Android

This chapter deals with the Android specific properties of the AusweisApp2 SDK. The AusweisApp2 core is encapsulated into an Android service which is running in the background without a user interface. This service is interfaced via an Android specific interprocess communication (IPC) mechanism. The basics of this mechanism - the Android Interface Definition Language (AIDL) - are introduced in the following section. Subsequent sections deal with the SDK interface itself and explain which steps are necessary in order to talk to the AusweisApp2 SDK.

The AusweisApp2 is available as an integrated and as an external variant. The integrated version is provided as an AAR package that can automatically be fetched by Android’s default build system gradle. The external variant is available as an APK in Google’s PlayStore. It is required that the user has manually installed the AusweisApp2 like any other app to connect to the external variant.

Important

The integrated variant is available in maven central for free. If you need enterprise support feel free to contact us.

Important

Please note that only Android devices that feature Extended Length NFC communication are supported by the SDK. This can be checked via the Android API methods getMaxTransceiveLength() and isExtendedLengthApduSupported().

https://developer.android.com/reference/android/nfc/tech/NfcA https://developer.android.com/reference/android/nfc/tech/IsoDep

Integrated

The integrated SDK is distributed as an AAR package that contains native arm64-v8a libraries only. The AAR package is available in the default repository of Android. The following listing shows the required mavenCentral in build.gradle.

buildscript {
    repositories {
        mavenCentral()
    }
}

The integrated AusweisApp2 will be fetched automatically as a dependency by your app/build.gradle file. It is recommended to always use the latest version (1.22.3) of AusweisApp2.

dependencies {
    implementation 'com.governikus:ausweisapp:1.x.y'
}

Note

All artifacts are signed with the following key (available on all public key servers): 0x699BF3055B0A49224EFDE7C72D7479A531451088

See also

The AAR package provides an AndroidManifest.xml to register required permissions and the background service. If your own AndroidManifest has conflicts with our provided file you can add some attributes to resolve those conflicts.

https://developer.android.com/studio/build/manifest-merge.html

App Bundle

The integrated AusweisApp2 uses native libraries which need to be extracted when used in an App Bundle, otherwise the SDK will not work correctly.

Add the following statement to your app’s gradle.properties file:

android.bundle.enableUncompressedNativeLibs=false

Logging

The AusweisApp2 uses default logging of Android and has its own log file. It is recommended to collect that log file if an error occurs in your application to receive better support.

The log file is in your application path:

/data/data/your.application.name/files/AusweisApp2.XXXXXX.log

The XXXXXX characters will be replaced by an automatically generated portion of the filename to avoid conflicts with previous instances.

A new log file will be created for each new instance of the AusweisApp2 and will be deleted after a correct shutdown. In case of old or multiple log files, it is highly probable that the previous instance crashed.

The AusweisApp2 deletes any log files that are older than 14 days.

Initialization of the Android Application

The integrated SDK creates a fork of the Android “main” Application if started. Due to this, the Application is instantiated a second time. Thus, it must ensure that any initialization (e.g. Firebase connections) is only carried out once. To do so the following snippet may prove useful:

public class MyAwesomeApp extends Application
{
    private static final String AA2_PROCESS = "ausweisapp2_service";

    @Override
    public void onCreate()
    {
        super.onCreate();
        if (isAA2Process())
            return;

        // Perform one-time initialization of YOUR app, e.g. Firebase connection
    }

    private boolean isAA2Process()
    {
        if (Build.VERSION.SDK_INT >= 28)
        {
            return Application.getProcessName().endsWith(AA2_PROCESS);
        }

        final int pid = android.os.Process.myPid();
        ActivityManager manager = (ActivityManager) getSystemService(ACTIVITY_SERVICE);
        for (ActivityManager.RunningAppProcessInfo appProcess : manager.getRunningAppProcesses())
        {
            if (appProcess.pid == pid)
            {
                return appProcess.processName.endsWith(AA2_PROCESS);
            }
        }
        return false;
    }
}

External

The APK is available in Google’s PlayStore and needs to be installed by the user. The external SDK is distributed as 32-bit and 64-bit.

Important

Please note that the external SDK will no longer be available with AusweisApp2 1.24.0. Only the integrated SDK will still be available.

Security

The following listing provides information about the solution to provide a secure connection to AusweisApp2.

