diff --git a/launcher/src/main/java/com/skcraft/launcher/install/FileVerify.java b/launcher/src/main/java/com/skcraft/launcher/install/FileVerify.java
new file mode 100644
index 0000000..d8a313a
--- /dev/null
+++ b/launcher/src/main/java/com/skcraft/launcher/install/FileVerify.java
@@ -0,0 +1,42 @@
+package com.skcraft.launcher.install;
+
+import com.skcraft.launcher.Launcher;
+import com.skcraft.launcher.LauncherException;
+import com.skcraft.launcher.util.FileUtils;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.java.Log;
+
+import java.io.File;
+
+@RequiredArgsConstructor
+@Log
+public class FileVerify implements InstallTask {
+	private final File target;
+	private final String name;
+	private final String hash;
+
+	@Override
+	public void execute(Launcher launcher) throws Exception {
+		log.info("Verifying file " + name);
+
+		String actualHash = FileUtils.getShaHash(target);
+		if (!actualHash.equals(hash)) {
+			String message = String.format(
+					"File %s (%s) is corrupt (invalid hash)\n" +
+					"Expected '%s'\nGot '%s'",
+					name, target.getAbsolutePath(), hash, actualHash);
+
+			throw new LauncherException(message, message);
+		}
+	}
+
+	@Override
+	public double getProgress() {
+		return -1;
+	}
+
+	@Override
+	public String getStatus() {
+		return "Verifying " + name;
+	}
+}
diff --git a/launcher/src/main/java/com/skcraft/launcher/model/minecraft/Library.java b/launcher/src/main/java/com/skcraft/launcher/model/minecraft/Library.java
index 5ebc43c..8c48832 100644
--- a/launcher/src/main/java/com/skcraft/launcher/model/minecraft/Library.java
+++ b/launcher/src/main/java/com/skcraft/launcher/model/minecraft/Library.java
@@ -145,6 +145,7 @@ public class Library {
         private String path;
         private String url;
         private String sha1;
+        private int size;
     }
 
     @Data
diff --git a/launcher/src/main/java/com/skcraft/launcher/update/BaseUpdater.java b/launcher/src/main/java/com/skcraft/launcher/update/BaseUpdater.java
index 2af4d07..81a939b 100644
--- a/launcher/src/main/java/com/skcraft/launcher/update/BaseUpdater.java
+++ b/launcher/src/main/java/com/skcraft/launcher/update/BaseUpdater.java
@@ -147,14 +147,19 @@ public abstract class BaseUpdater {
     }
 
     protected void installJar(@NonNull Installer installer,
+                              @NonNull VersionManifest.Artifact artifact,
                               @NonNull File jarFile,
                               @NonNull URL url) throws InterruptedException {
         // If the JAR does not exist, install it
         if (!jarFile.exists()) {
-            List<File> targets = new ArrayList<File>();
+            long size = artifact.getSize();
+            if (size <= 0) size = JAR_SIZE_ESTIMATE;
 
-            File tempFile = installer.getDownloader().download(url, "", JAR_SIZE_ESTIMATE, jarFile.getName());
+            File tempFile = installer.getDownloader().download(url, "", size, jarFile.getName());
             installer.queue(new FileMover(tempFile, jarFile));
+            if (artifact.getHash() != null) {
+                installer.queue(new FileVerify(jarFile, jarFile.getName(), artifact.getHash()));
+            }
             log.info("Installing " + jarFile.getName() + " from " + url);
         }
     }
@@ -217,7 +222,12 @@ public abstract class BaseUpdater {
             if (library.matches(environment)) {
                 checkInterrupted();
 
-                String path = library.getPath(environment);
+                Library.Artifact artifact = library.getArtifact(environment);
+                String path = artifact.getPath();
+
+                long size = artifact.getSize();
+                if (size <= 0) size = LIBRARY_SIZE_ESTIMATE;
+
                 File targetFile = new File(librariesDir, path);
 
                 if (!targetFile.exists()) {
@@ -230,10 +240,13 @@ public abstract class BaseUpdater {
                         }
                     }
 
-                    File tempFile = installer.getDownloader().download(urls, "", LIBRARY_SIZE_ESTIMATE,
+                    File tempFile = installer.getDownloader().download(urls, "", size,
                             library.getName() + ".jar");
                     log.info("Fetching " + path + " from " + urls);
-                    installer.queue(new FileMover( tempFile, targetFile));
+                    installer.queue(new FileMover(tempFile, targetFile));
+                    if (artifact.getSha1() != null) {
+                        installer.queue(new FileVerify(targetFile, library.getName(), artifact.getSha1()));
+                    }
                 }
             }
         }
diff --git a/launcher/src/main/java/com/skcraft/launcher/update/Updater.java b/launcher/src/main/java/com/skcraft/launcher/update/Updater.java
index 083dfde..745c20a 100644
--- a/launcher/src/main/java/com/skcraft/launcher/update/Updater.java
+++ b/launcher/src/main/java/com/skcraft/launcher/update/Updater.java
@@ -176,9 +176,10 @@ public class Updater extends BaseUpdater implements Callable<Instance>, Progress
 
         // Install the .jar
         File jarPath = launcher.getJarPath(version);
-        URL jarSource = url(version.getDownloads().get("client").getUrl());
+        VersionManifest.Artifact clientJar = version.getDownloads().get("client");
+        URL jarSource = url(clientJar.getUrl());
         log.info("JAR at " + jarPath.getAbsolutePath() + ", fetched from " + jarSource);
-        installJar(installer, jarPath, jarSource);
+        installJar(installer, clientJar, jarPath, jarSource);
 
         // Download libraries
         log.info("Enumerating libraries to download...");