diff --git a/app/build.gradle b/app/build.gradle
index 4e7094f..01ba6f9 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -29,6 +29,7 @@ dependencies {
     compile 'ch.dissem.jabit:jabit-domain:0.2.1-SNAPSHOT'
     compile 'ch.dissem.jabit:jabit-networking:0.2.1-SNAPSHOT'
     compile 'ch.dissem.jabit:jabit-security-spongy:0.2.1-SNAPSHOT'
+    compile 'ch.dissem.jabit:jabit-extensions:0.2.1-SNAPSHOT'
 
     compile 'org.slf4j:slf4j-android:1.7.12'
 
diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index f0fbceb..979915d 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -18,7 +18,7 @@
         android:label="@string/app_name"
         android:theme="@style/AppTheme">
         <activity
-            android:name=".MessageListActivity"
+            android:name=".MainActivity"
             android:label="@string/app_name">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
@@ -29,28 +29,28 @@
         <activity
             android:name=".MessageDetailActivity"
             android:label="@string/title_message_detail"
-            android:parentActivityName=".MessageListActivity"
+            android:parentActivityName=".MainActivity"
             tools:ignore="UnusedAttribute">
             <meta-data
                 android:name="android.support.PARENT_ACTIVITY"
-                android:value=".MessageListActivity" />
+                android:value=".MainActivity" />
         </activity>
         <activity
             android:name=".SubscriptionDetailActivity"
             android:label="@string/title_subscription_detail"
-            android:parentActivityName=".MessageListActivity"
+            android:parentActivityName=".MainActivity"
             tools:ignore="UnusedAttribute">
             <meta-data
                 android:name="android.support.PARENT_ACTIVITY"
-                android:value=".MessageListActivity" />
+                android:value=".MainActivity" />
         </activity>
         <activity
             android:name=".ComposeMessageActivity"
             android:label="Compose"
-            android:parentActivityName=".MessageListActivity">
+            android:parentActivityName=".MainActivity">
             <meta-data
                 android:name="android.support.PARENT_ACTIVITY"
-                android:value=".MessageListActivity" />
+                android:value=".MainActivity" />
 
             <intent-filter>
                 <action android:name="android.intent.action.SENDTO" />
@@ -79,7 +79,7 @@
         <activity
             android:name=".SettingsActivity"
             android:label="@string/settings"
-            android:parentActivityName=".MessageListActivity">
+            android:parentActivityName=".MainActivity">
             <intent-filter>
                 <action android:name="android.intent.action.MANAGE_NETWORK_USAGE" />
 
diff --git a/app/src/main/assets/db/migration/V1.2__Create_table_message.sql b/app/src/main/assets/db/migration/V1.2__Create_table_message.sql
index 79e32f2..ae72a6f 100644
--- a/app/src/main/assets/db/migration/V1.2__Create_table_message.sql
+++ b/app/src/main/assets/db/migration/V1.2__Create_table_message.sql
@@ -8,6 +8,7 @@ CREATE TABLE Message (
   sent                    INTEGER,
   received                INTEGER,
   status                  VARCHAR(20)   NOT NULL,
+  initial_hash            BINARY(64)    UNIQUE,
 
   FOREIGN KEY (sender)    REFERENCES Address (address),
   FOREIGN KEY (recipient) REFERENCES Address (address)
diff --git a/app/src/main/assets/db/migration/V2.0__Update_table_message.sql b/app/src/main/assets/db/migration/V2.0__Update_table_message.sql
new file mode 100644
index 0000000..2b39a6e
--- /dev/null
+++ b/app/src/main/assets/db/migration/V2.0__Update_table_message.sql
@@ -0,0 +1,7 @@
+-- This is done in V1.2, as SQLite doesn't support ADD CONSTRAINT and a proper migration
+-- wasn't really necessary yet.
+--
+-- This file is here to reduce confusion regarding to the original migration files.
+
+--ALTER TABLE Message ADD COLUMN initial_hash BINARY(64);
+--ALTER TABLE Message ADD CONSTRAINT initial_hash_unique UNIQUE(initial_hash);
\ No newline at end of file
diff --git a/app/src/main/assets/db/migration/V2.1__Create_table_POW.sql b/app/src/main/assets/db/migration/V2.1__Create_table_POW.sql
new file mode 100644
index 0000000..b39c6c5
--- /dev/null
+++ b/app/src/main/assets/db/migration/V2.1__Create_table_POW.sql
@@ -0,0 +1,7 @@
+CREATE TABLE POW (
+  initial_hash          BINARY(64)    PRIMARY KEY,
+  data                  BLOB          NOT NULL,
+  version               BIGINT        NOT NULL,
+  nonce_trials_per_byte BIGINT        NOT NULL,
+  extra_bytes           BIGINT        NOT NULL
+);
diff --git a/app/src/main/java/ch/dissem/apps/abit/MessageListActivity.java b/app/src/main/java/ch/dissem/apps/abit/MainActivity.java
similarity index 88%
rename from app/src/main/java/ch/dissem/apps/abit/MessageListActivity.java
rename to app/src/main/java/ch/dissem/apps/abit/MainActivity.java
index b21acb1..5d7bee8 100644
--- a/app/src/main/java/ch/dissem/apps/abit/MessageListActivity.java
+++ b/app/src/main/java/ch/dissem/apps/abit/MainActivity.java
@@ -42,12 +42,16 @@ import org.slf4j.LoggerFactory;
 import java.io.Serializable;
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
 
 import ch.dissem.apps.abit.listener.ActionBarListener;
 import ch.dissem.apps.abit.listener.ListSelectionListener;
 import ch.dissem.apps.abit.service.BitmessageService;
 import ch.dissem.apps.abit.service.Singleton;
 import ch.dissem.apps.abit.synchronization.Authenticator;
+import ch.dissem.apps.abit.synchronization.SyncAdapter;
+import ch.dissem.apps.abit.util.Preferences;
 import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.entity.Plaintext;
 import ch.dissem.bitmessage.entity.valueobject.Label;
@@ -77,13 +81,12 @@ import static ch.dissem.apps.abit.synchronization.StubProvider.AUTHORITY;
  * to listen for item selections.
  * </p>
  */
-public class MessageListActivity extends AppCompatActivity
+public class MainActivity extends AppCompatActivity
         implements ListSelectionListener<Serializable>, ActionBarListener {
     public static final String EXTRA_SHOW_MESSAGE = "ch.dissem.abit.ShowMessage";
     public static final String ACTION_SHOW_INBOX = "ch.dissem.abit.ShowInbox";
 
-    private static final Logger LOG = LoggerFactory.getLogger(MessageListActivity.class);
-    private static final long SYNC_FREQUENCY = 15 * 60; // seconds
+    private static final Logger LOG = LoggerFactory.getLogger(MainActivity.class);
     private static final int ADD_IDENTITY = 1;
 
     /**
@@ -99,8 +102,8 @@ public class MessageListActivity extends AppCompatActivity
     private static ServiceConnection connection = new ServiceConnection() {
         @Override
         public void onServiceConnected(ComponentName name, IBinder service) {
-            MessageListActivity.service = new Messenger(service);
-            MessageListActivity.bound = true;
+            MainActivity.service = new Messenger(service);
+            MainActivity.bound = true;
         }
 
         @Override
@@ -121,8 +124,10 @@ public class MessageListActivity extends AppCompatActivity
         super.onCreate(savedInstanceState);
         messageRepo = Singleton.getMessageRepository(this);
         addressRepo = Singleton.getAddressRepository(this);
-
-        selectedLabel = messageRepo.getLabels().get(0);
+        List<Label> labels = messageRepo.getLabels();
+        if (selectedLabel == null) {
+            selectedLabel = labels.get(0);
+        }
 
         setContentView(R.layout.activity_message_list);
 
@@ -130,7 +135,8 @@ public class MessageListActivity extends AppCompatActivity
         setSupportActionBar(toolbar);
 
         MessageListFragment listFragment = new MessageListFragment();
-        getSupportFragmentManager().beginTransaction().replace(R.id.item_list, listFragment).commit();
+        getSupportFragmentManager().beginTransaction().replace(R.id.item_list, listFragment)
+                .commit();
 
         if (findViewById(R.id.message_detail_container) != null) {
             // The detail container view will be present only in the
@@ -144,7 +150,7 @@ public class MessageListActivity extends AppCompatActivity
             listFragment.setActivateOnItemClick(true);
         }
 
-        createDrawer(toolbar);
+        createDrawer(toolbar, labels);
 
         Singleton.getMessageListener(this).resetNotification();
 
@@ -153,21 +159,10 @@ public class MessageListActivity extends AppCompatActivity
             onItemSelected(getIntent().getSerializableExtra(EXTRA_SHOW_MESSAGE));
         }
 
-        createSyncAccount();
-    }
-
-    private void createSyncAccount() {
-        // Create account, if it's missing. (Either first run, or user has deleted account.)
-        Account account = new Account(Authenticator.ACCOUNT_NAME, Authenticator.ACCOUNT_TYPE);
-
-        if (AccountManager.get(this).addAccountExplicitly(account, null, null)) {
-            // Inform the system that this account supports sync
-            ContentResolver.setIsSyncable(account, AUTHORITY, 1);
-            // Inform the system that this account is eligible for auto sync when the network is up
-            ContentResolver.setSyncAutomatically(account, AUTHORITY, true);
-            // Recommend a schedule for automatic synchronization. The system may modify this based
-            // on other scheduled syncs and network utilization.
-            ContentResolver.addPeriodicSync(account, AUTHORITY, new Bundle(), SYNC_FREQUENCY);
+        if (Preferences.useTrustedNode(this)) {
+            SyncAdapter.startSync(this);
+        } else {
+            SyncAdapter.stopSync(this);
         }
     }
 
@@ -185,7 +180,7 @@ public class MessageListActivity extends AppCompatActivity
         }
     }
 
-    private void createDrawer(Toolbar toolbar) {
+    private void createDrawer(Toolbar toolbar, Collection<Label> labels) {
         final ArrayList<IProfile> profiles = new ArrayList<>();
         for (BitmessageAddress identity : addressRepo.getIdentities()) {
             LOG.info("Adding identity " + identity.getAddress());
@@ -216,10 +211,12 @@ public class MessageListActivity extends AppCompatActivity
                 .withProfiles(profiles)
                 .withOnAccountHeaderListener(new AccountHeader.OnAccountHeaderListener() {
                     @Override
-                    public boolean onProfileChanged(View view, IProfile profile, boolean currentProfile) {
+                    public boolean onProfileChanged(View view, IProfile profile, boolean
+                            currentProfile) {
                         if (profile.getIdentifier() == ADD_IDENTITY) {
                             try {
-                                Message message = Message.obtain(null, BitmessageService.MSG_CREATE_IDENTITY);
+                                Message message = Message.obtain(null, BitmessageService
+                                        .MSG_CREATE_IDENTITY);
                                 message.replyTo = messenger;
                                 service.send(message);
                             } catch (RemoteException e) {
@@ -236,14 +233,15 @@ public class MessageListActivity extends AppCompatActivity
                     }
                 })
                 .build();
-        if (profiles.size() > 0) {
+        if (profiles.size() > 2) { // There's always the add and manage identity items
             accountHeader.setActiveProfile(profiles.get(0), true);
         }
         incomingHandler.updateAccountHeader(accountHeader);
 