  • Data between two apps connected via AIDL as a bound service cannot be grabbed by an attacker. Android will send the data to the corresponding app directly. There is no broadcast like an implicit intent.

  • An attacker cannot bind to an already bound service as AusweisApp2 will accept only one connection at the same time.

  • An attacker cannot resume a connection after the previous app disconnects because AusweisApp2 will reset the internal state if an app connects with another session ID.

  • An attacker cannot grab the session ID of the previous app because AusweisApp2 uses multiple sources of secure random number generator.

  • An attacker cannot fake AusweisApp2 for other apps because the connection via AIDL is bound with package name “com.governikus.ausweisapp2”. Google ensures that there is no other app in Google Play Store with that package name. Also the client app can check the fingerprint of signature certificate used for that package name.

Verify the authenticity of AusweisApp2

The following section deals with the cryptographic verification of the SDK’s authenticity. This step is necessary to ensure that the SDK has not been modified in a malicious way.

Fingerprint

In order to verify that the AusweisApp2 SDK is authentic and has not been modified in a malicious way, it is required to verify its authenticity before establishing a connection with it. Each Android application is signed by a distribution certificate which ensures its authenticity. During the installation of an application Android verifies that it has been correctly signed with the supplied distribution certificate. So everything the client has to do in order to verify the authenticity is to verify that the correct certificate has been used. The SHA256 fingerprint of the authentic SDK certificate is the following:

B0 2A C7 6B 50 A4 97 AE 81 0A EA C2 25 98 18 7B 3D 42 90 27 7D 08 51 A7 FA 8E 1A EA 5A 97 98 70

Example

The following example code demonstrates how the certificate hash value of a signed application on Android can be verified.

import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.content.pm.Signature;

public class AusweisApp2Validator
{
  private static final String PACKAGE = "com.governikus.ausweisapp2";
  private static final String FINGERPRINT = "..."; // see above

  public boolean isValid()
  {
    final PackageManager m = getPackageManager();
    PackageInfo info;
    try {
      info = m.getPackageInfo(PACKAGE, PackageManager.GET_SIGNATURES);
    } catch (PackageManager.NameNotFoundException e) {
      return false;
    }

    // What the API names signatures are actually the signing certificates.
    Signature certificates[] =  info.signatures;
    String computed = computeHashHexString("SHA256", certificates[0]);

    return FINGERPRINT.equalsIgnoreCase(computed);
  }
}

Visibility in AndroidManifest.xml

The client application needs to set the queries element as shown in the listing below in order to access the AusweisApp2 since Android 11.

<queries>
  <package android:name="com.governikus.ausweisapp2" />
</queries>

Note

The integrated variant already provides an AndroidManifest.xml with prepared queries.

Import the AIDL files

Android provides an interprocess communication (IPC) mechanism which is based on messages consisting of primitive types. In order to abstract from this very basic mechanism, there is the Android Interface Definition Language (AIDL). It allows the definition of Java like interfaces. The Android SDK generates the necessary interface implementations from supplied AIDL files in order to perform IPC, as if this function had been called directly in the current process.

In order to interact with the AusweisApp2 SDK there are two AIDL interfaces. The first one is given to the client application by the SDK and allows the client to establish a session with the SDK, to send JSON commands to the SDK and to pass discovered NFC tags to the SDK.

The second AIDL interface is given to the SDK by the client application. It enables the client to receive the intial session parameters as well as JSON messages from the SDK. Furthermore it has a function which is called when an existing connection with the SDK is dropped by the SDK. Both interfaces are listed below and you need to import them into your build environment.

Important

It is required that you place the AIDL files under subdirectory “aidl/com.governikus.ausweisapp2”. Also the interface methods names must be exactly the same.

Note

If you implement the integrated variant beside the external variant you do not need to manually add AIDL files as the AAR package already provides those interfaces.

Interface

package com.governikus.ausweisapp2;

import com.governikus.ausweisapp2.IAusweisApp2SdkCallback;
import android.nfc.Tag;

interface IAusweisApp2Sdk
{
    boolean connectSdk(IAusweisApp2SdkCallback pCallback);
    boolean send(String pSessionId, String pMessageFromClient);
    boolean updateNfcTag(String pSessionId, in Tag pTag);
}

Callback

package com.governikus.ausweisapp2;

interface IAusweisApp2SdkCallback
{
    void sessionIdGenerated(String pSessionId, boolean pIsSecureSessionId);
    void receive(String pJson);
    void sdkDisconnected();
}

Background service

The integrated and external variants use the same method to establish a connection to the AusweisApp2 SDK. The AusweisApp2 SDK is a background service in the external AusweisApp2 or an embedded background service in your own application.

