MVP of PIN lock feature

This only shows the settings if a screen lock is available.
If the setting is activated, it shows a drawer item for locking.
Once locked, the screen lock secret needs to be entered to enter the
app again.

First part of #1247

bramble-api/src/main/java/org/briarproject/bramble/api/account/AccountManager.java

@@ -67,4 +67,8 @@ public interface AccountManager {
 	 * and stored.
 	 */
 	boolean changePassword(String oldPassword, String newPassword);
+
+	boolean isLocked();
+
+	void setLocked(boolean locked);
 }

bramble-core/src/main/java/org/briarproject/bramble/account/AccountManagerImpl.java

@@ -45,6 +45,7 @@ class AccountManagerImpl implements AccountManager {
 
 	@Nullable
 	private volatile SecretKey databaseKey = null;
+	private volatile boolean locked = false;
 
 	@Inject
 	AccountManagerImpl(DatabaseConfig databaseConfig, CryptoComponent crypto,
@@ -218,4 +219,16 @@ class AccountManagerImpl implements AccountManager {
 			return key != null && encryptAndStoreDatabaseKey(key, newPassword);
 		}
 	}
+
+	@Override
+	public boolean isLocked() {
+		return locked;
+	}
+
+	@Override
+	public void setLocked(boolean locked) {
+		synchronized (stateChangeLock) {
+			this.locked = locked;
+		}
+	}
 }

briar-android/src/main/AndroidManifest.xml

@@ -402,5 +402,11 @@
 			android:theme="@android:style/Theme.NoDisplay">
 		</activity>
 
+		<activity
+			android:name=".android.login.UnlockActivity"
+			android:label="@string/lock_unlock"
+			android:launchMode="singleTask"
+			android:theme="@style/BriarTheme.NoActionBar"/>
+
 	</application>
 </manifest>

briar-android/src/main/java/org/briarproject/briar/android/TestingConstants.java

@@ -40,4 +40,9 @@ public interface TestingConstants {
 	 * Feature flag for enabling the sign-in reminder in release builds.
 	 */
 	boolean FEATURE_FLAG_SIGN_IN_REMINDER = IS_DEBUG_BUILD;
+
+	/**
+	 * Feature flag for enabling the PIN lock in release builds.
+	 */
+	boolean FEATURE_FLAG_PIN_LOCK = IS_DEBUG_BUILD;
 }

briar-android/src/main/java/org/briarproject/briar/android/activity/ActivityComponent.java

@@ -37,6 +37,7 @@ import org.briarproject.briar.android.login.OpenDatabaseActivity;
 import org.briarproject.briar.android.login.PasswordActivity;
 import org.briarproject.briar.android.login.PasswordFragment;
 import org.briarproject.briar.android.login.SetupActivity;
+import org.briarproject.briar.android.login.UnlockActivity;
 import org.briarproject.briar.android.navdrawer.NavDrawerActivity;
 import org.briarproject.briar.android.panic.PanicPreferencesActivity;
 import org.briarproject.briar.android.panic.PanicResponderActivity;
@@ -163,6 +164,8 @@ public interface ActivityComponent {
 
 	void inject(StartupFailureActivity activity);
 
+	void inject(UnlockActivity activity);
+
 	// Fragments
 	void inject(AuthorNameFragment fragment);
 

briar-android/src/main/java/org/briarproject/briar/android/activity/BriarActivity.java

@@ -16,6 +16,7 @@ import org.briarproject.briar.android.controller.BriarController;
 import org.briarproject.briar.android.controller.DbController;
 import org.briarproject.briar.android.controller.handler.UiResultHandler;
 import org.briarproject.briar.android.login.PasswordActivity;
+import org.briarproject.briar.android.login.UnlockActivity;
 import org.briarproject.briar.android.logout.ExitActivity;
 
 import java.util.logging.Logger;
@@ -30,6 +31,7 @@ import static android.content.Intent.FLAG_ACTIVITY_NO_ANIMATION;
 import static android.os.Build.VERSION.SDK_INT;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_DOZE_WHITELISTING;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_PASSWORD;
+import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_UNLOCK;
 import static org.briarproject.briar.android.util.UiUtils.getDozeWhitelistingIntent;
 import static org.briarproject.briar.android.util.UiUtils.isSamsung7;
 