         ArrayList<IDrawerItem> drawerItems = new ArrayList<>();
-        for (Label label : messageRepo.getLabels()) {
-            PrimaryDrawerItem item = new PrimaryDrawerItem().withName(label.toString()).withTag(label);
+        for (Label label : labels) {
+            PrimaryDrawerItem item = new PrimaryDrawerItem().withName(label.toString()).withTag
+                    (label);
             if (label.getType() == null) {
                 item.withIcon(CommunityMaterial.Icon.cmd_label);
             } else {
@@ -297,17 +295,21 @@ public class MessageListActivity extends AppCompatActivity
                                 .withChecked(BitmessageService.isRunning())
                                 .withOnCheckedChangeListener(new OnCheckedChangeListener() {
                                     @Override
-                                    public void onCheckedChanged(IDrawerItem drawerItem, CompoundButton buttonView, boolean isChecked) {
+                                    public void onCheckedChanged(IDrawerItem drawerItem,
+                                                                 CompoundButton buttonView,
+                                                                 boolean isChecked) {
                                         if (messenger != null) {
                                             if (isChecked) {
                                                 try {
-                                                    service.send(Message.obtain(null, MSG_START_NODE));
+                                                    service.send(Message.obtain(null,
+                                                            MSG_START_NODE));
                                                 } catch (RemoteException e) {
                                                     LOG.error(e.getMessage(), e);
                                                 }
                                             } else {
                                                 try {
-                                                    service.send(Message.obtain(null, MSG_STOP_NODE));
+                                                    service.send(Message.obtain(null,
+                                                            MSG_STOP_NODE));
                                                 } catch (RemoteException e) {
                                                     LOG.error(e.getMessage(), e);
                                                 }
@@ -318,7 +320,8 @@ public class MessageListActivity extends AppCompatActivity
                 )
                 .withOnDrawerItemClickListener(new Drawer.OnDrawerItemClickListener() {
                     @Override
-                    public boolean onItemClick(AdapterView<?> adapterView, View view, int i, long l, IDrawerItem item) {
+                    public boolean onItemClick(AdapterView<?> adapterView, View view, int i, long
+                            l, IDrawerItem item) {
                         if (item.getTag() instanceof Label) {
                             selectedLabel = (Label) item.getTag();
                             showSelectedLabel();
@@ -327,15 +330,18 @@ public class MessageListActivity extends AppCompatActivity
                             Nameable<?> ni = (Nameable<?>) item;
                             switch (ni.getNameRes()) {
                                 case R.string.contacts_and_subscriptions:
-                                    if (!(getSupportFragmentManager().findFragmentById(R.id.item_list) instanceof SubscriptionListFragment)) {
+                                    if (!(getSupportFragmentManager().findFragmentById(R.id
+                                            .item_list) instanceof SubscriptionListFragment)) {
                                         changeList(new SubscriptionListFragment());
                                     } else {
                                         ((SubscriptionListFragment) getSupportFragmentManager()
                                                 .findFragmentById(R.id.item_list)).updateList();
                                     }
+
                                     break;
                                 case R.string.settings:
-                                    startActivity(new Intent(MessageListActivity.this, SettingsActivity.class));
+                                    startActivity(new Intent(MainActivity.this, SettingsActivity
+                                            .class));
                                     break;
                                 case R.string.archive:
                                     selectedLabel = null;
@@ -353,7 +359,8 @@ public class MessageListActivity extends AppCompatActivity
     }
 
     private void showSelectedLabel() {
-        if (getSupportFragmentManager().findFragmentById(R.id.item_list) instanceof MessageListFragment) {
+        if (getSupportFragmentManager().findFragmentById(R.id.item_list) instanceof
+                MessageListFragment) {
             ((MessageListFragment) getSupportFragmentManager()
                     .findFragmentById(R.id.item_list)).updateList(selectedLabel);
         } else {
@@ -381,7 +388,8 @@ public class MessageListActivity extends AppCompatActivity
             else if (item instanceof BitmessageAddress)
                 fragment = new SubscriptionDetailFragment();
             else
-                throw new IllegalArgumentException("Plaintext or BitmessageAddress expected, but was "
+                throw new IllegalArgumentException("Plaintext or BitmessageAddress expected, but " +
+                        "was "
                         + item.getClass().getSimpleName());
             fragment.setArguments(arguments);
             getSupportFragmentManager().beginTransaction()
@@ -396,7 +404,8 @@ public class MessageListActivity extends AppCompatActivity
             else if (item instanceof BitmessageAddress)
                 detailIntent = new Intent(this, SubscriptionDetailActivity.class);
             else
-                throw new IllegalArgumentException("Plaintext or BitmessageAddress expected, but was "
+                throw new IllegalArgumentException("Plaintext or BitmessageAddress expected, but " +
+                        "was "
                         + item.getClass().getSimpleName());
 
             detailIntent.putExtra(MessageDetailFragment.ARG_ITEM, item);
@@ -417,7 +426,8 @@ public class MessageListActivity extends AppCompatActivity
     @Override
     protected void onStart() {
         super.onStart();
-        bindService(new Intent(this, BitmessageService.class), connection, Context.BIND_AUTO_CREATE);
+        bindService(new Intent(this, BitmessageService.class), connection, Context
+                .BIND_AUTO_CREATE);
     }
 
     @Override
@@ -459,8 +469,10 @@ public class MessageListActivity extends AppCompatActivity
                                 .withEmail(identity.getAddress())
                                 .withTag(identity);
                         if (accountHeader.getProfiles() != null) {
-                            //we know that there are 2 setting elements. set the new profile above them ;)
-                            accountHeader.addProfile(newProfile, accountHeader.getProfiles().size() - 2);
+                            //we know that there are 2 setting elements. set the new profile
+                            // above them ;)
+                            accountHeader.addProfile(newProfile, accountHeader.getProfiles().size
+                                    () - 2);
                         } else {
                             accountHeader.addProfiles(newProfile);
                         }
diff --git a/app/src/main/java/ch/dissem/apps/abit/MessageDetailActivity.java b/app/src/main/java/ch/dissem/apps/abit/MessageDetailActivity.java
index fae5692..22012c5 100644
--- a/app/src/main/java/ch/dissem/apps/abit/MessageDetailActivity.java
+++ b/app/src/main/java/ch/dissem/apps/abit/MessageDetailActivity.java
@@ -3,7 +3,6 @@ package ch.dissem.apps.abit;
 import android.content.Intent;
 import android.os.Bundle;
 import android.support.v4.app.NavUtils;
-import android.support.v7.app.ActionBarActivity;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.Toolbar;
 import android.view.MenuItem;
@@ -13,7 +12,7 @@ import android.view.MenuItem;
  * An activity representing a single Message detail screen. This
  * activity is only used on handset devices. On tablet-size devices,
  * item details are presented side-by-side with a list of items
- * in a {@link MessageListActivity}.
+ * in a {@link MainActivity}.
  * <p/>
  * This activity is mostly just a 'shell' activity containing nothing
  * more than a {@link MessageDetailFragment}.
@@ -64,7 +63,7 @@ public class MessageDetailActivity extends AppCompatActivity {
             //
             // http://developer.android.com/design/patterns/navigation.html#up-vs-back
             //
-            NavUtils.navigateUpTo(this, new Intent(this, MessageListActivity.class));
+            NavUtils.navigateUpTo(this, new Intent(this, MainActivity.class));
             return true;
         }
         return super.onOptionsItemSelected(item);
diff --git a/app/src/main/java/ch/dissem/apps/abit/MessageDetailFragment.java b/app/src/main/java/ch/dissem/apps/abit/MessageDetailFragment.java
index 6ca9ce0..438e2f2 100644
--- a/app/src/main/java/ch/dissem/apps/abit/MessageDetailFragment.java
+++ b/app/src/main/java/ch/dissem/apps/abit/MessageDetailFragment.java
@@ -5,7 +5,6 @@ import android.os.Bundle;
 import android.support.v4.app.Fragment;
 import android.text.util.Linkify;
 import android.text.util.Linkify.TransformFilter;
-import android.util.Patterns;
 import android.view.LayoutInflater;
 import android.view.Menu;
 import android.view.MenuInflater;
@@ -19,7 +18,6 @@ import com.mikepenz.google_material_typeface_library.GoogleMaterial;
 
 import java.util.Iterator;
 import java.util.regex.Matcher;
-import java.util.regex.Pattern;
 
 import ch.dissem.apps.abit.service.Singleton;
 import ch.dissem.apps.abit.util.Drawables;
@@ -28,8 +26,6 @@ import ch.dissem.bitmessage.entity.Plaintext;
 import ch.dissem.bitmessage.entity.valueobject.Label;
 import ch.dissem.bitmessage.ports.MessageRepository;
 
-import static android.text.util.Linkify.ALL;
-import static android.text.util.Linkify.EMAIL_ADDRESSES;
 import static android.text.util.Linkify.WEB_URLS;
 import static ch.dissem.apps.abit.util.Constants.BITMESSAGE_ADDRESS_PATTERN;
 import static ch.dissem.apps.abit.util.Constants.BITMESSAGE_URL_SCHEMA;
@@ -37,7 +33,7 @@ import static ch.dissem.apps.abit.util.Constants.BITMESSAGE_URL_SCHEMA;
 
 /**
  * A fragment representing a single Message detail screen.
- * This fragment is either contained in a {@link MessageListActivity}
+ * This fragment is either contained in a {@link MainActivity}
  * in two-pane mode (on tablets) or a {@link MessageDetailActivity}
  * on handsets.
  */
diff --git a/app/src/main/java/ch/dissem/apps/abit/MessageListFragment.java b/app/src/main/java/ch/dissem/apps/abit/MessageListFragment.java
index 9e6d1db..e844320 100644
--- a/app/src/main/java/ch/dissem/apps/abit/MessageListFragment.java
+++ b/app/src/main/java/ch/dissem/apps/abit/MessageListFragment.java
@@ -1,7 +1,5 @@
 package ch.dissem.apps.abit;
 
-import android.annotation.SuppressLint;
-import android.content.Context;
 import android.content.Intent;
 import android.graphics.Typeface;
 import android.os.Bundle;
@@ -55,7 +53,7 @@ public class MessageListFragment extends AbstractItemListFragment<Plaintext> {
     public void onResume() {
         super.onResume();
 
-        updateList(((MessageListActivity) getActivity()).getSelectedLabel());
+        doUpdateList(((MainActivity) getActivity()).getSelectedLabel());
     }
 
     @Override
@@ -64,6 +62,10 @@ public class MessageListFragment extends AbstractItemListFragment<Plaintext> {
 
         if (!isVisible()) return;
 
+        doUpdateList(label);
+    }
+
+    private void doUpdateList(Label label) {
         setListAdapter(new ArrayAdapter<Plaintext>(
                 getActivity(),
                 android.R.layout.simple_list_item_activated_1,
@@ -114,7 +116,7 @@ public class MessageListFragment extends AbstractItemListFragment<Plaintext> {
             @Override
             public void onClick(View view) {
                 Intent intent = new Intent(getActivity().getApplicationContext(), ComposeMessageActivity.class);
-                intent.putExtra(ComposeMessageActivity.EXTRA_IDENTITY, ((MessageListActivity)getActivity()).getSelectedIdentity());
+                intent.putExtra(ComposeMessageActivity.EXTRA_IDENTITY, ((MainActivity) getActivity()).getSelectedIdentity());
                 startActivity(intent);
             }
         });
diff --git a/app/src/main/java/ch/dissem/apps/abit/OpenBitmessageLinkActivity.java b/app/src/main/java/ch/dissem/apps/abit/OpenBitmessageLinkActivity.java
index 732ee2e..138ab77 100644
--- a/app/src/main/java/ch/dissem/apps/abit/OpenBitmessageLinkActivity.java
+++ b/app/src/main/java/ch/dissem/apps/abit/OpenBitmessageLinkActivity.java
@@ -43,7 +43,6 @@ import ch.dissem.bitmessage.entity.BitmessageAddress;
 import static ch.dissem.apps.abit.service.BitmessageService.DATA_FIELD_ADDRESS;
 import static ch.dissem.apps.abit.service.BitmessageService.MSG_ADD_CONTACT;
 import static ch.dissem.apps.abit.service.BitmessageService.MSG_SUBSCRIBE;
-import static ch.dissem.apps.abit.service.BitmessageService.MSG_SUBSCRIBE_AND_ADD_CONTACT;
 
 public class OpenBitmessageLinkActivity extends AppCompatActivity {
     private static final Logger LOG = LoggerFactory.getLogger(OpenBitmessageLinkActivity.class);
@@ -106,7 +105,7 @@ public class OpenBitmessageLinkActivity extends AppCompatActivity {
 
                 final int what;
                 if (subscribe.isChecked())
-                    what = MSG_SUBSCRIBE_AND_ADD_CONTACT;
+                    what = MSG_SUBSCRIBE;
                 else
                     what = MSG_ADD_CONTACT;
 
@@ -155,7 +154,8 @@ public class OpenBitmessageLinkActivity extends AppCompatActivity {
     @Override
     protected void onStart() {
         super.onStart();
-        bindService(new Intent(this, BitmessageService.class), connection, Context.BIND_AUTO_CREATE);
+        bindService(new Intent(this, BitmessageService.class), connection, Context
+                .BIND_AUTO_CREATE);
     }
 
     @Override
diff --git a/app/src/main/java/ch/dissem/apps/abit/SettingsActivity.java b/app/src/main/java/ch/dissem/apps/abit/SettingsActivity.java
index 670e299..f7f7acb 100644
--- a/app/src/main/java/ch/dissem/apps/abit/SettingsActivity.java
+++ b/app/src/main/java/ch/dissem/apps/abit/SettingsActivity.java
@@ -5,7 +5,7 @@ import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.Toolbar;
 
 /**
- * Created by chris on 14.07.15.
+ * @author Christian Basler
  */
 public class SettingsActivity extends AppCompatActivity {
     @Override
diff --git a/app/src/main/java/ch/dissem/apps/abit/SettingsFragment.java b/app/src/main/java/ch/dissem/apps/abit/SettingsFragment.java
index ffba5a9..75bd0bc 100644
--- a/app/src/main/java/ch/dissem/apps/abit/SettingsFragment.java
+++ b/app/src/main/java/ch/dissem/apps/abit/SettingsFragment.java
@@ -1,13 +1,22 @@
 package ch.dissem.apps.abit;
 
+import android.content.Context;
+import android.content.SharedPreferences;
 import android.os.Bundle;
-import android.preference.PreferenceActivity;
 import android.preference.PreferenceFragment;
+import android.preference.PreferenceManager;
+
+import ch.dissem.apps.abit.synchronization.SyncAdapter;
+
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_SERVER_POW;
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_TRUSTED_NODE;
 
