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The Bouncy Castle Java APIs for the OpenPGP Protocol. 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.openpgp;

import java.io.InputStream;

import org.bouncycastle.bcpg.AEADEncDataPacket;
import org.bouncycastle.bcpg.BCPGInputStream;
import org.bouncycastle.bcpg.InputStreamPacket;
import org.bouncycastle.bcpg.SymmetricEncIntegrityPacket;
import org.bouncycastle.bcpg.UnsupportedPacketVersionException;
import org.bouncycastle.openpgp.operator.PGPDataDecryptor;
import org.bouncycastle.openpgp.operator.PGPDataDecryptorFactory;

public class PGPSymmetricKeyEncryptedData
    extends PGPEncryptedData
{
    protected PGPSymmetricKeyEncryptedData(InputStreamPacket encData)
    {
        super(encData);
    }

    protected InputStream createDecryptionStream(PGPDataDecryptorFactory dataDecryptorFactory, PGPSessionKey sessionKey)
        throws PGPException
    {
        // OpenPGP v5
        if (encData instanceof AEADEncDataPacket)
        {
            AEADEncDataPacket aeadData = (AEADEncDataPacket)encData;

            if (aeadData.getAlgorithm() != sessionKey.getAlgorithm())
            {
                throw new PGPException("session key and AEAD algorithm mismatch");
            }

            PGPDataDecryptor dataDecryptor = dataDecryptorFactory.createDataDecryptor(
                aeadData, sessionKey);
            BCPGInputStream encIn = encData.getInputStream();

            return new BCPGInputStream(dataDecryptor.getInputStream(encIn));
        }
        else if (encData instanceof SymmetricEncIntegrityPacket)
        {
            SymmetricEncIntegrityPacket seipd = (SymmetricEncIntegrityPacket)encData;

            // OpenPGP v4 (SEIPD v1 with integrity protection)
            if (seipd.getVersion() == SymmetricEncIntegrityPacket.VERSION_1)
            {
                PGPDataDecryptor dataDecryptor = dataDecryptorFactory.createDataDecryptor(true, sessionKey.getAlgorithm(), sessionKey.getKey());
                return getDataStream(true, dataDecryptor);
            }

            // OpenPGP v6 (AEAD with SEIPD v2)
            else if (seipd.getVersion() == SymmetricEncIntegrityPacket.VERSION_2)
            {
                PGPDataDecryptor dataDecryptor = dataDecryptorFactory.createDataDecryptor(seipd, sessionKey);
                return new BCPGInputStream(dataDecryptor.getInputStream(encData.getInputStream()));
            }

            // Unsupported
            else
            {
                throw new UnsupportedPacketVersionException("Unsupported SEIPD packet version: " + seipd.getVersion());
            }
        }
        // OpenPGP v3,v4 (SED without integrity protection)
        else
        {
            PGPDataDecryptor dataDecryptor = dataDecryptorFactory.createDataDecryptor(false, sessionKey.getAlgorithm(), sessionKey.getKey());
            return getDataStream(false, dataDecryptor);
        }
    }

    private InputStream getDataStream(
        boolean withIntegrityPacket,
        PGPDataDecryptor dataDecryptor)
        throws PGPException
    {
        try
        {
            BCPGInputStream encIn = encData.getInputStream();
            encIn.mark(dataDecryptor.getBlockSize() + 2); // iv + 2 octets checksum
            if (processSymmetricEncIntegrityPacketDataStream(withIntegrityPacket, dataDecryptor, encIn))
            {
                encIn.reset();
                throw new PGPDataValidationException("data check failed.");
            }

            return encStream;
        }
        catch (PGPException e)
        {
            throw e;
        }
        catch (Exception e)
        {
            throw new PGPException("Exception creating cipher", e);
        }
    }
}