@@ -55,6 +57,8 @@ public abstract class BriarActivity extends BaseActivity {
 		if (request == REQUEST_PASSWORD) {
 			if (result == RESULT_OK) briarController.startAndBindService();
 			else supportFinishAfterTransition();
+		} else if (request == REQUEST_UNLOCK) {
+			if (result != RESULT_OK) supportFinishAfterTransition();
 		}
 	}
 
@@ -64,6 +68,9 @@ public abstract class BriarActivity extends BaseActivity {
 		if (!briarController.accountSignedIn() && !isFinishing()) {
 			Intent i = new Intent(this, PasswordActivity.class);
 			startActivityForResult(i, REQUEST_PASSWORD);
+		} else if(briarController.isLocked()) {
+			Intent i = new Intent(this, UnlockActivity.class);
+			startActivityForResult(i, REQUEST_UNLOCK);
 		} else if (SDK_INT >= 23) {
 			briarController.hasDozed(new UiResultHandler<Boolean>(this) {
 				@Override

briar-android/src/main/java/org/briarproject/briar/android/activity/RequestCodes.java

@@ -12,5 +12,6 @@ public interface RequestCodes {
 	int REQUEST_PERMISSION_CAMERA = 8;
 	int REQUEST_DOZE_WHITELISTING = 9;
 	int REQUEST_ENABLE_BLUETOOTH = 10;
+	int REQUEST_UNLOCK = 11;
 
 }

briar-android/src/main/java/org/briarproject/briar/android/controller/BriarController.java

@@ -8,6 +8,8 @@ public interface BriarController extends ActivityLifecycleController {
 
 	boolean accountSignedIn();
 
+	boolean isLocked();
+
 	/**
 	 * Returns true via the handler when the app has dozed
 	 * without being white-listed.

briar-android/src/main/java/org/briarproject/briar/android/controller/BriarControllerImpl.java

@@ -87,6 +87,11 @@ public class BriarControllerImpl implements BriarController {
 		return accountManager.hasDatabaseKey();
 	}
 
+	@Override
+	public boolean isLocked() {
+		return accountManager.isLocked();
+	}
+
 	@Override
 	public void hasDozed(ResultHandler<Boolean> handler) {
 		if (!dozeWatchdog.getAndResetDozeFlag()

briar-android/src/main/java/org/briarproject/briar/android/login/UnlockActivity.java

@@ -0,0 +1,81 @@
+package org.briarproject.briar.android.login;
+
+import android.app.KeyguardManager;
+import android.content.Context;
+import android.content.Intent;
+import android.os.Bundle;
+import android.support.annotation.Nullable;
+import android.support.annotation.RequiresApi;
+import android.support.v4.app.ActivityCompat;
+import android.widget.Button;
+
+import org.briarproject.bramble.api.account.AccountManager;
+import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
+import org.briarproject.briar.R;
+import org.briarproject.briar.android.activity.ActivityComponent;
+import org.briarproject.briar.android.activity.BaseActivity;
+
+import java.util.logging.Logger;
+
+import javax.inject.Inject;
+
+import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_UNLOCK;
+
+@RequiresApi(21)
+@MethodsNotNullByDefault
+@ParametersNotNullByDefault
+public class UnlockActivity extends BaseActivity {
+
+	private static final Logger LOG =
+			Logger.getLogger(UnlockActivity.class.getSimpleName());
+
+	@Inject
+	AccountManager accountManager;
+
+	@Override
+	public void injectActivity(ActivityComponent component) {
+		component.inject(this);
+	}
+
+	public void onCreate(@Nullable Bundle state) {
+		super.onCreate(state);
+		setContentView(R.layout.activity_unlock);
+
+		Button button = findViewById(R.id.unlock);
+		button.setOnClickListener(view -> requestKeyguardUnlock());
+		requestKeyguardUnlock();
+	}
+
+	@Override
+	protected void onActivityResult(int requestCode, int resultCode,
+			Intent data) {
+		super.onActivityResult(requestCode, resultCode, data);
+		if (requestCode == REQUEST_UNLOCK && resultCode == RESULT_OK) {
+			unlock();
+		}
+	}
+
+	private void requestKeyguardUnlock() {
+		KeyguardManager keyguardManager = (KeyguardManager) getSystemService(
+				Context.KEYGUARD_SERVICE);
+		assert keyguardManager != null;
+		Intent intent = keyguardManager
+				.createConfirmDeviceCredentialIntent(getString(R.string.lock_unlock),
+						null);
+		if (intent == null) {
+			// the user must have removed the screen lock since locked
+			LOG.warning("Unlocking without keyguard");
+			unlock();
+		} else {
+			startActivityForResult(intent, REQUEST_UNLOCK);
+		}
+	}
+
+	private void unlock() {
+		accountManager.setLocked(false);
+		setResult(RESULT_OK);
+		ActivityCompat.finishAfterTransition(this);
+	}
+
+}