 /**
- * Created by chris on 14.07.15.
+ * @author Christian Basler
  */
-public class SettingsFragment extends PreferenceFragment {
+public class SettingsFragment
+        extends PreferenceFragment
+        implements SharedPreferences.OnSharedPreferenceChangeListener {
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
@@ -15,4 +24,32 @@ public class SettingsFragment extends PreferenceFragment {
         // Load the preferences from an XML resource
         addPreferencesFromResource(R.xml.preferences);
     }
+
+    @Override
+    public void onAttach(Context ctx) {
+        super.onAttach(ctx);
+        PreferenceManager.getDefaultSharedPreferences(ctx)
+                .registerOnSharedPreferenceChangeListener(this);
+    }
+
+    @Override
+    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
+        switch (key) {
+            case PREFERENCE_TRUSTED_NODE:
+                String node = sharedPreferences.getString(PREFERENCE_TRUSTED_NODE, null);
+                if (node != null) {
+                    SyncAdapter.startSync(getActivity());
+                } else {
+                    SyncAdapter.stopSync(getActivity());
+                }
+                break;
+            case PREFERENCE_SERVER_POW:
+                if (sharedPreferences.getBoolean(PREFERENCE_SERVER_POW, false)) {
+                    SyncAdapter.startPowSync(getActivity());
+                } else {
+                    SyncAdapter.stopPowSync(getActivity());
+                }
+                break;
+        }
+    }
 }
\ No newline at end of file
diff --git a/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailActivity.java b/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailActivity.java
index 57bf32b..f8c8faf 100644
--- a/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailActivity.java
+++ b/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailActivity.java
@@ -28,7 +28,7 @@ import android.view.MenuItem;
  * An activity representing a single Subscription detail screen. This
  * activity is only used on handset devices. On tablet-size devices,
  * item details are presented side-by-side with a list of items
- * in a {@link MessageListActivity}.
+ * in a {@link MainActivity}.
  * <p/>
  * This activity is mostly just a 'shell' activity containing nothing
  * more than a {@link SubscriptionDetailFragment}.
@@ -79,7 +79,7 @@ public class SubscriptionDetailActivity extends AppCompatActivity {
             //
             // http://developer.android.com/design/patterns/navigation.html#up-vs-back
             //
-            NavUtils.navigateUpTo(this, new Intent(this, MessageListActivity.class));
+            NavUtils.navigateUpTo(this, new Intent(this, MainActivity.class));
             return true;
         }
         return super.onOptionsItemSelected(item);
diff --git a/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailFragment.java b/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailFragment.java
index 50d7c76..6d7c3b4 100644
--- a/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailFragment.java
+++ b/app/src/main/java/ch/dissem/apps/abit/SubscriptionDetailFragment.java
@@ -34,7 +34,7 @@ import ch.dissem.bitmessage.entity.BitmessageAddress;
 
 /**
  * A fragment representing a single Message detail screen.
- * This fragment is either contained in a {@link MessageListActivity}
+ * This fragment is either contained in a {@link MainActivity}
  * in two-pane mode (on tablets) or a {@link MessageDetailActivity}
  * on handsets.
  */
diff --git a/app/src/main/java/ch/dissem/apps/abit/SubscriptionListFragment.java b/app/src/main/java/ch/dissem/apps/abit/SubscriptionListFragment.java
index 4da7c4e..b84aa54 100644
--- a/app/src/main/java/ch/dissem/apps/abit/SubscriptionListFragment.java
+++ b/app/src/main/java/ch/dissem/apps/abit/SubscriptionListFragment.java
@@ -16,6 +16,7 @@
 
 package ch.dissem.apps.abit;
 
+import android.content.Context;
 import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.view.LayoutInflater;
@@ -25,6 +26,7 @@ import android.widget.ArrayAdapter;
 import android.widget.ImageView;
 import android.widget.TextView;
 
+import ch.dissem.apps.abit.listener.ActionBarListener;
 import ch.dissem.apps.abit.service.Singleton;
 import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.entity.valueobject.Label;
@@ -100,12 +102,18 @@ public class SubscriptionListFragment extends AbstractItemListFragment<Bitmessag
         });
     }
 
+    @Override
+    public void onAttach(Context ctx) {
+        super.onAttach(ctx);
+        if (ctx instanceof ActionBarListener){
+            ((ActionBarListener) ctx).updateTitle(getString(R.string.contacts_and_subscriptions));
+        }
+    }
+
     @Nullable
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
-        View rootView = inflater.inflate(R.layout.fragment_contact_list, container, false);
-
-        return rootView;
+        return inflater.inflate(R.layout.fragment_contact_list, container, false);
     }
 
     @Override
diff --git a/app/src/main/java/ch/dissem/apps/abit/adapter/AndroidSecurity.java b/app/src/main/java/ch/dissem/apps/abit/adapter/AndroidSecurity.java
new file mode 100644
index 0000000..e495ff1
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/adapter/AndroidSecurity.java
@@ -0,0 +1,34 @@
+package ch.dissem.apps.abit.adapter;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.preference.PreferenceManager;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+
+import ch.dissem.apps.abit.util.PRNGFixes;
+import ch.dissem.bitmessage.entity.BitmessageAddress;
+import ch.dissem.bitmessage.entity.ObjectMessage;
+import ch.dissem.bitmessage.entity.Plaintext;
+import ch.dissem.bitmessage.entity.PlaintextHolder;
+import ch.dissem.bitmessage.entity.payload.Broadcast;
+import ch.dissem.bitmessage.entity.valueobject.Label;
+import ch.dissem.bitmessage.factory.Factory;
+import ch.dissem.bitmessage.ports.ProofOfWorkEngine;
+import ch.dissem.bitmessage.security.sc.SpongySecurity;
+import ch.dissem.bitmessage.utils.UnixTime;
+
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_SERVER_POW;
+import static ch.dissem.bitmessage.entity.Plaintext.Status.SENT;
+import static ch.dissem.bitmessage.entity.Plaintext.Type.BROADCAST;
+import static ch.dissem.bitmessage.utils.UnixTime.DAY;
+
+/**
+ * @author Christian Basler
+ */
+public class AndroidSecurity extends SpongySecurity {
+    public AndroidSecurity() {
+        PRNGFixes.apply();
+    }
+}
diff --git a/app/src/main/java/ch/dissem/apps/abit/adapter/SwitchingProofOfWorkEngine.java b/app/src/main/java/ch/dissem/apps/abit/adapter/SwitchingProofOfWorkEngine.java
new file mode 100644
index 0000000..d129e07
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/adapter/SwitchingProofOfWorkEngine.java
@@ -0,0 +1,52 @@
+package ch.dissem.apps.abit.adapter;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.preference.PreferenceManager;
+
+import java.util.Arrays;
+
+import ch.dissem.apps.abit.util.Preferences;
+import ch.dissem.bitmessage.InternalContext;
+import ch.dissem.bitmessage.ports.ProofOfWorkEngine;
+
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_SERVER_POW;
+
+/**
+ * Switches between two {@link ProofOfWorkEngine}s depending on the configuration.
+ *
+ * @author Christian Basler
+ */
+public class SwitchingProofOfWorkEngine implements ProofOfWorkEngine, InternalContext.ContextHolder {
+    private final Context ctx;
+    private final String preference;
+    private final ProofOfWorkEngine option;
+    private final ProofOfWorkEngine fallback;
+
+    public SwitchingProofOfWorkEngine(Context ctx, String preference,
+                                      ProofOfWorkEngine option, ProofOfWorkEngine fallback) {
+        this.ctx = ctx;
+        this.preference = preference;
+        this.option = option;
+        this.fallback = fallback;
+    }
+
+    @Override
+    public void calculateNonce(byte[] initialHash, byte[] target, Callback callback) {
+        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(ctx);
+        if (preferences.getBoolean(preference, false)) {
+            option.calculateNonce(initialHash, target, callback);
+        } else {
+            fallback.calculateNonce(initialHash, target, callback);
+        }
+    }
+
+    @Override
+    public void setContext(InternalContext context) {
+        for (ProofOfWorkEngine e : Arrays.asList(option, fallback)) {
+            if (e instanceof InternalContext.ContextHolder) {
+                ((InternalContext.ContextHolder) e).setContext(context);
+            }
+        }
+    }
+}
diff --git a/app/src/main/java/ch/dissem/apps/abit/listener/MessageListener.java b/app/src/main/java/ch/dissem/apps/abit/listener/MessageListener.java
index edf17b3..16acfbd 100644
--- a/app/src/main/java/ch/dissem/apps/abit/listener/MessageListener.java
+++ b/app/src/main/java/ch/dissem/apps/abit/listener/MessageListener.java
@@ -18,25 +18,13 @@ package ch.dissem.apps.abit.listener;
 
 import android.annotation.TargetApi;
 import android.app.NotificationManager;
-import android.app.PendingIntent;
 import android.content.Context;
-import android.content.Intent;
 import android.database.Cursor;
-import android.graphics.Bitmap;
-import android.graphics.Canvas;
-import android.graphics.Typeface;
 import android.net.Uri;
 import android.os.Build;
 import android.provider.ContactsContract;
 import android.support.v7.app.NotificationCompat;
-import android.text.Spannable;
-import android.text.SpannableString;
-import android.text.Spanned;
-import android.text.style.StyleSpan;
 
-import ch.dissem.apps.abit.Identicon;
-import ch.dissem.apps.abit.MessageListActivity;
-import ch.dissem.apps.abit.R;
 import ch.dissem.apps.abit.notification.NewMessageNotification;
 import ch.dissem.bitmessage.BitmessageContext;
 import ch.dissem.bitmessage.entity.Plaintext;
diff --git a/app/src/main/java/ch/dissem/apps/abit/notification/NetworkNotification.java b/app/src/main/java/ch/dissem/apps/abit/notification/NetworkNotification.java
index bff6e92..6236546 100644
--- a/app/src/main/java/ch/dissem/apps/abit/notification/NetworkNotification.java
+++ b/app/src/main/java/ch/dissem/apps/abit/notification/NetworkNotification.java
@@ -10,7 +10,7 @@ import android.support.v7.app.NotificationCompat;
 import java.util.Timer;
 import java.util.TimerTask;
 
-import ch.dissem.apps.abit.MessageListActivity;
+import ch.dissem.apps.abit.MainActivity;
 import ch.dissem.apps.abit.R;
 import ch.dissem.bitmessage.BitmessageContext;
 import ch.dissem.bitmessage.utils.Property;
@@ -64,7 +64,7 @@ public class NetworkNotification extends AbstractNotification {
             }
             builder.setContentText(info);
         }
-        Intent showMessageIntent = new Intent(ctx, MessageListActivity.class);
+        Intent showMessageIntent = new Intent(ctx, MainActivity.class);
         PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 1, showMessageIntent, 0);
         builder.setContentIntent(pendingIntent);
         notification = builder.build();
diff --git a/app/src/main/java/ch/dissem/apps/abit/notification/NewMessageNotification.java b/app/src/main/java/ch/dissem/apps/abit/notification/NewMessageNotification.java
index 4e66ab1..efe1f8e 100644
--- a/app/src/main/java/ch/dissem/apps/abit/notification/NewMessageNotification.java
+++ b/app/src/main/java/ch/dissem/apps/abit/notification/NewMessageNotification.java
@@ -13,7 +13,7 @@ import android.text.style.StyleSpan;
 import java.util.LinkedList;
 
 import ch.dissem.apps.abit.Identicon;
-import ch.dissem.apps.abit.MessageListActivity;
+import ch.dissem.apps.abit.MainActivity;
 import ch.dissem.apps.abit.R;
 import ch.dissem.bitmessage.entity.Plaintext;
 
@@ -38,8 +38,8 @@ public class NewMessageNotification extends AbstractNotification {
                 .setStyle(new NotificationCompat.BigTextStyle().bigText(bigText))
                 .setContentInfo("Info");
 
-        Intent showMessageIntent = new Intent(ctx, MessageListActivity.class);
-        showMessageIntent.putExtra(MessageListActivity.EXTRA_SHOW_MESSAGE, plaintext);
+        Intent showMessageIntent = new Intent(ctx, MainActivity.class);
+        showMessageIntent.putExtra(MainActivity.EXTRA_SHOW_MESSAGE, plaintext);
         PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, showMessageIntent, PendingIntent.FLAG_UPDATE_CURRENT);
         builder.setContentIntent(pendingIntent);
 
@@ -66,8 +66,8 @@ public class NewMessageNotification extends AbstractNotification {
         }
         builder.setStyle(inboxStyle);
 
-        Intent intent = new Intent(ctx, MessageListActivity.class);
-        intent.setAction(MessageListActivity.ACTION_SHOW_INBOX);
+        Intent intent = new Intent(ctx, MainActivity.class);
+        intent.setAction(MainActivity.ACTION_SHOW_INBOX);
         PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 1, intent, 0);
         builder.setContentIntent(pendingIntent);
         notification = builder.build();
diff --git a/app/src/main/java/ch/dissem/apps/abit/notification/ProofOfWorkNotification.java b/app/src/main/java/ch/dissem/apps/abit/notification/ProofOfWorkNotification.java
index f917a83..b0b33b4 100644
--- a/app/src/main/java/ch/dissem/apps/abit/notification/ProofOfWorkNotification.java
+++ b/app/src/main/java/ch/dissem/apps/abit/notification/ProofOfWorkNotification.java
@@ -5,7 +5,7 @@ import android.content.Context;
 import android.content.Intent;
 import android.support.v7.app.NotificationCompat;
 
-import ch.dissem.apps.abit.MessageListActivity;
+import ch.dissem.apps.abit.MainActivity;
 import ch.dissem.apps.abit.R;
 
