diff --git a/src/main/java/de/bixilon/minosoft/Minosoft.java b/src/main/java/de/bixilon/minosoft/Minosoft.java
index aba9f2eee..46b8de543 100644
--- a/src/main/java/de/bixilon/minosoft/Minosoft.java
+++ b/src/main/java/de/bixilon/minosoft/Minosoft.java
@@ -34,6 +34,7 @@ import de.bixilon.minosoft.util.task.Task;
 import de.bixilon.minosoft.util.task.TaskImportance;
 import javafx.application.Platform;
 import javafx.scene.control.Dialog;
+import javafx.stage.Stage;
 
 import java.io.IOException;
 import java.util.*;
@@ -85,11 +86,19 @@ public final class Minosoft {
                     System.exit(1);
                 }
             }
+            // hide all other gui parts
+            StartProgressWindow.hideDialog();
+            Launcher.exit();
             Platform.runLater(() -> {
                 Dialog<Boolean> dialog = new Dialog<>();
                 dialog.setTitle("Critical Error");
                 dialog.setHeaderText("An error occurred while starting Minosoft");
-                dialog.setContentText(exception.getMessage());
+                dialog.setContentText(exception.getLocalizedMessage());
+
+                Stage stage = (Stage) dialog.getDialogPane().getScene().getWindow();
+                stage.setAlwaysOnTop(true);
+                stage.toFront();
+                dialog.setOnCloseRequest(dialogEvent -> System.exit(1));
                 dialog.showAndWait();
                 System.exit(1);
             });
@@ -101,7 +110,7 @@ public final class Minosoft {
 
         taskWorker.addTask(new Task(progress -> {
             progress.countUp();
-            LocaleManager.load(config.getString(ConfigurationPaths.LANGUAGE));
+            LocaleManager.load(config.getString(ConfigurationPaths.GENERAL_LANGUAGE));
             progress.countDown();
 
         }, "Minosoft Language", "", Priorities.HIGH, TaskImportance.REQUIRED));
@@ -143,7 +152,7 @@ public final class Minosoft {
 
         taskWorker.addTask(new Task(progress -> {
             progress.countUp();
-            MinecraftLocaleManager.load(config.getString(ConfigurationPaths.LANGUAGE));
+            MinecraftLocaleManager.load(config.getString(ConfigurationPaths.GENERAL_LANGUAGE));
             progress.countDown();
 
         }, "Mojang language", "", Priorities.HIGH, TaskImportance.REQUIRED, new HashSet<>(Collections.singleton("Assets"))));
diff --git a/src/main/java/de/bixilon/minosoft/config/Configuration.java b/src/main/java/de/bixilon/minosoft/config/Configuration.java
index 438f4e4af..aca3dab48 100644
--- a/src/main/java/de/bixilon/minosoft/config/Configuration.java
+++ b/src/main/java/de/bixilon/minosoft/config/Configuration.java
@@ -91,6 +91,7 @@ public class Configuration {
     public boolean getBoolean(ConfigurationPaths path) {
         return switch (path) {
             case NETWORK_FAKE_CLIENT_BRAND -> config.getAsJsonObject("network").get("fake-network-brand").getAsBoolean();
+            case DEBUG_VERIFY_ASSETS -> config.getAsJsonObject("debug").get("verify-assets").getAsBoolean();
             default -> throw new RuntimeException(String.format("Illegal boolean value: %s", path));
         };
     }
@@ -98,6 +99,7 @@ public class Configuration {
     public void putBoolean(ConfigurationPaths path, boolean value) {
         switch (path) {
             case NETWORK_FAKE_CLIENT_BRAND -> config.getAsJsonObject("network").addProperty("fake-network-brand", value);
+            case DEBUG_VERIFY_ASSETS -> config.getAsJsonObject("debug").addProperty("verify-assets", value);
             default -> throw new RuntimeException(String.format("Illegal boolean value: %s", path));
         }
     }
@@ -122,7 +124,7 @@ public class Configuration {
         return switch (path) {
             case ACCOUNT_SELECTED -> config.getAsJsonObject("accounts").get("selected").getAsString();
             case GENERAL_LOG_LEVEL -> config.getAsJsonObject("general").get("log-level").getAsString();
-            case LANGUAGE -> config.getAsJsonObject("general").get("language").getAsString();
+            case GENERAL_LANGUAGE -> config.getAsJsonObject("general").get("language").getAsString();
             case MAPPINGS_URL -> config.getAsJsonObject("download").getAsJsonObject("urls").get("mappings").getAsString();
             case CLIENT_TOKEN -> config.getAsJsonObject("accounts").get("client-token").getAsString();
             default -> throw new RuntimeException(String.format("Illegal String value: %s", path));
@@ -133,7 +135,7 @@ public class Configuration {
         switch (path) {
             case ACCOUNT_SELECTED -> config.getAsJsonObject("accounts").addProperty("selected", value);
             case GENERAL_LOG_LEVEL -> config.getAsJsonObject("general").addProperty("log-level", value);
-            case LANGUAGE -> config.getAsJsonObject("general").addProperty("language", value);
+            case GENERAL_LANGUAGE -> config.getAsJsonObject("general").addProperty("language", value);
             case MAPPINGS_URL -> config.getAsJsonObject("download").getAsJsonObject("urls").addProperty("mappings", value);
             case CLIENT_TOKEN -> config.getAsJsonObject("accounts").addProperty("client-token", value);
             default -> throw new RuntimeException(String.format("Illegal String value: %s", path));
diff --git a/src/main/java/de/bixilon/minosoft/config/ConfigurationPaths.java b/src/main/java/de/bixilon/minosoft/config/ConfigurationPaths.java
index 3adf3fb1b..2e22acbf3 100644
--- a/src/main/java/de/bixilon/minosoft/config/ConfigurationPaths.java
+++ b/src/main/java/de/bixilon/minosoft/config/ConfigurationPaths.java
@@ -21,5 +21,6 @@ public enum ConfigurationPaths {
     CLIENT_TOKEN,
     MAPPINGS_URL,
     ACCOUNT_SELECTED,
-    LANGUAGE,
+    GENERAL_LANGUAGE,
+    DEBUG_VERIFY_ASSETS,
 }
\ No newline at end of file
diff --git a/src/main/java/de/bixilon/minosoft/data/assets/AssetsManager.java b/src/main/java/de/bixilon/minosoft/data/assets/AssetsManager.java
index 0bd9b1fde..e8bc4382d 100644
--- a/src/main/java/de/bixilon/minosoft/data/assets/AssetsManager.java
+++ b/src/main/java/de/bixilon/minosoft/data/assets/AssetsManager.java
@@ -19,6 +19,8 @@ import com.google.gson.JsonObject;
 import com.google.gson.JsonParser;
 import com.google.gson.stream.JsonReader;
 import de.bixilon.minosoft.Config;
+import de.bixilon.minosoft.Minosoft;
+import de.bixilon.minosoft.config.ConfigurationPaths;
 import de.bixilon.minosoft.logging.Log;
 import de.bixilon.minosoft.logging.LogLevels;
 import de.bixilon.minosoft.util.CountUpAndDownLatch;
@@ -146,7 +148,17 @@ public class AssetsManager {
 