briar-android/src/main/java/org/briarproject/briar/android/navdrawer/NavDrawerActivity.java

@@ -7,6 +7,7 @@ import android.os.Bundle;
 import android.support.annotation.NonNull;
 import android.support.design.widget.NavigationView;
 import android.support.design.widget.NavigationView.OnNavigationItemSelectedListener;
+import android.support.v4.app.ActivityCompat;
 import android.support.v4.app.FragmentManager;
 import android.support.v4.app.FragmentTransaction;
 import android.support.v4.content.ContextCompat;
@@ -59,6 +60,7 @@ import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_PASSW
 import static org.briarproject.briar.android.navdrawer.NavDrawerController.ExpiryWarning.NO;
 import static org.briarproject.briar.android.navdrawer.NavDrawerController.ExpiryWarning.UPDATE;
 import static org.briarproject.briar.android.util.UiUtils.getDaysUntilExpiry;
+import static org.briarproject.briar.android.util.UiUtils.hasScreenLock;
 
 public class NavDrawerActivity extends BriarActivity implements
 		BaseFragmentListener, TransportStateListener,
@@ -152,6 +154,14 @@ public class NavDrawerActivity extends BriarActivity implements
 	public void onStart() {
 		super.onStart();
 		updateTransports();
+		if (hasScreenLock(this)) {
+			controller.isLockable(new UiResultHandler<Boolean>(this) {
+				@Override
+				public void onResultUi(@NonNull Boolean lockable) {
+					setLockVisible(lockable);
+				}
+			});
+		} else setLockVisible(false);
 		controller.showExpiryWarning(new UiResultHandler<ExpiryWarning>(this) {
 			@Override
 			public void onResultUi(ExpiryWarning expiry) {
@@ -213,9 +223,15 @@ public class NavDrawerActivity extends BriarActivity implements
 	public boolean onNavigationItemSelected(@NonNull MenuItem item) {
 		drawerLayout.closeDrawer(START);
 		clearBackStack();
-		loadFragment(item.getItemId());
-		// Don't display the Settings item as checked
-		return item.getItemId() != R.id.nav_btn_settings;
+		if (item.getItemId() == R.id.nav_btn_lock) {
+			controller.lock();
+			ActivityCompat.finishAfterTransition(this);
+			return false;
+		} else {
+			loadFragment(item.getItemId());
+			// Don't display the Settings item as checked
+			return item.getItemId() != R.id.nav_btn_settings;
+		}
 	}
 
 	@Override
@@ -301,6 +317,11 @@ public class NavDrawerActivity extends BriarActivity implements
 		// Do nothing for now
 	}
 
+	private void setLockVisible(boolean visible) {
+		MenuItem item = navigation.getMenu().findItem(R.id.nav_btn_lock);
+		if (item != null) item.setVisible(visible);
+	}
+
 	@SuppressWarnings("ConstantConditions")
 	private void showExpiryWarning(ExpiryWarning expiry) {
 		int daysUntilExpiry = getDaysUntilExpiry();

briar-android/src/main/java/org/briarproject/briar/android/navdrawer/NavDrawerController.java