 /**
@@ -18,7 +18,7 @@ public class ProofOfWorkNotification extends AbstractNotification {
         super(ctx);
         NotificationCompat.Builder builder = new NotificationCompat.Builder(ctx);
 
-        Intent showMessageIntent = new Intent(ctx, MessageListActivity.class);
+        Intent showMessageIntent = new Intent(ctx, MainActivity.class);
         PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, showMessageIntent, PendingIntent.FLAG_UPDATE_CURRENT);
 
         builder.setVisibility(NotificationCompat.VISIBILITY_PUBLIC)
diff --git a/app/src/main/java/ch/dissem/apps/abit/pow/ServerPowEngine.java b/app/src/main/java/ch/dissem/apps/abit/pow/ServerPowEngine.java
new file mode 100644
index 0000000..a8b76af
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/pow/ServerPowEngine.java
@@ -0,0 +1,81 @@
+package ch.dissem.apps.abit.pow;
+
+import android.content.Context;
+import android.support.annotation.NonNull;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadFactory;
+
+import ch.dissem.apps.abit.service.Singleton;
+import ch.dissem.apps.abit.synchronization.SyncAdapter;
+import ch.dissem.apps.abit.util.Preferences;
+import ch.dissem.bitmessage.InternalContext;
+import ch.dissem.bitmessage.entity.BitmessageAddress;
+import ch.dissem.bitmessage.extensions.CryptoCustomMessage;
+import ch.dissem.bitmessage.extensions.pow.ProofOfWorkRequest;
+import ch.dissem.bitmessage.ports.ProofOfWorkEngine;
+
+import static ch.dissem.bitmessage.extensions.pow.ProofOfWorkRequest.Request.CALCULATE;
+import static ch.dissem.bitmessage.utils.Singleton.security;
+
+/**
+ * @author Christian Basler
+ */
+public class ServerPowEngine implements ProofOfWorkEngine, InternalContext
+        .ContextHolder {
+    private static final Logger LOG = LoggerFactory.getLogger(ServerPowEngine.class);
+
+    private final Context ctx;
+    private InternalContext context;
+
+    private final ExecutorService pool;
+
+    public ServerPowEngine(Context ctx) {
+        this.ctx = ctx;
+        pool = Executors.newCachedThreadPool(new ThreadFactory() {
+            @Override
+            public Thread newThread(@NonNull Runnable r) {
+                Thread thread = Executors.defaultThreadFactory().newThread(r);
+                thread.setPriority(Thread.MIN_PRIORITY);
+                return thread;
+            }
+        });
+    }
+
+    @Override
+    public void calculateNonce(final byte[] initialHash, final byte[] target, Callback callback) {
+        pool.execute(new Runnable() {
+            @Override
+            public void run() {
+                BitmessageAddress identity = Singleton.getIdentity(ctx);
+                if (identity == null) throw new RuntimeException("No Identity for calculating POW");
+
+                ProofOfWorkRequest request = new ProofOfWorkRequest(identity, initialHash,
+                        CALCULATE, target);
+                SyncAdapter.startPowSync(ctx);
+                try {
+                    CryptoCustomMessage<ProofOfWorkRequest> cryptoMsg = new CryptoCustomMessage<>
+                            (request);
+                    cryptoMsg.signAndEncrypt(
+                            identity,
+                            security().createPublicKey(identity.getPublicDecryptionKey())
+                    );
+                    context.getNetworkHandler().send(
+                            Preferences.getTrustedNode(ctx), Preferences.getTrustedNodePort(ctx),
+                            cryptoMsg);
+                } catch (Exception e) {
+                    LOG.error(e.getMessage(), e);
+                }
+            }
+        });
+    }
+
+    @Override
+    public void setContext(InternalContext context) {
+        this.context = context;
+    }
+}
diff --git a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidAddressRepository.java b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidAddressRepository.java
index 9d490df..abf123a 100644
--- a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidAddressRepository.java
+++ b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidAddressRepository.java
@@ -19,6 +19,7 @@ package ch.dissem.apps.abit.repository;
 import android.content.ContentValues;
 import android.database.Cursor;
 import android.database.sqlite.SQLiteDatabase;
+
 import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.entity.payload.Pubkey;
 import ch.dissem.bitmessage.entity.payload.V3Pubkey;
@@ -27,6 +28,7 @@ import ch.dissem.bitmessage.entity.valueobject.PrivateKey;
 import ch.dissem.bitmessage.factory.Factory;
 import ch.dissem.bitmessage.ports.AddressRepository;
 import ch.dissem.bitmessage.utils.Encode;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -118,27 +120,30 @@ public class AndroidAddressRepository implements AddressRepository {
                 COLUMN_SUBSCRIBED
         };
 
+        SQLiteDatabase db = sql.getReadableDatabase();
+        Cursor c = db.query(
+                TABLE_NAME, projection,
+                where,
+                null, null, null, null
+        );
         try {
-            SQLiteDatabase db = sql.getReadableDatabase();
-            Cursor c = db.query(
-                    TABLE_NAME, projection,
-                    where,
-                    null, null, null, null
-            );
             c.moveToFirst();
             while (!c.isAfterLast()) {
                 BitmessageAddress address;
 
                 byte[] privateKeyBytes = c.getBlob(c.getColumnIndex(COLUMN_PRIVATE_KEY));
                 if (privateKeyBytes != null) {
-                    PrivateKey privateKey = PrivateKey.read(new ByteArrayInputStream(privateKeyBytes));
+                    PrivateKey privateKey = PrivateKey.read(new ByteArrayInputStream
+                            (privateKeyBytes));
                     address = new BitmessageAddress(privateKey);
                 } else {
                     address = new BitmessageAddress(c.getString(c.getColumnIndex(COLUMN_ADDRESS)));
                     byte[] publicKeyBytes = c.getBlob(c.getColumnIndex(COLUMN_PUBLIC_KEY));
                     if (publicKeyBytes != null) {
-                        Pubkey pubkey = Factory.readPubkey(address.getVersion(), address.getStream(),
-                                new ByteArrayInputStream(publicKeyBytes), publicKeyBytes.length, false);
+                        Pubkey pubkey = Factory.readPubkey(address.getVersion(), address
+                                        .getStream(),
+                                new ByteArrayInputStream(publicKeyBytes), publicKeyBytes.length,
+                                false);
                         if (address.getVersion() == 4 && pubkey instanceof V3Pubkey) {
                             pubkey = new V4Pubkey((V3Pubkey) pubkey);
                         }
@@ -153,8 +158,9 @@ public class AndroidAddressRepository implements AddressRepository {
             }
         } catch (IOException e) {
             LOG.error(e.getMessage(), e);
+        } finally {
+            c.close();
         }
-
         return result;
     }
 
@@ -173,9 +179,14 @@ public class AndroidAddressRepository implements AddressRepository {
 
     private boolean exists(BitmessageAddress address) {
         SQLiteDatabase db = sql.getReadableDatabase();
-        Cursor cursor = db.rawQuery("SELECT COUNT(*) FROM Address WHERE address='" + address.getAddress() + "'", null);
-        cursor.moveToFirst();
-        return cursor.getInt(0) > 0;
+        Cursor cursor = db.rawQuery("SELECT COUNT(*) FROM Address WHERE address='" + address
+                .getAddress() + "'", null);
+        try {
+            cursor.moveToFirst();
+            return cursor.getInt(0) > 0;
+        } finally {
+            cursor.close();
+        }
     }
 
     private void update(BitmessageAddress address) throws IOException {
@@ -194,7 +205,8 @@ public class AndroidAddressRepository implements AddressRepository {
             values.put(COLUMN_PRIVATE_KEY, Encode.bytes(address.getPrivateKey()));
             values.put(COLUMN_SUBSCRIBED, address.isSubscribed());
 
-            int update = db.update(TABLE_NAME, values, "address = '" + address.getAddress() + "'", null);
+            int update = db.update(TABLE_NAME, values, "address = '" + address.getAddress() +
+                    "'", null);
             if (update < 0) {
                 LOG.error("Could not update address " + address);
             }
diff --git a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidInventory.java b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidInventory.java
index 5e5f1c5..56ece70 100644
--- a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidInventory.java
+++ b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidInventory.java
@@ -74,15 +74,20 @@ public class AndroidInventory implements Inventory {
         SQLiteDatabase db = sql.getReadableDatabase();
         Cursor c = db.query(
                 TABLE_NAME, projection,
-                (includeExpired ? "" : "expires > " + now() + " AND ") + "stream IN (" + join(streams) + ")",
+                (includeExpired ? "" : "expires > " + now() + " AND ") + "stream IN (" + join
+                        (streams) + ")",
                 null, null, null, null
         );
-        c.moveToFirst();
         List<InventoryVector> result = new LinkedList<>();
-        while (!c.isAfterLast()) {
-            byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_HASH));
-            result.add(new InventoryVector(blob));
-            c.moveToNext();
+        try {
+            c.moveToFirst();
+            while (!c.isAfterLast()) {
+                byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_HASH));
+                result.add(new InventoryVector(blob));
+                c.moveToNext();
+            }
+        } finally {
+            c.close();
         }
         return result;
     }
@@ -108,15 +113,19 @@ public class AndroidInventory implements Inventory {
                 "hash = X'" + vector + "'",
                 null, null, null, null
         );
-        c.moveToFirst();
-        if (c.isAfterLast()) {
-            LOG.info("Object requested that we don't have. IV: " + vector);
-            return null;
-        }
+        try {
+            c.moveToFirst();
+            if (c.isAfterLast()) {
+                LOG.info("Object requested that we don't have. IV: " + vector);
+                return null;
+            }
 
-        int version = c.getInt(c.getColumnIndex(COLUMN_VERSION));
-        byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_DATA));
-        return Factory.getObjectMessage(version, new ByteArrayInputStream(blob), blob.length);
+            int version = c.getInt(c.getColumnIndex(COLUMN_VERSION));
+            byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_DATA));
+            return Factory.getObjectMessage(version, new ByteArrayInputStream(blob), blob.length);
+        } finally {
+            c.close();
+        }
     }
 
     @Override
@@ -144,13 +153,18 @@ public class AndroidInventory implements Inventory {
                 where.toString(),
                 null, null, null, null
         );
-        c.moveToFirst();
         List<ObjectMessage> result = new LinkedList<>();
-        while (!c.isAfterLast()) {
-            int objectVersion = c.getInt(c.getColumnIndex(COLUMN_VERSION));
-            byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_DATA));
-            result.add(Factory.getObjectMessage(objectVersion, new ByteArrayInputStream(blob), blob.length));
-            c.moveToNext();
+        try {
+            c.moveToFirst();
+            while (!c.isAfterLast()) {
+                int objectVersion = c.getInt(c.getColumnIndex(COLUMN_VERSION));
+                byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_DATA));
+                result.add(Factory.getObjectMessage(objectVersion, new ByteArrayInputStream(blob),
+                        blob.length));
+                c.moveToNext();
+            }
+        } finally {
+            c.close();
         }
         return result;
     }
@@ -187,7 +201,11 @@ public class AndroidInventory implements Inventory {
                 "hash = X'" + object.getInventoryVector() + "'",
                 null, null, null, null
         );
-        return c.getCount() > 0;
+        try {
+            return c.getCount() > 0;
+        } finally {
+            c.close();
+        }
     }
 