     private static boolean verifyAssetHash(String hash) {
         // file does not exist
-        return getAssetSize(hash) != -1;// ToDo
+        if (getAssetSize(hash) == -1) {
+            return false;
+        }
+        if (!Minosoft.config.getBoolean(ConfigurationPaths.DEBUG_VERIFY_ASSETS)) {
+            return true;
+        }
+        try {
+            return hash.equals(Util.sha1Gzip(new File(getAssetDiskPath(hash))));
+        } catch (IOException ignored) {
+        }
+        return false;
     }
 
     public static void generateJarAssets() throws IOException {
diff --git a/src/main/java/de/bixilon/minosoft/gui/main/Launcher.java b/src/main/java/de/bixilon/minosoft/gui/main/Launcher.java
index 09e941856..6eae8ca5f 100644
--- a/src/main/java/de/bixilon/minosoft/gui/main/Launcher.java
+++ b/src/main/java/de/bixilon/minosoft/gui/main/Launcher.java
@@ -33,11 +33,16 @@ import java.io.IOException;
 import java.util.concurrent.CountDownLatch;
 
 public class Launcher {
+    private static Stage stage;
+    private static boolean exit = false;
 
     public static void start() throws Exception {
         Log.info("Starting launcher...");
         CountDownLatch latch = new CountDownLatch(1);
         Platform.runLater(() -> {
+            if (exit) {
+                return;
+            }
             Stage stage = new Stage();
 
             GUITools.versionList.setCellFactory(new Callback<>() {
@@ -73,14 +78,27 @@ public class Launcher {
 
             stage.setTitle(LocaleManager.translate(Strings.MAIN_WINDOW_TITLE));
             stage.getIcons().add(GUITools.logo);
-            stage.show();
             stage.setOnCloseRequest(windowEvent -> System.exit(0));
             if (Minosoft.getSelectedAccount() == null) {
                 MainWindow.manageAccounts();
             }
+            if (exit) {
+                return;
+            }
+            stage.show();
+            Launcher.stage = stage;
             latch.countDown();
         });
         latch.await();
         Log.info("Launcher started!");
     }
+
+    public static void exit() {
+        exit = true;
+        if (stage == null) {
+            return;
+        }
+
+        Platform.runLater(() -> stage.close());
+    }
 }
\ No newline at end of file
diff --git a/src/main/java/de/bixilon/minosoft/gui/main/StartProgressWindow.java b/src/main/java/de/bixilon/minosoft/gui/main/StartProgressWindow.java
index ea2a8797e..0f3ca694f 100644
--- a/src/main/java/de/bixilon/minosoft/gui/main/StartProgressWindow.java
+++ b/src/main/java/de/bixilon/minosoft/gui/main/StartProgressWindow.java
@@ -29,26 +29,33 @@ import java.util.concurrent.atomic.AtomicReference;
 
 public class StartProgressWindow extends Application {
     public static CountDownLatch toolkitLatch = new CountDownLatch(2);
+    private static Dialog<Boolean> progressDialog;
+    private static boolean exit = false;
 