@@ -14,6 +14,10 @@ public interface NavDrawerController extends ActivityLifecycleController {
 
 	boolean isTransportRunning(TransportId transportId);
 
+	void isLockable(ResultHandler<Boolean> handler);
+
+	void lock();
+
 	void showExpiryWarning(ResultHandler<ExpiryWarning> handler);
 
 	void expiryWarningDismissed();

briar-android/src/main/java/org/briarproject/briar/android/navdrawer/NavDrawerControllerImpl.java

@@ -3,6 +3,7 @@ package org.briarproject.briar.android.navdrawer;
 import android.app.Activity;
 import android.content.Context;
 
+import org.briarproject.bramble.api.account.AccountManager;
 import org.briarproject.bramble.api.db.DatabaseExecutor;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.event.Event;
@@ -36,6 +37,7 @@ import static org.briarproject.briar.android.controller.BriarControllerImpl.DOZE
 import static org.briarproject.briar.android.navdrawer.NavDrawerController.ExpiryWarning.NO;
 import static org.briarproject.briar.android.navdrawer.NavDrawerController.ExpiryWarning.SHOW;
 import static org.briarproject.briar.android.navdrawer.NavDrawerController.ExpiryWarning.UPDATE;
+import static org.briarproject.briar.android.settings.SettingsFragment.PREF_SCREEN_LOCK;
 import static org.briarproject.briar.android.settings.SettingsFragment.SETTINGS_NAMESPACE;
 import static org.briarproject.briar.android.util.UiUtils.needsDozeWhitelisting;
 
@@ -51,6 +53,7 @@ public class NavDrawerControllerImpl extends DbControllerImpl
 
 	private final PluginManager pluginManager;
 	private final SettingsManager settingsManager;
+	private final AccountManager accountManager;
 	private final EventBus eventBus;
 
 	private volatile TransportStateListener listener;
@@ -58,10 +61,12 @@ public class NavDrawerControllerImpl extends DbControllerImpl
 	@Inject
 	NavDrawerControllerImpl(@DatabaseExecutor Executor dbExecutor,
 			LifecycleManager lifecycleManager, PluginManager pluginManager,
-			SettingsManager settingsManager, EventBus eventBus) {
+			SettingsManager settingsManager, AccountManager accountManager,
+			EventBus eventBus) {
 		super(dbExecutor, lifecycleManager);
 		this.pluginManager = pluginManager;
 		this.settingsManager = settingsManager;
+		this.accountManager = accountManager;
 		this.eventBus = eventBus;
 	}
 
@@ -107,6 +112,25 @@ public class NavDrawerControllerImpl extends DbControllerImpl
 				() -> listener.stateUpdate(id, enabled));
 	}
 
+	@Override
+	public void isLockable(ResultHandler<Boolean> handler) {
+		runOnDbThread(() -> {
+			try {
+				Settings settings =
+						settingsManager.getSettings(SETTINGS_NAMESPACE);
+				boolean ask = settings.getBoolean(PREF_SCREEN_LOCK, false);
+				handler.onResult(ask);
+			} catch (DbException e) {
+				logException(LOG, WARNING, e);
+			}
+		});
+	}
+
+	@Override
+	public void lock() {
+		accountManager.setLocked(true);
+	}
+
 	@Override
 	public void showExpiryWarning(ResultHandler<ExpiryWarning> handler) {
 		if (!IS_DEBUG_BUILD && !IS_BETA_BUILD) {

briar-android/src/main/java/org/briarproject/briar/android/settings/SettingsFragment.java