     @Override
diff --git a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidMessageRepository.java b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidMessageRepository.java
index eba1b0d..f33c91b 100644
--- a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidMessageRepository.java
+++ b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidMessageRepository.java
@@ -30,6 +30,7 @@ import ch.dissem.bitmessage.entity.valueobject.InventoryVector;
 import ch.dissem.bitmessage.entity.valueobject.Label;
 import ch.dissem.bitmessage.ports.MessageRepository;
 import ch.dissem.bitmessage.utils.Encode;
+import ch.dissem.bitmessage.utils.Strings;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -58,6 +59,7 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
     private static final String COLUMN_SENT = "sent";
     private static final String COLUMN_RECEIVED = "received";
     private static final String COLUMN_STATUS = "status";
+    private static final String COLUMN_INITIAL_HASH = "initial_hash";
 
     private static final String JOIN_TABLE_NAME = "Message_Label";
     private static final String JT_COLUMN_MESSAGE = "message_id";
@@ -112,10 +114,14 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
                 null, null, null,
                 LBL_COLUMN_ORDER
         );
-        c.moveToFirst();
-        while (!c.isAfterLast()) {
-            result.add(getLabel(c));
-            c.moveToNext();
+        try {
+            c.moveToFirst();
+            while (!c.isAfterLast()) {
+                result.add(getLabel(c));
+                c.moveToNext();
+            }
+        } finally {
+            c.close();
         }
         return result;
     }
@@ -124,27 +130,31 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
         String typeName = c.getString(c.getColumnIndex(LBL_COLUMN_TYPE));
         Label.Type type = typeName == null ? null : Label.Type.valueOf(typeName);
         String text;
-        switch (type) {
-            case INBOX:
-                text = ctx.getString(R.string.inbox);
-                break;
-            case DRAFT:
-                text = ctx.getString(R.string.draft);
-                break;
-            case SENT:
-                text = ctx.getString(R.string.sent);
-                break;
-            case UNREAD:
-                text = ctx.getString(R.string.unread);
-                break;
-            case TRASH:
-                text = ctx.getString(R.string.trash);
-                break;
-            case BROADCAST:
-                text = ctx.getString(R.string.broadcasts);
-                break;
-            default:
-                text = c.getString(c.getColumnIndex(LBL_COLUMN_LABEL));
+        if (type == null) {
+            text = c.getString(c.getColumnIndex(LBL_COLUMN_LABEL));
+        } else {
+            switch (type) {
+                case INBOX:
+                    text = ctx.getString(R.string.inbox);
+                    break;
+                case DRAFT:
+                    text = ctx.getString(R.string.draft);
+                    break;
+                case SENT:
+                    text = ctx.getString(R.string.sent);
+                    break;
+                case UNREAD:
+                    text = ctx.getString(R.string.unread);
+                    break;
+                case TRASH:
+                    text = ctx.getString(R.string.trash);
+                    break;
+                case BROADCAST:
+                    text = ctx.getString(R.string.broadcasts);
+                    break;
+                default:
+                    text = c.getString(c.getColumnIndex(LBL_COLUMN_LABEL));
+            }
         }
         Label label = new Label(
                 text,
@@ -158,7 +168,8 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
     public int countUnread(Label label) {
         String where;
         if (label != null) {
-            where = "id IN (SELECT message_id FROM Message_Label WHERE label_id=" + label.getId() + ") AND ";
+            where = "id IN (SELECT message_id FROM Message_Label WHERE label_id=" + label.getId()
+                    + ") AND ";
         } else {
             where = "";
         }
@@ -169,13 +180,32 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
                         "SELECT id FROM Label WHERE type = '" + Label.Type.UNREAD.name() + "'))",
                 null, null, null, null
         );
-        return c.getColumnCount();
+        try {
+            return c.getColumnCount();
+        } finally {
+            c.close();
+        }
+    }
+
+    @Override
+    public Plaintext getMessage(byte[] initialHash) {
+        List<Plaintext> results = find("initial_hash=X'" + Strings.hex(initialHash) + "'");
+        switch (results.size()) {
+            case 0:
+                return null;
+            case 1:
+                return results.get(0);
+            default:
+                throw new RuntimeException("This shouldn't happen, found " + results.size() +
+                        " messages, one or none was expected");
+        }
     }
 
     @Override
     public List<Plaintext> findMessages(Label label) {
         if (label != null) {
-            return find("id IN (SELECT message_id FROM Message_Label WHERE label_id=" + label.getId() + ")");
+            return find("id IN (SELECT message_id FROM Message_Label WHERE label_id=" + label
+                    .getId() + ")");
         } else {
             return find("id NOT IN (SELECT message_id FROM Message_Label)");
         }
@@ -183,7 +213,8 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
 
     @Override
     public List<Plaintext> findMessages(Plaintext.Status status, BitmessageAddress recipient) {
-        return find("status='" + status.name() + "' AND recipient='" + recipient.getAddress() + "'");
+        return find("status='" + status.name() + "' AND recipient='" + recipient.getAddress() +
+                "'");
     }
 
     @Override
@@ -213,34 +244,41 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
                 COLUMN_STATUS
         };
 
+        SQLiteDatabase db = sql.getReadableDatabase();
+        Cursor c = db.query(
+                TABLE_NAME, projection,
+                where,
+                null, null, null,
+                COLUMN_RECEIVED + " DESC"
+        );
         try {
-            SQLiteDatabase db = sql.getReadableDatabase();
-            Cursor c = db.query(
-                    TABLE_NAME, projection,
-                    where,
-                    null, null, null,
-                    COLUMN_RECEIVED + " DESC"
-            );
             c.moveToFirst();
             while (!c.isAfterLast()) {
                 byte[] iv = c.getBlob(c.getColumnIndex(COLUMN_IV));
                 byte[] data = c.getBlob(c.getColumnIndex(COLUMN_DATA));
-                Plaintext.Type type = Plaintext.Type.valueOf(c.getString(c.getColumnIndex(COLUMN_TYPE)));
-                Plaintext.Builder builder = Plaintext.readWithoutSignature(type, new ByteArrayInputStream(data));
+                Plaintext.Type type = Plaintext.Type.valueOf(c.getString(c.getColumnIndex
+                        (COLUMN_TYPE)));
+                Plaintext.Builder builder = Plaintext.readWithoutSignature(type, new
+                        ByteArrayInputStream(data));
                 long id = c.getLong(c.getColumnIndex(COLUMN_ID));
                 builder.id(id);
                 builder.IV(new InventoryVector(iv));
-                builder.from(bmc.getAddressRepo().getAddress(c.getString(c.getColumnIndex(COLUMN_SENDER))));
-                builder.to(bmc.getAddressRepo().getAddress(c.getString(c.getColumnIndex(COLUMN_RECIPIENT))));
+                builder.from(bmc.getAddressRepository().getAddress(c.getString(c.getColumnIndex
+                        (COLUMN_SENDER))));
+                builder.to(bmc.getAddressRepository().getAddress(c.getString(c.getColumnIndex
+                        (COLUMN_RECIPIENT))));
                 builder.sent(c.getLong(c.getColumnIndex(COLUMN_SENT)));
                 builder.received(c.getLong(c.getColumnIndex(COLUMN_RECEIVED)));
-                builder.status(Plaintext.Status.valueOf(c.getString(c.getColumnIndex(COLUMN_STATUS))));
+                builder.status(Plaintext.Status.valueOf(c.getString(c.getColumnIndex
+                        (COLUMN_STATUS))));
                 builder.labels(findLabels(id));
                 result.add(builder.build());
                 c.moveToNext();
             }
         } catch (IOException e) {
             LOG.error(e.getMessage(), e);
+        } finally {
+            c.close();
         }
         return result;
     }
@@ -257,12 +295,13 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
 
             // save from address if necessary
             if (message.getId() == null) {
-                BitmessageAddress savedAddress = bmc.getAddressRepo().getAddress(message.getFrom().getAddress());
+                BitmessageAddress savedAddress = bmc.getAddressRepository().getAddress(message
+                        .getFrom().getAddress());
                 if (savedAddress == null || savedAddress.getPrivateKey() == null) {
                     if (savedAddress != null && savedAddress.getAlias() != null) {
                         message.getFrom().setAlias(savedAddress.getAlias());
                     }
-                    bmc.getAddressRepo().save(message.getFrom());
+                    bmc.getAddressRepository().save(message.getFrom());
                 }
             }
 
@@ -295,7 +334,8 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
 
     private void insert(SQLiteDatabase db, Plaintext message) throws IOException {
         ContentValues values = new ContentValues();
-        values.put(COLUMN_IV, message.getInventoryVector() == null ? null : message.getInventoryVector().getHash());
+        values.put(COLUMN_IV, message.getInventoryVector() == null ? null : message
+                .getInventoryVector().getHash());
         values.put(COLUMN_TYPE, message.getType().name());
         values.put(COLUMN_SENDER, message.getFrom().getAddress());
         values.put(COLUMN_RECIPIENT, message.getTo() == null ? null : message.getTo().getAddress());
@@ -303,13 +343,15 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
         values.put(COLUMN_SENT, message.getSent());
         values.put(COLUMN_RECEIVED, message.getReceived());
         values.put(COLUMN_STATUS, message.getStatus() == null ? null : message.getStatus().name());
+        values.put(COLUMN_INITIAL_HASH, message.getInitialHash());
         long id = db.insertOrThrow(TABLE_NAME, null, values);
         message.setId(id);
     }
 
     private void update(SQLiteDatabase db, Plaintext message) throws IOException {
         ContentValues values = new ContentValues();
-        values.put(COLUMN_IV, message.getInventoryVector() == null ? null : message.getInventoryVector().getHash());
+        values.put(COLUMN_IV, message.getInventoryVector() == null ? null : message
+                .getInventoryVector().getHash());
         values.put(COLUMN_TYPE, message.getType().name());
         values.put(COLUMN_SENDER, message.getFrom().getAddress());
         values.put(COLUMN_RECIPIENT, message.getTo() == null ? null : message.getTo().getAddress());
@@ -317,6 +359,7 @@ public class AndroidMessageRepository implements MessageRepository, InternalCont
         values.put(COLUMN_SENT, message.getSent());
         values.put(COLUMN_RECEIVED, message.getReceived());
         values.put(COLUMN_STATUS, message.getStatus() == null ? null : message.getStatus().name());
+        values.put(COLUMN_INITIAL_HASH, message.getInitialHash());
         db.update(TABLE_NAME, values, "id = " + message.getId(), null);
     }
 
diff --git a/app/src/main/java/ch/dissem/apps/abit/repository/AndroidProofOfWorkRepository.java b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidProofOfWorkRepository.java
new file mode 100644
index 0000000..2273df0
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/repository/AndroidProofOfWorkRepository.java
@@ -0,0 +1,133 @@
+package ch.dissem.apps.abit.repository;
+
+import android.content.ContentValues;
+import android.database.Cursor;
+import android.database.sqlite.SQLiteConstraintException;
+import android.database.sqlite.SQLiteDatabase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+
+import ch.dissem.bitmessage.entity.ObjectMessage;
+import ch.dissem.bitmessage.factory.Factory;
+import ch.dissem.bitmessage.ports.ProofOfWorkRepository;
+import ch.dissem.bitmessage.utils.Encode;
+import ch.dissem.bitmessage.utils.Strings;
+
+import static ch.dissem.bitmessage.utils.Singleton.security;
+
+/**
+ * @author Christian Basler
+ */
+public class AndroidProofOfWorkRepository implements ProofOfWorkRepository {
+    private static final Logger LOG = LoggerFactory.getLogger(AndroidProofOfWorkRepository.class);
+
+    private static final String TABLE_NAME = "POW";
+    private static final String COLUMN_INITIAL_HASH = "initial_hash";
+    private static final String COLUMN_DATA = "data";
+    private static final String COLUMN_VERSION = "version";
+    private static final String COLUMN_NONCE_TRIALS_PER_BYTE = "nonce_trials_per_byte";
+    private static final String COLUMN_EXTRA_BYTES = "extra_bytes";
+
+    private final SqlHelper sql;
+
+    public AndroidProofOfWorkRepository(SqlHelper sql) {
+        this.sql = sql;
+    }
+
+    @Override
+    public Item getItem(byte[] initialHash) {
+        // Define a projection that specifies which columns from the database
+        // you will actually use after this query.
+        String[] projection = {
+                COLUMN_DATA,
+                COLUMN_VERSION,
+                COLUMN_NONCE_TRIALS_PER_BYTE,
+                COLUMN_EXTRA_BYTES
+        };
+
+        SQLiteDatabase db = sql.getReadableDatabase();
+        Cursor c = db.query(
+                TABLE_NAME, projection,
+                "initial_hash = X'" + Strings.hex(initialHash) + "'",
+                null, null, null, null
+        );
+        try {
+            c.moveToFirst();
+            if (!c.isAfterLast()) {
+                int version = c.getInt(c.getColumnIndex(COLUMN_VERSION));
+                byte[] blob = c.getBlob(c.getColumnIndex(COLUMN_DATA));
+                return new Item(
+                        Factory.getObjectMessage(version, new ByteArrayInputStream(blob), blob
+                                .length),
+                        c.getLong(c.getColumnIndex(COLUMN_NONCE_TRIALS_PER_BYTE)),
+                        c.getLong(c.getColumnIndex(COLUMN_EXTRA_BYTES))
+                );
+            }
+        } finally {
+            c.close();
+        }
+        throw new RuntimeException("Object requested that we don't have. Initial hash: " +
+                Strings.hex(initialHash));
+    }
+
+    @Override
+    public List<byte[]> getItems() {
+        // Define a projection that specifies which columns from the database
+        // you will actually use after this query.
+        String[] projection = {
+                COLUMN_INITIAL_HASH
+        };
+
+        SQLiteDatabase db = sql.getReadableDatabase();
+        Cursor c = db.query(
+                TABLE_NAME, projection,
+                null, null, null, null, null
+        );
+        List<byte[]> result = new LinkedList<>();
+        try {
+            c.moveToFirst();
+            while (!c.isAfterLast()) {
+                byte[] initialHash = c.getBlob(c.getColumnIndex(COLUMN_INITIAL_HASH));
+                result.add(initialHash);
+                c.moveToNext();
+            }
+        } finally {
+            c.close();
+        }
+        return result;
+    }
+
+    @Override
+    public void putObject(ObjectMessage object, long nonceTrialsPerByte, long extraBytes) {
+        try {
+            SQLiteDatabase db = sql.getWritableDatabase();
+            // Create a new map of values, where column names are the keys
+            ContentValues values = new ContentValues();
+            values.put(COLUMN_INITIAL_HASH, security().getInitialHash(object));
+            values.put(COLUMN_DATA, Encode.bytes(object));
+            values.put(COLUMN_VERSION, object.getVersion());
+            values.put(COLUMN_NONCE_TRIALS_PER_BYTE, nonceTrialsPerByte);
+            values.put(COLUMN_EXTRA_BYTES, extraBytes);
+
+            db.insertOrThrow(TABLE_NAME, null, values);
+        } catch (SQLiteConstraintException e) {
+            LOG.trace(e.getMessage(), e);
+        } catch (IOException e) {
+            LOG.error(e.getMessage(), e);
+        }
+    }
+
+    @Override
+    public void removeObject(byte[] initialHash) {
+        SQLiteDatabase db = sql.getWritableDatabase();
+        db.delete(TABLE_NAME,
+                "initial_hash = X'" + Strings.hex(initialHash) + "'",
+                null);
+    }
+}
diff --git a/app/src/main/java/ch/dissem/apps/abit/repository/SqlHelper.java b/app/src/main/java/ch/dissem/apps/abit/repository/SqlHelper.java
index f79bbb2..9b5083a 100644
--- a/app/src/main/java/ch/dissem/apps/abit/repository/SqlHelper.java
+++ b/app/src/main/java/ch/dissem/apps/abit/repository/SqlHelper.java
@@ -26,7 +26,7 @@ import ch.dissem.apps.abit.util.Assets;
  */
 public class SqlHelper extends SQLiteOpenHelper {
     // If you change the database schema, you must increment the database version.
-    public static final int DATABASE_VERSION = 1;
+    public static final int DATABASE_VERSION = 2;
     public static final String DATABASE_NAME = "jabit.db";
 
     protected final Context ctx;
@@ -38,7 +38,7 @@ public class SqlHelper extends SQLiteOpenHelper {
 
