feat: Check the HMAC during decryption

lib/src/helpers.dart

@@ -7,3 +7,16 @@ List<int> concat(List<List<int>> inputs) {
 
   return tmp;
 }
+
+/// Compares the two lists [a] and [b] and return true if [a] and [b] are index-by-index
+/// equal. Returns false, if they are not "equal";
+bool listsEqual(List<int> a, List<int> b) {
+  // TODO(Unknown): Do we need to use a constant time comparison?
+  if (a.length != b.length) return false;
+
+  for (var i = 0; i < a.length; i++) {
+    if (a[i] != b[i]) return false;
+  }
+
+  return true;
+}