解密文件与facebooks隐藏库

Question

我正在尝试使用隐藏的facebook库来加密和解密某些音频文件。 因此加密阶段没有问题，但随后的解密阶段始终以以下错误结束：

com.facebook.crypto.cipher.NativeGCMCipherException: decryptFinal
   at com.facebook.crypto.cipher.NativeGCMCipher.decryptFinal(NativeGCMCipher.java:108)
   at com.facebook.crypto.streams.NativeGCMCipherInputStream.ensureTagValid(NativeGCMCipherInputStream.java:126)
   at com.facebook.crypto.streams.NativeGCMCipherInputStream.read(NativeGCMCipherInputStream.java:91)
   at com.facebook.crypto.streams.NativeGCMCipherInputStream.read(NativeGCMCipherInputStream.java:76)

这是我的代码：

public class Main extends Activity {

private Button btnEncrypt, btnDecrypt, btnPlay, btnStop;
private final String mediaPath = Environment.getExternalStorageDirectory().getPath();
private File file;
private String media;
private NativeCryptoLibrary mNativeCryptoLibrary;
private Crypto crypto;
private keyChain key;
private Entity entity;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btnEncrypt = (Button) findViewById(R.id.btnEncrypt);
    btnDecrypt = (Button) findViewById(R.id.btnDecrypt);

    mNativeCryptoLibrary = new SystemNativeCryptoLibrary();
    key = new keyChain();

    crypto = new Crypto(key, mNativeCryptoLibrary);

    if (!crypto.isAvailable()) {
        Log.e("ERROR: ", "crypto not available!");
        return;
    }

    media = mediaPath+"/example.mp3";
    file = new File(media);
    entity = new Entity("TEST");

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    btnEncrypt.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            OutputStream fileStream, outputStream;
            try {
                final byte[] encrypt = new byte[(int) file.length()];
                fileStream = new BufferedOutputStream(new FileOutputStream(media));
                // encrypt
                outputStream = crypto.getCipherOutputStream(fileStream, entity);
                outputStream.write(encrypt);
                outputStream.close();
            } catch (FileNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (CryptoInitializationException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (KeyChainException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    });

    btnDecrypt.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            FileInputStream fileStream;
            OutputStream out;
            try {
                fileStream = new FileInputStream(media);
                InputStream inputStream = crypto.getCipherInputStream(fileStream, entity);

                out = new FileOutputStream (Environment.getExternalStorageDirectory().getPath()+"/decrypt.mp3");
                int read;
                byte[] buffer = new byte[1024];
                while ((read = inputStream.read(buffer)) != -1) {
                    out.write(buffer, 0, read);
                }
                inputStream.close();
                out.close();
            } catch (FileNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (CryptoInitializationException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (KeyChainException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        return rootView;
    }
}

}

我发现这篇文章Problems with facebooks conceal library但我仍无法解决我的问题。