     @Override
     public void onCreate(SQLiteDatabase db) {
-        onUpgrade(db, 0, 1);
+        onUpgrade(db, 0, 2);
     }
 
     @Override
@@ -48,6 +48,9 @@ public class SqlHelper extends SQLiteOpenHelper {
                 executeMigration(db, "V1.0__Create_table_inventory");
                 executeMigration(db, "V1.1__Create_table_address");
                 executeMigration(db, "V1.2__Create_table_message");
+            case 1:
+                // executeMigration(db, "V2.0__Update_table_message");
+                executeMigration(db, "V2.1__Create_table_POW");
             default:
                 // Nothing to do. Let's assume we won't upgrade from a version that's newer than DATABASE_VERSION.
         }
diff --git a/app/src/main/java/ch/dissem/apps/abit/service/BitmessageService.java b/app/src/main/java/ch/dissem/apps/abit/service/BitmessageService.java
index 55f54d5..d2184fd 100644
--- a/app/src/main/java/ch/dissem/apps/abit/service/BitmessageService.java
+++ b/app/src/main/java/ch/dissem/apps/abit/service/BitmessageService.java
@@ -34,7 +34,6 @@ public class BitmessageService extends Service {
     public static final int MSG_CREATE_IDENTITY = 10;
     public static final int MSG_SUBSCRIBE = 20;
     public static final int MSG_ADD_CONTACT = 21;
-    public static final int MSG_SUBSCRIBE_AND_ADD_CONTACT = 23;
     public static final int MSG_SEND_MESSAGE = 30;
     public static final int MSG_SEND_BROADCAST = 31;
     public static final int MSG_START_NODE = 100;
@@ -122,6 +121,13 @@ public class BitmessageService extends Service {
                     }
                     break;
                 }
+                case MSG_ADD_CONTACT: {
+                    Serializable data = msg.getData().getSerializable(DATA_FIELD_ADDRESS);
+                    if (data instanceof BitmessageAddress) {
+                        bmc.addContact((BitmessageAddress) data);
+                    }
+                    break;
+                }
                 case MSG_SEND_MESSAGE: {
                     Serializable identity = msg.getData().getSerializable(DATA_FIELD_IDENTITY);
                     Serializable address = msg.getData().getSerializable(DATA_FIELD_ADDRESS);
diff --git a/app/src/main/java/ch/dissem/apps/abit/service/ProofOfWorkService.java b/app/src/main/java/ch/dissem/apps/abit/service/ProofOfWorkService.java
index 7d86ef8..7cb7087 100644
--- a/app/src/main/java/ch/dissem/apps/abit/service/ProofOfWorkService.java
+++ b/app/src/main/java/ch/dissem/apps/abit/service/ProofOfWorkService.java
@@ -73,9 +73,9 @@ public class ProofOfWorkService extends Service {
             service.startForeground(ONGOING_NOTIFICATION_ID, notification.getNotification());
             engine.calculateNonce(initialHash, target, new ProofOfWorkEngine.Callback() {
                 @Override
-                public void onNonceCalculated(byte[] nonce) {
+                public void onNonceCalculated(byte[] initialHash, byte[] nonce) {
                     try {
-                        callback.onNonceCalculated(nonce);
+                        callback.onNonceCalculated(initialHash, nonce);
                     } finally {
                         service.stopForeground(true);
                         service.stopSelf();
diff --git a/app/src/main/java/ch/dissem/apps/abit/service/ServicePowEngine.java b/app/src/main/java/ch/dissem/apps/abit/service/ServicePowEngine.java
index 16d2aaa..eaffbdf 100644
--- a/app/src/main/java/ch/dissem/apps/abit/service/ServicePowEngine.java
+++ b/app/src/main/java/ch/dissem/apps/abit/service/ServicePowEngine.java
@@ -53,8 +53,8 @@ public class ServicePowEngine implements ProofOfWorkEngine, ProofOfWorkEngine.Ca
     }
 
     @Override
-    public void onNonceCalculated(byte[] bytes) {
-        callback.onNonceCalculated(bytes);
+    public void onNonceCalculated(byte[] initialHash, byte[] bytes) {
+        callback.onNonceCalculated(initialHash, bytes);
         ctx.unbindService(connection);
     }
 
diff --git a/app/src/main/java/ch/dissem/apps/abit/service/Singleton.java b/app/src/main/java/ch/dissem/apps/abit/service/Singleton.java
index a178519..51d9311 100644
--- a/app/src/main/java/ch/dissem/apps/abit/service/Singleton.java
+++ b/app/src/main/java/ch/dissem/apps/abit/service/Singleton.java
@@ -2,17 +2,27 @@ package ch.dissem.apps.abit.service;
 
 import android.content.Context;
 
+import java.util.List;
+
+import ch.dissem.apps.abit.adapter.AndroidSecurity;
+import ch.dissem.apps.abit.adapter.SwitchingProofOfWorkEngine;
 import ch.dissem.apps.abit.listener.MessageListener;
+import ch.dissem.apps.abit.pow.ServerPowEngine;
 import ch.dissem.apps.abit.repository.AndroidAddressRepository;
 import ch.dissem.apps.abit.repository.AndroidInventory;
 import ch.dissem.apps.abit.repository.AndroidMessageRepository;
+import ch.dissem.apps.abit.repository.AndroidProofOfWorkRepository;
 import ch.dissem.apps.abit.repository.SqlHelper;
+import ch.dissem.apps.abit.util.Constants;
 import ch.dissem.bitmessage.BitmessageContext;
+import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.networking.DefaultNetworkHandler;
 import ch.dissem.bitmessage.ports.AddressRepository;
 import ch.dissem.bitmessage.ports.MemoryNodeRegistry;
 import ch.dissem.bitmessage.ports.MessageRepository;
-import ch.dissem.bitmessage.security.sc.SpongySecurity;
+import ch.dissem.bitmessage.ports.ProofOfWorkRepository;
+
+import static ch.dissem.bitmessage.utils.UnixTime.DAY;
 
 /**
  * Provides singleton objects across the application.
@@ -21,6 +31,8 @@ public class Singleton {
     public static final Object lock = new Object();
     private static BitmessageContext bitmessageContext;
     private static MessageListener messageListener;
+    private static BitmessageAddress identity;
+    private static AndroidProofOfWorkRepository powRepo;
 
     public static BitmessageContext getBitmessageContext(Context context) {
         if (bitmessageContext == null) {
@@ -28,15 +40,23 @@ public class Singleton {
                 if (bitmessageContext == null) {
                     final Context ctx = context.getApplicationContext();
                     SqlHelper sqlHelper = new SqlHelper(ctx);
+                    powRepo = new AndroidProofOfWorkRepository(sqlHelper);
                     bitmessageContext = new BitmessageContext.Builder()
-                            .proofOfWorkEngine(new ServicePowEngine(ctx))
-                            .security(new SpongySecurity())
+                            .proofOfWorkEngine(new SwitchingProofOfWorkEngine(
+                                    ctx, Constants.PREFERENCE_SERVER_POW,
+                                    new ServerPowEngine(ctx),
+                                    new ServicePowEngine(ctx)
+                            ))
+                            .security(new AndroidSecurity())
                             .nodeRegistry(new MemoryNodeRegistry())
                             .inventory(new AndroidInventory(sqlHelper))
                             .addressRepo(new AndroidAddressRepository(sqlHelper))
                             .messageRepo(new AndroidMessageRepository(sqlHelper, ctx))
+                            .powRepo(powRepo)
                             .networkHandler(new DefaultNetworkHandler())
                             .listener(getMessageListener(ctx))
+                            .doNotSendPubkeyOnIdentityCreation()
+                            .pubkeyTTL(2 * DAY)
                             .build();
                 }
             }
@@ -62,4 +82,24 @@ public class Singleton {
     public static AddressRepository getAddressRepository(Context ctx) {
         return getBitmessageContext(ctx).addresses();
     }
+
+    public static ProofOfWorkRepository getProofOfWorkRepository(Context ctx) {
+        if (powRepo == null) getBitmessageContext(ctx);
+        return powRepo;
+    }
+
+    public static BitmessageAddress getIdentity(Context ctx) {
+        if (identity == null) {
+            synchronized (Singleton.class) {
+                if (identity == null) {
+                    List<BitmessageAddress> identities = getBitmessageContext(ctx).addresses()
+                            .getIdentities();
+                    if (identities.size() > 0) {
+                        identity = identities.get(0);
+                    }
+                }
+            }
+        }
+        return identity;
+    }
 }
diff --git a/app/src/main/java/ch/dissem/apps/abit/synchronization/Authenticator.java b/app/src/main/java/ch/dissem/apps/abit/synchronization/Authenticator.java
index 6d4eec5..009b35d 100644
--- a/app/src/main/java/ch/dissem/apps/abit/synchronization/Authenticator.java
+++ b/app/src/main/java/ch/dissem/apps/abit/synchronization/Authenticator.java
@@ -12,8 +12,8 @@ import android.os.Bundle;
  * of its methods
  */
 public class Authenticator extends AbstractAccountAuthenticator {
-    public static final String ACCOUNT_NAME = "Bitmessage";
-    public static final String ACCOUNT_TYPE = "ch.dissem.bitmessage";
+    public static final Account ACCOUNT_SYNC = new Account("Bitmessage", "ch.dissem.bitmessage");
+    public static final Account ACCOUNT_POW = new Account("Proof of Work ", "ch.dissem.bitmessage");
 
     // Simple constructor
     public Authenticator(Context context) {
diff --git a/app/src/main/java/ch/dissem/apps/abit/synchronization/SyncAdapter.java b/app/src/main/java/ch/dissem/apps/abit/synchronization/SyncAdapter.java
index 7654833..682e165 100644
--- a/app/src/main/java/ch/dissem/apps/abit/synchronization/SyncAdapter.java
+++ b/app/src/main/java/ch/dissem/apps/abit/synchronization/SyncAdapter.java
@@ -1,24 +1,34 @@
 package ch.dissem.apps.abit.synchronization;
 
 import android.accounts.Account;
+import android.accounts.AccountManager;
 import android.content.AbstractThreadedSyncAdapter;
 import android.content.ContentProviderClient;
+import android.content.ContentResolver;
 import android.content.Context;
-import android.content.SharedPreferences;
 import android.content.SyncResult;
 import android.os.Bundle;
-import android.preference.PreferenceManager;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.net.InetAddress;
-import java.net.UnknownHostException;
+import java.util.List;
 
-import ch.dissem.apps.abit.R;
-import ch.dissem.apps.abit.notification.ErrorNotification;
 import ch.dissem.apps.abit.service.Singleton;
+import ch.dissem.apps.abit.util.Preferences;
 import ch.dissem.bitmessage.BitmessageContext;
+import ch.dissem.bitmessage.entity.BitmessageAddress;
+import ch.dissem.bitmessage.entity.CustomMessage;
+import ch.dissem.bitmessage.extensions.CryptoCustomMessage;
+import ch.dissem.bitmessage.extensions.pow.ProofOfWorkRequest;
+import ch.dissem.bitmessage.ports.ProofOfWorkRepository;
+
+import static ch.dissem.apps.abit.synchronization.Authenticator.ACCOUNT_POW;
+import static ch.dissem.apps.abit.synchronization.Authenticator.ACCOUNT_SYNC;
+import static ch.dissem.apps.abit.synchronization.StubProvider.AUTHORITY;
+import static ch.dissem.bitmessage.extensions.pow.ProofOfWorkRequest.Request.CALCULATE;
+import static ch.dissem.bitmessage.extensions.pow.ProofOfWorkRequest.Request.COMPLETE;
+import static ch.dissem.bitmessage.utils.Singleton.security;
 
 /**
  * Sync Adapter to synchronize with the Bitmessage network - fetches
@@ -27,6 +37,8 @@ import ch.dissem.bitmessage.BitmessageContext;
 public class SyncAdapter extends AbstractThreadedSyncAdapter {
     private final static Logger LOG = LoggerFactory.getLogger(SyncAdapter.class);
 
