Use Windows cert store.

src/main/java/net/fabricmc/installer/Main.java

@@ -26,6 +26,7 @@
 import net.fabricmc.installer.util.ArgumentParser;
 import net.fabricmc.installer.util.CrashDialog;
 import net.fabricmc.installer.util.MetaHandler;
+import net.fabricmc.installer.util.OperatingSystem;
 import net.fabricmc.installer.util.Reference;
 
 public class Main {
@@ -35,6 +36,11 @@ public class Main {
 	public static final List<Handler> HANDLERS = new ArrayList<>();
 
 	public static void main(String[] args) throws IOException {
+		if (OperatingSystem.CURRENT == OperatingSystem.WINDOWS) {
+			// Use the operating system cert store
+			System.setProperty("javax.net.ssl.trustStoreType", "WINDOWS-ROOT");
+		}
+
 		System.out.println("Loading Fabric Installer: " + Main.class.getPackage().getImplementationVersion());
 
 		HANDLERS.add(new ClientHandler());

src/main/java/net/fabricmc/installer/launcher/NativesHelper.java

@@ -26,8 +26,10 @@
 import java.util.Map;
 import java.util.Objects;
 
+import net.fabricmc.installer.util.OperatingSystem;
+
 public class NativesHelper {
-	private static final String OS_ID = getOS() + "-" + System.getProperty("os.arch").toLowerCase(Locale.ROOT);
+	private static final String OS_ID = OperatingSystem.CURRENT.name().toLowerCase(Locale.ROOT) + "-" + System.getProperty("os.arch").toLowerCase(Locale.ROOT);
 	private static final Map<String, String> NATIVES_MAP = getNativesMap();
 
 	private static boolean loaded = false;
@@ -49,18 +51,6 @@ private static Map<String, String> getNativesMap() {
 		return natives;
 	}
 
-	private static String getOS() {
-		String osName = System.getProperty("os.name").toLowerCase(Locale.ROOT);
-
-		if (osName.contains("win")) {
-			return "windows";
-		} else if (osName.contains("mac")) {
-			return "macos";
-		} else {
-			return "linux";
-		}
-	}
-
 	public static boolean loadSafelyIfCompatible() {
 		if (isCompatible()) {
 			try {

src/main/java/net/fabricmc/installer/util/OperatingSystem.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.installer.util;
+
+import java.util.Locale;
+
+public enum OperatingSystem {
+	WINDOWS,
+	MACOS,
+	LINUX;
+
+	public static final OperatingSystem CURRENT = getCurrent();
+
+	private static OperatingSystem getCurrent() {
+		String osName = System.getProperty("os.name").toLowerCase(Locale.ENGLISH);
+
+		if (osName.contains("win")) {
+			return WINDOWS;
+		} else if (osName.contains("mac")) {
+			return MACOS;
+		} else {
+			return LINUX;
+		}
+	}
+}

src/main/java/net/fabricmc/installer/util/Utils.java

@@ -61,16 +61,15 @@ public ResourceBundle newBundle(String baseName, Locale locale, String format, C
 	});
 
 	public static Path findDefaultInstallDir() {
-		String os = System.getProperty("os.name").toLowerCase(Locale.ENGLISH);
 		Path dir;
 
-		if (os.contains("win") && System.getenv("APPDATA") != null) {
+		if (OperatingSystem.CURRENT == OperatingSystem.WINDOWS && System.getenv("APPDATA") != null) {
 			dir = Paths.get(System.getenv("APPDATA")).resolve(".minecraft");
 		} else {
 			String home = System.getProperty("user.home", ".");
 			Path homeDir = Paths.get(home);
 
-			if (os.contains("mac")) {
+			if (OperatingSystem.CURRENT == OperatingSystem.MACOS) {
 				dir = homeDir.resolve("Library").resolve("Application Support").resolve("minecraft");
 			} else {
 				dir = homeDir.resolve(".minecraft");