@@ -82,10 +82,12 @@ import static org.briarproject.bramble.util.LogUtils.logException;
 import static org.briarproject.bramble.util.LogUtils.now;
 import static org.briarproject.bramble.util.StringUtils.join;
 import static org.briarproject.briar.android.TestingConstants.FEATURE_FLAG_DARK_THEME;
+import static org.briarproject.briar.android.TestingConstants.FEATURE_FLAG_PIN_LOCK;
 import static org.briarproject.briar.android.TestingConstants.FEATURE_FLAG_SIGN_IN_REMINDER;
 import static org.briarproject.briar.android.TestingConstants.IS_DEBUG_BUILD;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_RINGTONE;
 import static org.briarproject.briar.android.navdrawer.NavDrawerActivity.INTENT_SIGN_OUT;
+import static org.briarproject.briar.android.util.UiUtils.hasScreenLock;
 import static org.briarproject.briar.api.android.AndroidNotificationManager.BLOG_CHANNEL_ID;
 import static org.briarproject.briar.api.android.AndroidNotificationManager.CONTACT_CHANNEL_ID;
 import static org.briarproject.briar.api.android.AndroidNotificationManager.FORUM_CHANNEL_ID;
@@ -109,6 +111,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 	public static final String BT_NAMESPACE = BluetoothConstants.ID.getString();
 	public static final String TOR_NAMESPACE = TorConstants.ID.getString();
 	public static final String LANGUAGE = "pref_key_language";
+	public static final String PREF_SCREEN_LOCK = "pref_key_lock";
 	public static final String NOTIFY_SIGN_IN = "pref_key_notify_sign_in";
 	public static final String TOR_LOCATION = "pref_key_tor_location";
 
@@ -120,6 +123,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 	private ListPreference enableBluetooth;
 	private ListPreference torNetwork;
 	private CheckBoxPreference torBlocked;
+	private CheckBoxPreference screenLock;
 	private CheckBoxPreference notifyPrivateMessages;
 	private CheckBoxPreference notifyGroupMessages;
 	private CheckBoxPreference notifyForumPosts;
@@ -162,6 +166,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		setBlockedCountries();
 		CheckBoxPreference notifySignIn =
 				(CheckBoxPreference) findPreference(NOTIFY_SIGN_IN);
+		screenLock = (CheckBoxPreference) findPreference(PREF_SCREEN_LOCK);
 		notifyPrivateMessages = (CheckBoxPreference) findPreference(
 				"pref_key_notify_private_messages");
 		notifyGroupMessages = (CheckBoxPreference) findPreference(
@@ -200,6 +205,12 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		enableBluetooth.setOnPreferenceChangeListener(this);
 		torNetwork.setOnPreferenceChangeListener(this);
 		torBlocked.setOnPreferenceChangeListener(this);
+		if (getActivity() != null && hasScreenLock(getActivity())) {
+			screenLock.setVisible(FEATURE_FLAG_PIN_LOCK);
+			screenLock.setOnPreferenceChangeListener(this);
+		} else {
+			screenLock.setVisible(false);
+		}
 		if (SDK_INT >= 21) {
 			notifyLockscreen.setVisible(true);
 			notifyLockscreen.setOnPreferenceChangeListener(this);
@@ -340,8 +351,10 @@ public class SettingsFragment extends PreferenceFragmentCompat
 						PREF_TOR_NETWORK_ALWAYS);
 				boolean torBlockedSetting =
 						torSettings.getBoolean(PREF_TOR_DISABLE_BLOCKED, true);
-				displaySettings(btSetting, torNetworkSetting,
-						torBlockedSetting);
+				boolean screenLockSetting =
+						settings.getBoolean(PREF_SCREEN_LOCK, false);
+				displaySettings(btSetting, torNetworkSetting, torBlockedSetting,
+						screenLockSetting);
 			} catch (DbException e) {
 				logException(LOG, WARNING, e);
 			}
@@ -349,11 +362,12 @@ public class SettingsFragment extends PreferenceFragmentCompat
 	}
 
 	private void displaySettings(boolean btSetting, int torNetworkSetting,
-			boolean torBlockedSetting) {
+			boolean torBlockedSetting, boolean screenLockSetting) {
 		listener.runOnUiThreadUnlessDestroyed(() -> {
 			enableBluetooth.setValue(Boolean.toString(btSetting));
 			torNetwork.setValue(Integer.toString(torNetworkSetting));
 			torBlocked.setChecked(torBlockedSetting);
+			screenLock.setChecked(screenLockSetting);
 
