code: @RequestMapping(value = "/oauth/token_key", method = RequestMethod.GET)
      @ResponseBody
      public Map<String, String> getKey(Principal principal) {
          if ((principal == null || principal instanceof AnonymousAuthenticationToken) && !converter.isPublic()) {
              throw new AccessDeniedException("You need to authenticate to see a shared key");
          }
          Map<String, String> result = converter.getKey();
          return result;
      }
docs: Get the verification key for the token signatures. The principal has to
  be provided only if the key is secret
  (shared not public).
  
  @param principal the currently authenticated user if there is one
  @return the key used to verify tokens