public InputStream getDataStream(
        PublicKeyDataDecryptorFactory dataDecryptorFactory)
        throws PGPException
    {
        byte[] sessionData = dataDecryptorFactory.recoverSessionData(keyData.getAlgorithm(), keyData.getEncSessionKey());

        if (!confirmCheckSum(sessionData))
        {
            throw new PGPKeyValidationException("key checksum failed");
        }

        if (sessionData[0] != SymmetricKeyAlgorithmTags.NULL)
        {
            try
            {
                boolean      withIntegrityPacket = encData instanceof SymmetricEncIntegrityPacket;
                byte[]       sessionKey = new byte[sessionData.length - 3];

                System.arraycopy(sessionData, 1, sessionKey, 0, sessionKey.length);

                PGPDataDecryptor dataDecryptor = dataDecryptorFactory.createDataDecryptor(withIntegrityPacket, sessionData[0] & 0xff, sessionKey);

                encStream = new BCPGInputStream(dataDecryptor.getInputStream(encData.getInputStream()));

                if (withIntegrityPacket)
                {
                    truncStream = new TruncatedStream(encStream);

                    integrityCalculator = dataDecryptor.getIntegrityCalculator();

                    encStream = new TeeInputStream(truncStream, integrityCalculator.getOutputStream());
                }

                byte[] iv = new byte[dataDecryptor.getBlockSize()];

                for (int i = 0; i != iv.length; i++)
                {
                    int    ch = encStream.read();

                    if (ch < 0)
                    {
                        throw new EOFException("unexpected end of stream.");
                    }

                    iv[i] = (byte)ch;
                }

                int    v1 = encStream.read();
                int    v2 = encStream.read();

                if (v1 < 0 || v2 < 0)
                {
                    throw new EOFException("unexpected end of stream.");
                }

                //
                // some versions of PGP appear to produce 0 for the extra
                // bytes rather than repeating the two previous bytes
                //
                /*
                             * Commented out in the light of the oracle attack.
                            if (iv[iv.length - 2] != (byte)v1 && v1 != 0)
                            {
                                throw new PGPDataValidationException("data check failed.");
                            }

                            if (iv[iv.length - 1] != (byte)v2 && v2 != 0)
                            {
                                throw new PGPDataValidationException("data check failed.");
                            }
                            */

                return encStream;
            }
            catch (PGPException e)
            {
                throw e;
            }
            catch (Exception e)
            {
                throw new PGPException("Exception starting decryption", e);
            }
        }
        else
        {
            return encData.getInputStream();
        }
    }