 			if (SDK_INT < 26) {
 				notifyPrivateMessages.setChecked(settings.getBoolean(
@@ -414,6 +428,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		enableBluetooth.setEnabled(enabled);
 		torNetwork.setEnabled(enabled);
 		torBlocked.setEnabled(enabled);
+		screenLock.setEnabled(enabled);
 		notifyPrivateMessages.setEnabled(enabled);
 		notifyGroupMessages.setEnabled(enabled);
 		notifyForumPosts.setEnabled(enabled);
@@ -478,6 +493,10 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		} else if (preference == torBlocked) {
 			boolean torBlockedSetting = (Boolean) newValue;
 			storeTorBlockedSetting(torBlockedSetting);
+		} else if (preference == screenLock) {
+			Settings s = new Settings();
+			s.putBoolean(PREF_SCREEN_LOCK, (Boolean) newValue);
+			storeSettings(s);
 		} else if (preference == notifyPrivateMessages) {
 			Settings s = new Settings();
 			s.putBoolean(PREF_NOTIFY_PRIVATE, (Boolean) newValue);

briar-android/src/main/java/org/briarproject/briar/android/splash/SplashScreenActivity.java

@@ -14,6 +14,7 @@ import org.briarproject.briar.android.activity.ActivityComponent;
 import org.briarproject.briar.android.activity.BaseActivity;
 import org.briarproject.briar.android.login.OpenDatabaseActivity;
 import org.briarproject.briar.android.login.SetupActivity;
+import org.briarproject.briar.android.navdrawer.NavDrawerActivity;
 
 import java.util.logging.Logger;
 
@@ -44,7 +45,15 @@ public class SplashScreenActivity extends BaseActivity {
 		setContentView(R.layout.splash);
 
 		if (accountManager.hasDatabaseKey()) {
-			startActivity(new Intent(this, OpenDatabaseActivity.class));
+			Intent i;
+			if (accountManager.isLocked()) {
+				// The database is already open, so start main activity which
+				// will open the activity to unlock, then brings main to front.
+				i = new Intent(this, NavDrawerActivity.class);
+			} else {
+				i = new Intent(this, OpenDatabaseActivity.class);
+			}
+			startActivity(i);
 			finish();
 		} else {
 			new Handler().postDelayed(() -> {

briar-android/src/main/java/org/briarproject/briar/android/util/UiUtils.java

@@ -2,6 +2,7 @@ package org.briarproject.briar.android.util;
 
 import android.annotation.SuppressLint;
 import android.annotation.TargetApi;
+import android.app.KeyguardManager;
 import android.content.Context;
 import android.content.DialogInterface.OnClickListener;
 import android.content.Intent;
@@ -229,4 +230,13 @@ public class UiUtils {
 		return ContextCompat.getColor(ctx, color);
 	}
 
+	public static boolean hasScreenLock(Context ctx) {
+		KeyguardManager keyguardManager = (KeyguardManager) ctx
+				.getSystemService(Context.KEYGUARD_SERVICE);
+		if (keyguardManager == null || SDK_INT < 21) return false;
+		// check if there's a lock mechanism we can use, try to ignore SIM
+		return (SDK_INT < 23 && keyguardManager.isKeyguardSecure()) ||
+				(SDK_INT >= 23 && keyguardManager.isDeviceSecure());
+	}
+
 }