     public static void show(CountUpAndDownLatch progress) {
+        if (exit) {
+            return;
+        }
         new Thread(() -> {
             if (progress.getCount() == 0) {
                 return;
             }
             AtomicReference<ProgressBar> progressBar = new AtomicReference<>();
-            AtomicReference<Dialog<Boolean>> progressDialog = new AtomicReference<>();
             Platform.runLater(() -> {
-                progressDialog.set(new Dialog<>());
-                progressDialog.get().setTitle(LocaleManager.translate(Strings.MINOSOFT_STILL_STARTING_TITLE));
-                progressDialog.get().setHeaderText(LocaleManager.translate(Strings.MINOSOFT_STILL_STARTING_HEADER));
+                progressDialog = new Dialog<>();
+                progressDialog.setTitle(LocaleManager.translate(Strings.MINOSOFT_STILL_STARTING_TITLE));
+                progressDialog.setHeaderText(LocaleManager.translate(Strings.MINOSOFT_STILL_STARTING_HEADER));
                 GridPane grid = new GridPane();
                 progressBar.set(new ProgressBar());
                 progressBar.get().setProgress(1.0F - ((float) progress.getCount() / progress.getTotal()));
                 grid.add(progressBar.get(), 0, 0);
-                progressDialog.get().getDialogPane().setContent(grid);
-                progressDialog.get().show();
+                progressDialog.getDialogPane().setContent(grid);
+                if (exit) {
+                    return;
+                }
+                progressDialog.show();
 
-                Stage stage = (Stage) progressDialog.get().getDialogPane().getScene().getWindow();
+                Stage stage = (Stage) progressDialog.getDialogPane().getScene().getWindow();
                 stage.setAlwaysOnTop(true);
                 stage.toFront();
 
@@ -61,11 +68,7 @@ public class StartProgressWindow extends Application {
                 }
                 Platform.runLater(() -> progressBar.get().setProgress(1.0F - ((float) progress.getCount() / progress.getTotal())));
             }
-
-            Platform.runLater(() -> {
-                progressDialog.get().setResult(Boolean.TRUE);
-                progressDialog.get().hide();
-            });
+            hideDialog();
         }).start();
     }
 
@@ -77,6 +80,17 @@ public class StartProgressWindow extends Application {
         Log.debug("Initialized JavaFX Toolkit!");
     }
 
+    public static void hideDialog() {
+        exit = true;
+        if (progressDialog == null) {
+            return;
+        }
+        Platform.runLater(() -> {
+            progressDialog.setResult(Boolean.TRUE);
+            progressDialog.hide();
+        });
+    }
+
     @Override
     public void start(Stage stage) {
         toolkitLatch.countDown();
diff --git a/src/main/java/de/bixilon/minosoft/util/Util.java b/src/main/java/de/bixilon/minosoft/util/Util.java
index 27ac9f876..26de0a9bf 100644
--- a/src/main/java/de/bixilon/minosoft/util/Util.java
+++ b/src/main/java/de/bixilon/minosoft/util/Util.java
@@ -102,17 +102,40 @@ public final class Util {
     }
 
     public static String sha1(byte[] data) {
+        ByteArrayInputStream inputStream = new ByteArrayInputStream(data);
         try {
-            MessageDigest crypt = MessageDigest.getInstance("SHA-1");
-            crypt.reset();
-            crypt.update(data);
-            return byteArrayToHexString(crypt.digest());
-        } catch (NoSuchAlgorithmException e) {
+            return sha1(inputStream);
+        } catch (IOException e) {
             e.printStackTrace();
         }
         return null;
     }
 
+    public static String sha1(File file) throws IOException {
+        return sha1(new FileInputStream(file));
+    }
+
+    public static String sha1Gzip(File file) throws IOException {
+        return sha1(new GZIPInputStream(new FileInputStream(file)));
+    }
+
+    public static String sha1(InputStream inputStream) throws IOException {
+        try {
+            MessageDigest crypt = MessageDigest.getInstance("SHA-1");
+            crypt.reset();
+
+            byte[] buffer = new byte[4096];
+            int length;
+            while ((length = inputStream.read(buffer, 0, 4096)) != -1) {
+                crypt.update(buffer, 0, length);
+            }
+            return byteArrayToHexString(crypt.digest());
+        } catch (NoSuchAlgorithmException | FileNotFoundException e) {
+            e.printStackTrace();
+            throw new RuntimeException(e);
+        }
+    }
+
     public static String byteArrayToHexString(byte[] b) {
         StringBuilder result = new StringBuilder();
         for (byte value : b) {
diff --git a/src/main/resources/config/config.json b/src/main/resources/config/config.json
index 47c88d3c7..4a0d77d74 100644
--- a/src/main/resources/config/config.json
+++ b/src/main/resources/config/config.json
@@ -23,5 +23,8 @@
     "urls": {
       "mappings": "https://gitlab.com/Bixilon/minosoft/-/raw/master/data/mcdata/%s.tar.gz?inline=false"
     }
+  },
+  "debug": {
+    "verify-assets": true
   }
 }
\ No newline at end of file