
     * @return ASN.1 encoding of key encryption algorithm parameters.
     */
    public byte[] getKeyEncryptionAlgParams()
    {
        try
        {
            return encodeObj(keyEncAlg.getParameters());
        }
        catch (Exception e)
        {
            throw new RuntimeException("exception getting encryption parameters " + e);
        }
    }

    /**
     * Return the content digest calculated during the read of the content if one has been generated. This will
     * only happen if we are dealing with authenticated data and authenticated attributes are present.
     *
     * @return byte array containing the digest.
     */
    public byte[] getContentDigest()
    {
        if (secureReadable instanceof CMSEnvelopedHelper.CMSDigestAuthenticatedSecureReadable)
        {
            return ((CMSEnvelopedHelper.CMSDigestAuthenticatedSecureReadable)secureReadable).getDigest();
        }

        return null;
    }

    /**
     * Return the MAC calculated for the recipient. Note: this call is only meaningful once all
     * the content has been read.
     *
     * @return  byte array containing the mac.
     */
    public byte[] getMac()
    {
        if (resultMac == null)
        {
            if (operator.isMacBased())
            {
                if (additionalData != null)
                {
                    try
                    {
                        Streams.drain(operator.getInputStream(new ByteArrayInputStream(additionalData.getAuthAttributes().getEncoded(ASN1Encoding.DER))));
                    }
                    catch (IOException e)
                    {
                        throw new IllegalStateException("unable to drain input: " + e.getMessage());
                    }
                }
                resultMac = operator.getMac();
            }
        }

        return resultMac;
    }

    /**
     * Return the decrypted/encapsulated content in the EnvelopedData after recovering the content
     * encryption/MAC key using the passed in Recipient.
     *
     * @param recipient recipient object to use to recover content encryption key
     * @return  the content inside the EnvelopedData this RecipientInformation is associated with.
     * @throws CMSException if the content-encryption/MAC key cannot be recovered.
     */
    public byte[] getContent(
        Recipient recipient)
        throws CMSException
    {
        try
        {
            return CMSUtils.streamToByteArray(getContentStream(recipient).getContentStream());
        }
        catch (IOException e)
        {
            throw new CMSException("unable to parse internal stream: " + e.getMessage(), e);
        }
    }

    /**
     * Return the content type of the encapsulated data accessed by this recipient.
     *
     * @return the content type OID.
     */
    public ASN1ObjectIdentifier getContentType()
    {
        return secureReadable.getContentType();
    }

    /**
     * Return a CMSTypedStream representing the content in the EnvelopedData after recovering the content
     * encryption/MAC key using the passed in Recipient.
     *
     * @param recipient recipient object to use to recover content encryption key
     * @return  the content inside the EnvelopedData this RecipientInformation is associated with.
     * @throws CMSException if the content-encryption/MAC key cannot be recovered.
     */
    public CMSTypedStream getContentStream(Recipient recipient)
        throws CMSException, IOException
    {
        operator = getRecipientOperator(recipient);

        if (additionalData != null)
        {
            if (additionalData.isAead())
            {
                // TODO: this needs to be done after reading the encrypted data
                operator.getAADStream().write(additionalData.getAuthAttributes().getEncoded(ASN1Encoding.DER));

                return new CMSTypedStream(secureReadable.getContentType(), operator.getInputStream(secureReadable.getInputStream()));
            }
            else
            {
                return new CMSTypedStream(secureReadable.getContentType(), secureReadable.getInputStream());
            }
        }

        return new CMSTypedStream(secureReadable.getContentType(), operator.getInputStream(secureReadable.getInputStream()));
    }

    protected abstract RecipientOperator getRecipientOperator(Recipient recipient)
        throws CMSException, IOException;
}