diff --git a/src/auth.ts b/src/auth.ts
index 5d2ef9c..37369c0 100644
--- a/src/auth.ts
+++ b/src/auth.ts
@@ -7,7 +7,7 @@ import {
 import { Device } from "./device.ts";
 
 function toHex(data: Uint8Array): string {
-  return Array.from(data).map((b) => b.toString(16)).join("");
+  return Array.from(data).map((b) => b.toString(16).padStart(1, "0")).join("");
 }
 async function generateHMAC(key: string, message: string) {
   const encoder = new TextEncoder();
diff --git a/static/index.html b/static/index.html
index c73d1c7..0fbbb5a 100644
--- a/static/index.html
+++ b/static/index.html
@@ -82,7 +82,7 @@
   <script>
     function preventDefault(e) {alert(); e.preventDefault()}
     function toHex(data) {
-      return Array.from(data).map((b) => b.toString(16)).join("");
+      return Array.from(data).map((b) => b.toString(16).padStart(1, "0")).join("");
     }
     async function generateHMAC(key, message) {
       const encoder = new TextEncoder();