Binding to the service

In order to start the AusweisApp2 SDK it is necessary to bind to the Android service supplied by the SDK. This binding fulfils two purposes:

  • First it starts the SDK.

  • Second it enables the client to establish an IPC connection as mentioned above.

Due to the nature of an Android service, there can be only one instance of the SDK running. If multiple clients bind to the service, they are interacting with the same instance of the service. The service is terminated once all previously bound clients are unbound.

To differentiate between different connected clients, virtual sessions are used once the binding is completed. These sessions are discussed in a separate section, section Create session to AusweisApp2.

Create connection

First of all, in order to bind to the service, one needs to instantiate an Android ServiceConnection. Subsequently, the object is passed to the Android API and the contained methods are invoked by Android on service connection and disconnection.

import android.content.ServiceConnection;

// [...]

ServiceConnection mConnection = new ServiceConnection()
{
  @Override
  public void onServiceConnected(ComponentName className, IBinder service)
  {
      // ... details below
  }

  @Override
  public void onServiceDisconnected(ComponentName className)
  {
      // ... details below
  }
}

Bind service to raw connection

In order to perform the actual binding a directed Intent, which identifies the AusweisApp2 SDK, is created. This Intent is sent to the Android API along with the ServiceConnection created above. This API call either starts up the SDK if it is the first client, or connects to the running SDK instance if there is already another client bound.

If you use the external variant of AusweisApp2 you need to pass the package name of Governikus. Otherwise you need to pass your own package name as the integrated variant is a background service of your application.

import android.app.Activity;
import android.content.Context;
import android.content.Intent;

// [...]

String pkg = "com.governikus.ausweisapp2";

boolean useIntegrated = true; // use external or integrated
if (useIntegrated)
  pkg = getApplicationContext().getPackageName();

String name = "com.governikus.ausweisapp2.START_SERVICE";
Intent serviceIntent = new Intent(name);
serviceIntent.setPackage(pkg);
bindService(serviceIntent, mConnection, Context.BIND_AUTO_CREATE);

Redirect to Play Store

It is necessary that AusweisApp2 is installed in order to use the external SDK. It is recommended to check and display a message in case the user needs to install AusweisApp2 first. Also, the user should be redirected to the Play Store entry to find the app.

import android.content.ActivityNotFoundException;
import android.content.pm.ResolveInfo;
import android.net.Uri;
import java.util.List;

PackageManager m = getPackageManager();
List<ResolveInfo> list = m.queryIntentServices(serviceIntent, PackageManager.MATCH_ALL);

if (list == null || list.isEmpty())
{
  final String name = "com.governikus.ausweisapp2";
  try {
    startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("market://details?id=" + name)));
  } catch (ActivityNotFoundException e) {
    // Use the browser if Play Store is not installed, too!
    startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("https://play.google.com/store/apps/details?id=" + name)));
  }
}

Note

This is not necessary if you use the integrated variant.

Initializing the AIDL connection

Once the Android service of the AusweisApp2 SDK is successfully started and bound to by the client, the Android system calls the onServiceConnected method of the ServiceConnection created and supplied above. This method receives an instance of the IBinder Android service interface.

The IBinder is then used by the client application to initialize the auto generated AIDL stub in order to use the AIDL IPC mechanism. The used stub is supposed to be auto generated by the Android SDK if you have properly configured your build environment.

The stub initialization returns an instance of IAusweisApp2Sdk which is used to interact with the SDK. The example below stores this instance in the member variable mSdk.

import android.content.ComponentName;
import android.content.ServiceConnection;
import android.os.IBinder;

import com.governikus.ausweisapp2.IAusweisApp2Sdk;

// [...]

IAusweisApp2Sdk mSdk;

ServiceConnection mConnection = new ServiceConnection(){
  @Override
  public void onServiceConnected(ComponentName className, IBinder service)
  {
      try {
          mSdk = IAusweisApp2Sdk.Stub.asInterface(service);
      } catch (ClassCastException|RemoteException e) {
          // ...
      }
  }

  @Override
  public void onServiceDisconnected(ComponentName className)
  {
      mSdk = null;
  }
}

Create session to AusweisApp2

Once your client is bound to the AusweisApp2 SDK service and you have initialized the AIDL IPC mechanism, you are ready to use the actual SDK API.