briar-android/src/main/res/layout/activity_unlock.xml

@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="utf-8"?>
+<android.support.constraint.ConstraintLayout
+	xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:app="http://schemas.android.com/apk/res-auto"
+	xmlns:tools="http://schemas.android.com/tools"
+	android:layout_width="match_parent"
+	android:layout_height="match_parent"
+	android:orientation="vertical"
+	tools:context=".android.login.UnlockActivity">
+
+	<android.support.v7.widget.AppCompatImageView
+		android:id="@+id/image"
+		android:layout_width="150dp"
+		android:layout_height="150dp"
+		android:layout_margin="@dimen/margin_large"
+		android:src="@drawable/splash_screen"
+		app:layout_constraintBottom_toTopOf="@+id/is_locked"
+		app:layout_constraintEnd_toEndOf="parent"
+		app:layout_constraintStart_toStartOf="parent"
+		app:layout_constraintTop_toTopOf="parent"
+		app:layout_constraintVertical_chainStyle="spread"
+		app:tint="?attr/colorControlNormal"/>
+
+	<TextView
+		android:id="@+id/is_locked"
+		android:layout_width="wrap_content"
+		android:layout_height="wrap_content"
+		android:layout_margin="@dimen/margin_large"
+		android:gravity="center"
+		android:text="@string/lock_is_locked"
+		android:textSize="@dimen/text_size_xlarge"
+		app:layout_constraintBottom_toTopOf="@+id/unlock"
+		app:layout_constraintEnd_toEndOf="parent"
+		app:layout_constraintStart_toStartOf="parent"
+		app:layout_constraintTop_toBottomOf="@+id/image"
+		app:layout_constraintVertical_chainStyle="spread"/>
+
+	<Button
+		android:id="@+id/unlock"
+		style="@style/BriarButton.Default"
+		android:layout_width="match_parent"
+		android:layout_height="wrap_content"
+		android:layout_margin="@dimen/margin_large"
+		android:text="@string/lock_unlock"
+		app:layout_constraintBottom_toBottomOf="parent"
+		app:layout_constraintEnd_toEndOf="parent"
+		app:layout_constraintStart_toStartOf="parent"/>
+
+</android.support.constraint.ConstraintLayout>

briar-android/src/main/res/menu/navigation_drawer.xml

@@ -26,6 +26,11 @@
 			android:id="@+id/nav_btn_settings"
 			android:icon="@drawable/ic_settings_black_24dp"
 			android:title="@string/settings_button"/>
+		<item
+			android:id="@+id/nav_btn_lock"
+			android:icon="@drawable/startup_lock"
+			android:title="@string/lock_button"
+			android:visible="false"/>
 		<item
 			android:id="@+id/nav_btn_signout"
 			android:icon="@drawable/ic_signout"

briar-android/src/main/res/values/strings.xml

@@ -62,6 +62,7 @@
 	<string name="groups_button">Private Groups</string>
 	<string name="forums_button">Forums</string>
 	<string name="blogs_button">Blogs</string>
+	<string name="lock_button">Lock App</string>
 	<string name="settings_button">Settings</string>
 	<string name="sign_out_button">Sign Out</string>
 
@@ -356,6 +357,8 @@
 
 	<!-- Settings Security and Panic -->
 	<string name="security_settings_title">Security</string>
+	<string name="pref_lock_title">Screen Lock</string>
+	<string name="pref_lock_summary">Lock app access with Android screen lock or fingerprint</string>
 	<string name="change_password">Change password</string>
 	<string name="current_password">Current password</string>
 	<string name="choose_new_password">New password</string>
@@ -444,4 +447,10 @@
 	<string name="permission_camera_denied_toast">Camera permission was not granted</string>
 	<string name="qr_code">QR code</string>
 	<string name="show_qr_code_fullscreen">Show QR code fullscreen</string>
+
+	<!-- App Locking -->
+	<string name="lock_unlock">Unlock Briar</string>
+	<string name="lock_is_locked">Briar is locked</string>
+	<string name="lock_tap_to_unlock">Tap to unlock</string>
+
 </resources>

briar-android/src/main/res/xml/settings.xml

@@ -56,6 +56,13 @@
 		android:layout="@layout/preferences_category"
 		android:title="@string/security_settings_title">
 
+		<CheckBoxPreference
+			android:key="pref_key_lock"
+			android:persistent="false"
+			android:summary="@string/pref_lock_summary"
+			android:title="@string/pref_lock_title"
+			android:visibility="gone"/>
+
 		<Preference
 			android:key="pref_key_change_password"
 			android:title="@string/change_password">