code: public static String unsign(@NotNull String val, @NotNull Mac mac) {
          int idx = val.lastIndexOf('.');
  
          if (idx == -1) {
              return null;
          }
  
          String str = val.substring(0, idx);
          if (val.equals(sign(str, mac))) {
              return str;
          }
          return null;
      }
docs: Returns the original value is the signature is correct. Null otherwise.