Since the Android system does not allow to limit the number of clients which can connect to a service, the SDK API uses custom sessions to manage the connected clients. There is a maximum of one established session at a time.

In order to open a session with the SDK you need to pass an instance of IAusweisApp2SdkCallback to the connectSdk function of your previously acquired instance of IAusweisApp2Sdk. If your callback is accepted, the function returns true. Otherwise there is a problem with your supplied callback. Sessions will be disconnected once the IBinder instance of the connected client is invalidated, another communication error occurs or another Client connects. Please see Disconnect from SDK for instructions to gracefully disconnect from the SDK.

As mentioned above: If there already is a connected client and a second client attempts to connect, the first client is disconnected and the second client is granted exclusive access to the SDK. The first client is informed via its callback by sdkDisconnected. The second client is presented a fresh environment and it has no access to any data of the first client.

If you have successfully established a session, the sessionIdGenerated function of your callback is invoked. With this invocation you receive two arguments. pIsSecureSessionId is true if the SDK was able to gather enough entropy in order to generate a secure random session ID. If it is false, there is no session ID passed. There is nothing you can do about such an error. It results from a problem with the random number generator, which in turn is very likely the result of an ongoing local attack. The device should be considered manipulated and the user should be informed.

On success pSessionId holds the actual session ID generated by the SDK. This ID is used to identify your session and you need to pass it to all future SDK function invocations of this session.

The listing below shows an example for an instantiation of IAusweisApp2SdkCallback and establishing a session.

import com.governikus.ausweisapp2.IAusweisApp2Sdk;
import com.governikus.ausweisapp2.IAusweisApp2SdkCallback;

// [...]

LocalCallback mCallback = new LocalCallback();
class LocalCallback extends IAusweisApp2SdkCallback.Stub
{
  public String mSessionID = null;

  @Override
  public void sessionIdGenerated(
    String pSessionId, boolean pIsSecureSessionId) throws RemoteException
  {
      mSessionID = pSessionId;
  }

  @Override
  public void receive(String pJson) throws RemoteException
  {
      // handle message from SDK
  }
}

// [...]

try
{
  if (!mSdk.connectSdk(mCallback))
  {
      // already connected? Handle error...
  }
}
catch (RemoteException e)
{
    // handle exception
}

Send command

In order to send a JSON command to the AusweisApp2 SDK, you need to invoke the send function of your instance of IAusweisApp2Sdk. For this command to be processed by the SDK you need to supply the session ID which you have previously received. The listing below shows an example.

String cmd = "{\"cmd\": \"GET_INFO\"}";
try
{
  if (!mSdk.send(mCallback.mSessionID, cmd))
  {
      // disconnected? Handle error...
  }
}
catch (RemoteException e)
{
    // handle exception
}

Receive message

Messages from the AusweisApp2 SDK are passed to you via the same instance of IAusweisApp2SdkCallback in which you have received the session ID. The receive method is called each time the SDK sends a message.

Disconnect from SDK

In order to disconnect from the AusweisApp2 SDK you need to invalidate your instance of IBinder. There are two possibilities to do this. The first one is to unbind from the SDK Android service to undo your binding, like shown in the code listing below. The second one is to return false in the pingBinder function of your IBinder instance.

unbindService(mConnection);

Passing NFC tags to the SDK

NFC tags can only be detected by applications which have a foreground window on the Android platform. A common workaround for this problem is to equip background services with a transparent window which is shown to dispatch NFC tags.

However, if there are multiple applications installed, which are capable of dispatching NFC tags, the Android system will display an App Chooser for each discovered tag enabling the user to select the appropriate application to handle the NFC tag. To have such a chooser display the name and image of the client application instead of the SDK, the client application is required to dispatch discovered NFC tags and to pass them to the SDK.

Furthermore, this interface design enables the client application to do foreground dispatching of NFC tags. If the active application registers itself for foreground dispatching, it receives discovered NFC tags directly without Android displaying an App Chooser.

