MAPG-235 refactored challenge token generation and check

src/Repository/UserInChallengeRepository.php

@@ -57,10 +57,8 @@ class UserInChallengeRepository
         }
 
         // validate token string
-        foreach (str_split($token_str) as $char) {
-            if (!(('0' <= $char && $char <= '9') || ('a' <= $char && $char <= 'f'))) {
-                return null;
-            }
+        if (!ctype_xdigit($token_str)) {
+            return null;
         }
         // convert token to int
         $token = hexdec($token_str);