+    private static final long SYNC_FREQUENCY = 15 * 60; // seconds
+
     private final BitmessageContext bmc;
 
     /**
@@ -38,7 +50,17 @@ public class SyncAdapter extends AbstractThreadedSyncAdapter {
     }
 
     @Override
-    public void onPerformSync(Account account, Bundle extras, String authority, ContentProviderClient provider, SyncResult syncResult) {
+    public void onPerformSync(Account account, Bundle extras, String authority,
+                              ContentProviderClient provider, SyncResult syncResult) {
+        if (account.equals(Authenticator.ACCOUNT_SYNC))
+            syncData();
+        else if (account.equals(Authenticator.ACCOUNT_POW))
+            syncPOW();
+        else
+            throw new RuntimeException("Unknown " + account);
+    }
+
+    private void syncData() {
         // If the Bitmessage context acts as a full node, synchronization isn't necessary
         if (bmc.isRunning()) {
             LOG.info("Synchronization skipped, Abit is acting as a full node");
@@ -46,40 +68,103 @@ public class SyncAdapter extends AbstractThreadedSyncAdapter {
         }
         LOG.info("Synchronizing Bitmessage");
 
-        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(getContext());
-
-        String trustedNode = preferences.getString("trusted_node", null);
-        if (trustedNode == null) return;
-        trustedNode = trustedNode.trim();
-        if (trustedNode.isEmpty()) return;
-
-        int port;
-        if (trustedNode.matches("^(?![0-9a-fA-F]*:[0-9a-fA-F]*:).*(:[0-9]+)$")) {
-            int index = trustedNode.lastIndexOf(':');
-            String portString = trustedNode.substring(index + 1);
-            trustedNode = trustedNode.substring(0, index);
-            try {
-                port = Integer.parseInt(portString);
-            } catch (NumberFormatException e) {
-                new ErrorNotification(getContext())
-                        .setError(R.string.error_invalid_sync_port, portString)
-                        .show();
-                return;
-            }
-        } else {
-            port = 8444;
-        }
-        long timeoutInSeconds = Long.parseLong(preferences.getString("sync_timeout", "120"));
         try {
             LOG.info("Synchronization started");
-            bmc.synchronize(InetAddress.getByName(trustedNode), port, timeoutInSeconds, true);
+            bmc.synchronize(
+                    Preferences.getTrustedNode(getContext()),
+                    Preferences.getTrustedNodePort(getContext()),
+                    Preferences.getTimeoutInSeconds(getContext()),
+                    true);
             LOG.info("Synchronization finished");
-        } catch (UnknownHostException e) {
-            new ErrorNotification(getContext())
-                    .setError(R.string.error_invalid_sync_host)
-                    .show();
         } catch (RuntimeException e) {
             LOG.error(e.getMessage(), e);
         }
     }
+
+    private void syncPOW() {
+        // If the Bitmessage context acts as a full node, synchronization isn't necessary
+        LOG.info("Looking for completed POW");
+
+        try {
+            BitmessageAddress identity = Singleton.getIdentity(getContext());
+            byte[] privateKey = identity.getPrivateKey().getPrivateEncryptionKey();
+            byte[] signingKey = security().createPublicKey(identity.getPublicDecryptionKey());
+            ProofOfWorkRequest.Reader reader = new ProofOfWorkRequest.Reader(identity);
+            ProofOfWorkRepository powRepo = Singleton.getProofOfWorkRepository(getContext());
+            List<byte[]> items = powRepo.getItems();
+            for (byte[] initialHash : items) {
+                ProofOfWorkRepository.Item item = powRepo.getItem(initialHash);
+                byte[] target = security().getProofOfWorkTarget(item.object, item
+                        .nonceTrialsPerByte, item.extraBytes);
+                CryptoCustomMessage<ProofOfWorkRequest> cryptoMsg = new CryptoCustomMessage<>(
+                        new ProofOfWorkRequest(identity, initialHash, CALCULATE, target));
+                cryptoMsg.signAndEncrypt(identity, signingKey);
+                CustomMessage response = bmc.send(
+                        Preferences.getTrustedNode(getContext()),
+                        Preferences.getTrustedNodePort(getContext()),
+                        cryptoMsg
+                );
+                if (response.isError()) {
+                    LOG.error("Server responded with error: " + new String(response.getData(),
+                            "UTF-8"));
+                } else {
+                    ProofOfWorkRequest decryptedResponse = CryptoCustomMessage.read(
+                            response, reader).decrypt(privateKey);
+                    if (decryptedResponse.getRequest() == COMPLETE) {
+                        bmc.internals().getProofOfWorkService().onNonceCalculated(
+                                initialHash, decryptedResponse.getData());
+                    }
+                }
+            }
+            if (items.size() == 0) {
+                stopPowSync(getContext());
+            }
+            LOG.info("Synchronization finished");
+        } catch (Exception e) {
+            LOG.error(e.getMessage(), e);
+        }
+    }
+
+    public static void startSync(Context ctx) {
+        // Create account, if it's missing. (Either first run, or user has deleted account.)
+        Account account = addAccount(ctx, ACCOUNT_SYNC);
+
+        // Recommend a schedule for automatic synchronization. The system may modify this based
+        // on other scheduled syncs and network utilization.
+        ContentResolver.addPeriodicSync(account, AUTHORITY, new Bundle(), SYNC_FREQUENCY);
+    }
+
+    public static void stopSync(Context ctx) {
+        // Create account, if it's missing. (Either first run, or user has deleted account.)
+        Account account = addAccount(ctx, ACCOUNT_SYNC);
+
+        ContentResolver.removePeriodicSync(account, AUTHORITY, new Bundle());
+    }
+
+
+    public static void startPowSync(Context ctx) {
+        // Create account, if it's missing. (Either first run, or user has deleted account.)
+        Account account = addAccount(ctx, ACCOUNT_POW);
+
+        // Recommend a schedule for automatic synchronization. The system may modify this based
+        // on other scheduled syncs and network utilization.
+        ContentResolver.addPeriodicSync(account, AUTHORITY, new Bundle(), SYNC_FREQUENCY);
+    }
+
+    public static void stopPowSync(Context ctx) {
+        // Create account, if it's missing. (Either first run, or user has deleted account.)
+        Account account = addAccount(ctx, ACCOUNT_POW);
+
+        ContentResolver.removePeriodicSync(account, AUTHORITY, new Bundle());
+    }
+
+    private static Account addAccount(Context ctx, Account account) {
+        if (AccountManager.get(ctx).addAccountExplicitly(account, null, null)) {
+            // Inform the system that this account supports sync
+            ContentResolver.setIsSyncable(account, AUTHORITY, 1);
+            // Inform the system that this account is eligible for auto sync when the network is up
+            ContentResolver.setSyncAutomatically(account, AUTHORITY, true);
+        }
+        return account;
+    }
 }
diff --git a/app/src/main/java/ch/dissem/apps/abit/util/Constants.java b/app/src/main/java/ch/dissem/apps/abit/util/Constants.java
index 436dd6c..20fcbee 100644
--- a/app/src/main/java/ch/dissem/apps/abit/util/Constants.java
+++ b/app/src/main/java/ch/dissem/apps/abit/util/Constants.java
@@ -3,9 +3,14 @@ package ch.dissem.apps.abit.util;
 import java.util.regex.Pattern;
 