Permissions in AndroidManifest.xml

The client applications needs to register the NFC permission as shown in the listing below in order to access the NFC reader hardware.

<uses-permission android:name="android.permission.NFC"/>

Note

The integrated variant already provides an AndroidManifest.xml with prepared permissions.

Intent-Filter in AndroidManifest.xml

In order to be informed about attached NFC tags by Android, the client application is required to register an intent filter. The appropriate filter is shown in the listing below.

<intent-filter>
  <action android:name="android.nfc.action.TECH_DISCOVERED" />
</intent-filter>
<meta-data android:name="android.nfc.action.TECH_DISCOVERED" android:resource="@xml/nfc_tech_filter" />

NFC Technology Filter

Since there are many different kinds of NFC tags, Android requires the application to register a technology filter for the kind of tags the application wants to receive. The proper filter for the German eID card is shown in the listing below.

<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">
  <tech-list>
    <tech>android.nfc.tech.IsoDep</tech>
  </tech-list>
</resources>

Implementation

As it is common on the Android platform, information is sent to applications encapsulated in instances of the Intent class. In order to process newly discovered NFC tags, Intents which are given to the application need to be checked for the parcelable NFC extra as shown in the code listing below. Subsequently the client is required to send them to the AusweisApp2 SDK by calling the updateNfcTag method of the previously acquired IAusweisApp2Sdk instance. The listing below shows an example for the described process.

import android.content.Intent;
import android.nfc.NfcAdapter;
import android.nfc.Tag;

import com.governikus.ausweisapp2.IAusweisApp2Sdk;
import com.governikus.ausweisapp2.IAusweisApp2SdkCallback;

// [...]

void handleIntent(Intent intent)
{
  final Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
  if (tag != null)
  {
    try {
      mSdk.updateNfcTag(mCallback.mSessionID, tag);
    } catch (RemoteException e) {
      // ...
    }
  }
}

Dispatching NFC tags in foreground

As already mentioned under Passing NFC tags to the SDK, an App Chooser is displayed for discovered NFC tags by Android if multiple applications which are able to dispatch NFC tags are installed. An application can suppress this App Chooser if it registers itself for foreground dispatching at runtime. This way NFC tags are handled directly by the application without a chooser being displayed. Subsequently the client is required to send them to the AusweisApp2 SDK by calling the updateNfcTag method of the previously acquired IAusweisApp2Sdk instance. The required steps to handle NFC tags directly are shown in the code listing below by way of example.

import android.app.Activity;
import android.nfc.NfcAdapter;
import android.nfc.tech.IsoDep;
import java.util.Arrays;

import com.governikus.ausweisapp2.IAusweisApp2Sdk;

class ForegroundDispatcher
{
  private final Activity mActivity;
  private final NfcAdapter mAdapter;
  private final int mFlags;
  private final NfcAdapter.ReaderCallback mReaderCallback;

  ForegroundDispatcher(Activity pActivity, final IAusweisApp2Sdk pSdk, final String pSdkSessionID)
  {
    mActivity = pActivity;
    mAdapter = NfcAdapter.getDefaultAdapter(mActivity);
    mFlags = NfcAdapter.FLAG_READER_NFC_A | NfcAdapter.FLAG_READER_NFC_B | NfcAdapter.FLAG_READER_SKIP_NDEF_CHECK;
    mReaderCallback = new NfcAdapter.ReaderCallback()
    {
      public void onTagDiscovered(Tag pTag)
      {
        if (Arrays.asList(pTag.getTechList()).contains(IsoDep.class.getName()))
        {
          pSdk.updateNfcTag(pSdkSessionID, pTag);
        }
      }
    };
  }

  void enable()
  {
    if (mAdapter != null)
    {
      mAdapter.enableReaderMode(mActivity, mReaderCallback, mFlags, null);
    }
  }

  void disable()
  {
    if (mAdapter != null)
    {
      mAdapter.disableReaderMode(mActivity);
    }
  }
}

// [...]

ForegroundDispatcher mDispatcher = new ForegroundDispatcher(this);

The example implementation from above needs to be invoked when the application is brought to foreground and when it looses focus. An example for appropriate places are the onResume and the onPause methods of Activities as shown in the code listing below.

@Override
public void onResume()
{
  super.onResume();
  mDispatcher.enable();
}

@Override
public void onPause()
{
  super.onPause();
  mDispatcher.disable();
}