 /**
- * Created by chrigu on 16.11.15.
+ * @author Christian Basler
  */
 public class Constants {
+    public static final String PREFERENCE_WIFI_ONLY = "wifi_only";
+    public static final String PREFERENCE_TRUSTED_NODE = "trusted_node";
+    public static final String PREFERENCE_SYNC_TIMEOUT = "sync_timeout";
+    public static final String PREFERENCE_SERVER_POW = "server_pow";
+
     public static final String BITMESSAGE_URL_SCHEMA = "bitmessage:";
     public static final Pattern BITMESSAGE_ADDRESS_PATTERN = Pattern.compile("\\bBM-[a-zA-Z0-9]+\\b");
 }
diff --git a/app/src/main/java/ch/dissem/apps/abit/util/PRNGFixes.java b/app/src/main/java/ch/dissem/apps/abit/util/PRNGFixes.java
new file mode 100644
index 0000000..a17d0a2
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/util/PRNGFixes.java
@@ -0,0 +1,341 @@
+package ch.dissem.apps.abit.util;
+/*
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will Google be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, as long as the origin is not misrepresented.
+ */
+
+import android.os.Build;
+import android.os.Process;
+import android.util.Log;
+
+import java.io.ByteArrayOutputStream;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.UnsupportedEncodingException;
+import java.security.NoSuchAlgorithmException;
+import java.security.Provider;
+import java.security.SecureRandom;
+import java.security.SecureRandomSpi;
+import java.security.Security;
+
+/**
+ * Fixes for the output of the default PRNG having low entropy.
+ * <p/>
+ * The fixes need to be applied via {@link #apply()} before any use of Java
+ * Cryptography Architecture primitives. A good place to invoke them is in the
+ * application's {@code onCreate}.
+ *
+ * @see <a href="http://android-developers.blogspot.ch/2013/08/some-securerandom-thoughts.html">
+ * http://android-developers.blogspot.ch/2013/08/some-securerandom-thoughts.html</a>
+ */
+public final class PRNGFixes {
+
+    private static final int VERSION_CODE_JELLY_BEAN = 16;
+    private static final int VERSION_CODE_JELLY_BEAN_MR2 = 18;
+    private static final byte[] BUILD_FINGERPRINT_AND_DEVICE_SERIAL =
+            getBuildFingerprintAndDeviceSerial();
+
+    /**
+     * Hidden constructor to prevent instantiation.
+     */
+    private PRNGFixes() {
+    }
+
+    /**
+     * Applies all fixes.
+     *
+     * @throws SecurityException if a fix is needed but could not be applied.
+     */
+    public static void apply() {
+        applyOpenSSLFix();
+        installLinuxPRNGSecureRandom();
+    }
+
+    /**
+     * Applies the fix for OpenSSL PRNG having low entropy. Does nothing if the
+     * fix is not needed.
+     *
+     * @throws SecurityException if the fix is needed but could not be applied.
+     */
+    private static void applyOpenSSLFix() throws SecurityException {
+        if ((Build.VERSION.SDK_INT < VERSION_CODE_JELLY_BEAN)
+                || (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2)) {
+            // No need to apply the fix
+            return;
+        }
+
+        try {
+            // Mix in the device- and invocation-specific seed.
+            Class.forName("org.apache.harmony.xnet.provider.jsse.NativeCrypto")
+                    .getMethod("RAND_seed", byte[].class)
+                    .invoke(null, generateSeed());
+
+            // Mix output of Linux PRNG into OpenSSL's PRNG
+            int bytesRead = (Integer) Class.forName(
+                    "org.apache.harmony.xnet.provider.jsse.NativeCrypto")
+                    .getMethod("RAND_load_file", String.class, long.class)
+                    .invoke(null, "/dev/urandom", 1024);
+            if (bytesRead != 1024) {
+                throw new IOException(
+                        "Unexpected number of bytes read from Linux PRNG: "
+                                + bytesRead);
+            }
+        } catch (Exception e) {
+            throw new SecurityException("Failed to seed OpenSSL PRNG", e);
+        }
+    }
+
+    /**
+     * Installs a Linux PRNG-backed {@code SecureRandom} implementation as the
+     * default. Does nothing if the implementation is already the default or if
+     * there is not need to install the implementation.
+     *
+     * @throws SecurityException if the fix is needed but could not be applied.
+     */
+    private static void installLinuxPRNGSecureRandom()
+            throws SecurityException {
+        if (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2) {
+            // No need to apply the fix
+            return;
+        }
+
+        // Install a Linux PRNG-based SecureRandom implementation as the
+        // default, if not yet installed.
+        Provider[] secureRandomProviders =
+                Security.getProviders("SecureRandom.SHA1PRNG");
+        if ((secureRandomProviders == null)
+                || (secureRandomProviders.length < 1)
+                || (!LinuxPRNGSecureRandomProvider.class.equals(
+                secureRandomProviders[0].getClass()))) {
+            Security.insertProviderAt(new LinuxPRNGSecureRandomProvider(), 1);
+        }
+
+        // Assert that new SecureRandom() and
+        // SecureRandom.getInstance("SHA1PRNG") return a SecureRandom backed
+        // by the Linux PRNG-based SecureRandom implementation.
+        SecureRandom rng1 = new SecureRandom();
+        if (!LinuxPRNGSecureRandomProvider.class.equals(
+                rng1.getProvider().getClass())) {
+            throw new SecurityException(
+                    "new SecureRandom() backed by wrong Provider: "
+                            + rng1.getProvider().getClass());
+        }
+
+        SecureRandom rng2;
+        try {
+            rng2 = SecureRandom.getInstance("SHA1PRNG");
+        } catch (NoSuchAlgorithmException e) {
+            throw new SecurityException("SHA1PRNG not available", e);
+        }
+        if (!LinuxPRNGSecureRandomProvider.class.equals(
+                rng2.getProvider().getClass())) {
+            throw new SecurityException(
+                    "SecureRandom.getInstance(\"SHA1PRNG\") backed by wrong"
+                            + " Provider: " + rng2.getProvider().getClass());
+        }
+    }
+
+    /**
+     * {@code Provider} of {@code SecureRandom} engines which pass through
+     * all requests to the Linux PRNG.
+     */
+    private static class LinuxPRNGSecureRandomProvider extends Provider {
+
+        public LinuxPRNGSecureRandomProvider() {
+            super("LinuxPRNG",
+                    1.0,
+                    "A Linux-specific random number provider that uses"
+                            + " /dev/urandom");
+            // Although /dev/urandom is not a SHA-1 PRNG, some apps
+            // explicitly request a SHA1PRNG SecureRandom and we thus need to
+            // prevent them from getting the default implementation whose output
+            // may have low entropy.
+            put("SecureRandom.SHA1PRNG", LinuxPRNGSecureRandom.class.getName());
+            put("SecureRandom.SHA1PRNG ImplementedIn", "Software");
+        }
+    }
+
+    /**
+     * {@link SecureRandomSpi} which passes all requests to the Linux PRNG
+     * ({@code /dev/urandom}).
+     */
+    public static class LinuxPRNGSecureRandom extends SecureRandomSpi {
+
+        /*
+         * IMPLEMENTATION NOTE: Requests to generate bytes and to mix in a seed
+         * are passed through to the Linux PRNG (/dev/urandom). Instances of
+         * this class seed themselves by mixing in the current time, PID, UID,
+         * build fingerprint, and hardware serial number (where available) into
+         * Linux PRNG.
+         *
+         * Concurrency: Read requests to the underlying Linux PRNG are
+         * serialized (on sLock) to ensure that multiple threads do not get
+         * duplicated PRNG output.
+         */
+
+        private static final File URANDOM_FILE = new File("/dev/urandom");
+
+        private static final Object sLock = new Object();
+
+        /**
+         * Input stream for reading from Linux PRNG or {@code null} if not yet
+         * opened.
+         *
+         * @GuardedBy("sLock")
+         */
+        private static DataInputStream sUrandomIn;
+
+        /**
+         * Output stream for writing to Linux PRNG or {@code null} if not yet
+         * opened.
+         *
+         * @GuardedBy("sLock")
+         */
+        private static OutputStream sUrandomOut;
+
+        /**
+         * Whether this engine instance has been seeded. This is needed because
+         * each instance needs to seed itself if the client does not explicitly
+         * seed it.
+         */
+        private boolean mSeeded;
+
+        @Override
+        protected void engineSetSeed(byte[] bytes) {
+            try {
+                OutputStream out;
+                synchronized (sLock) {
+                    out = getUrandomOutputStream();
+                }
+                out.write(bytes);
+                out.flush();
+            } catch (IOException e) {
+                // On a small fraction of devices /dev/urandom is not writable.
+                // Log and ignore.
+                Log.w(PRNGFixes.class.getSimpleName(),
+                        "Failed to mix seed into " + URANDOM_FILE);
+            } finally {
+                mSeeded = true;
+            }
+        }
+
+        @Override
+        protected void engineNextBytes(byte[] bytes) {
+            if (!mSeeded) {
+                // Mix in the device- and invocation-specific seed.
+                engineSetSeed(generateSeed());
+            }
+
+            try {
+                DataInputStream in;
+                synchronized (sLock) {
+                    in = getUrandomInputStream();
+                }
+                synchronized (in) {
+                    in.readFully(bytes);
+                }
+            } catch (IOException e) {
+                throw new SecurityException(
+                        "Failed to read from " + URANDOM_FILE, e);
+            }
+        }
+
+        @Override
+        protected byte[] engineGenerateSeed(int size) {
+            byte[] seed = new byte[size];
+            engineNextBytes(seed);
+            return seed;
+        }
+
+        private DataInputStream getUrandomInputStream() {
+            synchronized (sLock) {
+                if (sUrandomIn == null) {
+                    // NOTE: Consider inserting a BufferedInputStream between
+                    // DataInputStream and FileInputStream if you need higher
+                    // PRNG output performance and can live with future PRNG
+                    // output being pulled into this process prematurely.
+                    try {
+                        sUrandomIn = new DataInputStream(
+                                new FileInputStream(URANDOM_FILE));
+                    } catch (IOException e) {
+                        throw new SecurityException("Failed to open "
+                                + URANDOM_FILE + " for reading", e);
+                    }
+                }
+                return sUrandomIn;
+            }
+        }
+
+        private OutputStream getUrandomOutputStream() throws IOException {
+            synchronized (sLock) {
+                if (sUrandomOut == null) {
+                    sUrandomOut = new FileOutputStream(URANDOM_FILE);
+                }
+                return sUrandomOut;
+            }
+        }
+    }
+
+    /**
+     * Generates a device- and invocation-specific seed to be mixed into the
+     * Linux PRNG.
+     */
+    private static byte[] generateSeed() {
+        try {
+            ByteArrayOutputStream seedBuffer = new ByteArrayOutputStream();
+            DataOutputStream seedBufferOut =
+                    new DataOutputStream(seedBuffer);
+            seedBufferOut.writeLong(System.currentTimeMillis());
+            seedBufferOut.writeLong(System.nanoTime());
+            seedBufferOut.writeInt(Process.myPid());
+            seedBufferOut.writeInt(Process.myUid());
+            seedBufferOut.write(BUILD_FINGERPRINT_AND_DEVICE_SERIAL);
+            seedBufferOut.close();
+            return seedBuffer.toByteArray();
+        } catch (IOException e) {
+            throw new SecurityException("Failed to generate seed", e);
+        }
+    }
+
+    /**
+     * Gets the hardware serial number of this device.
+     *
+     * @return serial number or {@code null} if not available.
+     */
+    private static String getDeviceSerialNumber() {
+        // We're using the Reflection API because Build.SERIAL is only available
+        // since API Level 9 (Gingerbread, Android 2.3).
+        try {
+            return (String) Build.class.getField("SERIAL").get(null);
+        } catch (Exception ignored) {
+            return null;
+        }
+    }
+
+    private static byte[] getBuildFingerprintAndDeviceSerial() {
+        StringBuilder result = new StringBuilder();
+        String fingerprint = Build.FINGERPRINT;
+        if (fingerprint != null) {
+            result.append(fingerprint);
+        }
+        String serial = getDeviceSerialNumber();
+        if (serial != null) {
+            result.append(serial);
+        }
+        try {
+            return result.toString().getBytes("UTF-8");
+        } catch (UnsupportedEncodingException e) {
+            throw new RuntimeException("UTF-8 encoding not supported");
+        }
+    }
+}
\ No newline at end of file
diff --git a/app/src/main/java/ch/dissem/apps/abit/util/Preferences.java b/app/src/main/java/ch/dissem/apps/abit/util/Preferences.java
new file mode 100644
index 0000000..20c5862
--- /dev/null
+++ b/app/src/main/java/ch/dissem/apps/abit/util/Preferences.java
@@ -0,0 +1,90 @@
+package ch.dissem.apps.abit.util;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.preference.PreferenceManager;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+
+import ch.dissem.apps.abit.R;
+import ch.dissem.apps.abit.notification.ErrorNotification;
+
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_SERVER_POW;
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_SYNC_TIMEOUT;
+import static ch.dissem.apps.abit.util.Constants.PREFERENCE_TRUSTED_NODE;
+
+/**
+ * Created by chrig on 01.12.2015.
+ */
+public class Preferences {
+    private static Logger LOG = LoggerFactory.getLogger(Preferences.class);
+
+    public static boolean useTrustedNode(Context ctx) {
+        String trustedNode = getPreference(ctx, PREFERENCE_TRUSTED_NODE);
+        return trustedNode == null || trustedNode.trim().isEmpty();
+    }
+
+    /**
+     * Warning, this method might do a network call and therefore can't be called from
+     * the UI thread.
+     */
+    public static InetAddress getTrustedNode(Context ctx) {
+        String trustedNode = getPreference(ctx, PREFERENCE_TRUSTED_NODE);
+        if (trustedNode == null) return null;
+        trustedNode = trustedNode.trim();
+        if (trustedNode.isEmpty()) return null;
+
+        if (trustedNode.matches("^(?![0-9a-fA-F]*:[0-9a-fA-F]*:).*(:[0-9]+)$")) {
+            int index = trustedNode.lastIndexOf(':');
+            trustedNode = trustedNode.substring(0, index);
+        }
+        try {
+            return InetAddress.getByName(trustedNode);
+        } catch (UnknownHostException e) {
+            new ErrorNotification(ctx)
+                    .setError(R.string.error_invalid_sync_host)
+                    .show();
+            LOG.error(e.getMessage(), e);
+            return null;
+        }
+    }
+
+    public static int getTrustedNodePort(Context ctx) {
+        String trustedNode = getPreference(ctx, PREFERENCE_TRUSTED_NODE);
+        if (trustedNode == null) return 8444;
+        trustedNode = trustedNode.trim();
+
+        if (trustedNode.matches("^(?![0-9a-fA-F]*:[0-9a-fA-F]*:).*(:[0-9]+)$")) {
+            int index = trustedNode.lastIndexOf(':');
+            String portString = trustedNode.substring(index + 1);
+            try {
+                return Integer.parseInt(portString);
+            } catch (NumberFormatException e) {
+                new ErrorNotification(ctx)
+                        .setError(R.string.error_invalid_sync_port, portString)
+                        .show();
+            }
+        }
+        return 8444;
+    }
+
+    public static long getTimeoutInSeconds(Context ctx) {
+        String preference = getPreference(ctx, PREFERENCE_SYNC_TIMEOUT);
+        return preference == null ? 120 : Long.parseLong(preference);
+    }
+
+    public static boolean isServerPOW(Context ctx) {
+        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(ctx);
+        return preferences.getBoolean(PREFERENCE_SERVER_POW, false);
+    }
+
+    private static String getPreference(Context ctx, String name) {
+        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(ctx);
+
+        return preferences.getString(name, null);
+    }
+}
diff --git a/app/src/main/res/layout/activity_open_bitmessage_link.xml b/app/src/main/res/layout/activity_open_bitmessage_link.xml
index 5f973d2..295dc87 100644
--- a/app/src/main/res/layout/activity_open_bitmessage_link.xml
+++ b/app/src/main/res/layout/activity_open_bitmessage_link.xml
@@ -1,21 +1,28 @@
 <?xml version="1.0" encoding="utf-8"?>
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
-    android:padding="16dp">
+    android:padding="24dp">
 
     <TextView
         android:id="@+id/address"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
+        android:layout_alignParentLeft="true"
+        android:layout_alignParentStart="true"
         android:text="BM-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
-        android:textAppearance="?android:attr/textAppearanceSmall" />
+        android:textSize="10dp"
+        tools:ignore="SpUsage" />
 
     <android.support.design.widget.TextInputLayout
         android:id="@+id/label_wrapper"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:layout_below="@+id/address">
+        android:layout_alignLeft="@+id/address"
+        android:layout_alignStart="@+id/address"
+        android:layout_below="@+id/address"
+        android:layout_marginTop="16dp">
 
         <EditText
             android:id="@+id/label"
@@ -30,8 +37,9 @@
         android:id="@+id/subscribe"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
+        android:layout_alignLeft="@+id/address"
+        android:layout_alignStart="@+id/address"
         android:layout_below="@+id/label_wrapper"
-        android:layout_centerHorizontal="true"
         android:layout_marginBottom="8dp"
         android:layout_marginTop="8dp"
         android:text="@string/subscribe" />
diff --git a/app/src/main/res/values-de/strings.xml b/app/src/main/res/values-de/strings.xml
index da9a684..19fda0d 100644
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -49,4 +49,6 @@
     <string name="compose_body_hint">Nachricht schreiben</string>
     <string name="contacts_and_subscriptions">Kontakte</string>
     <string name="subscribed">Abonniert</string>
+    <string name="server_pow">Server POW</string>
+    <string name="server_pow_summary">Der vertrauenswürdige Knoten macht den Proof of Work</string>
 </resources>
\ No newline at end of file
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 4ca446a..19fd4f8 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -49,4 +49,6 @@
     <string name="compose_body_hint">Write message</string>
     <string name="contacts_and_subscriptions">Contacts</string>
     <string name="subscribed">Subscribed</string>
+    <string name="server_pow">Server POW</string>
+    <string name="server_pow_summary">Trusted node does proof of work</string>
 </resources>
diff --git a/app/src/main/res/xml/preferences.xml b/app/src/main/res/xml/preferences.xml
index d026e3f..50fea51 100644
--- a/app/src/main/res/xml/preferences.xml
+++ b/app/src/main/res/xml/preferences.xml
@@ -24,4 +24,11 @@
         android:key="sync_timeout"
         android:summary="@string/sync_timeout_summary"
         android:title="@string/sync_timeout" />
+    <SwitchPreference
+        android:defaultValue="false"
+        android:key="server_pow"
+        android:dependency="trusted_node"
+        android:title="@string/server_pow"
+        android:summary="@string/server_pow_summary"
+        />
 </PreferenceScreen>
\ No newline at end of file
diff --git a/build.gradle b/build.gradle
index 7acef02..f0f1e2b 100644
--- a/build.gradle
+++ b/build.gradle
@@ -9,7 +9,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.5.0-beta1'
+        classpath 'com.android.tools.build:gradle:1.5.0'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files