docs: @Nullable
      private String getCredentials(String header) {
          if (header == null) {
              return null;
          }
  
          final int space = header.indexOf(' ');
          if (space <= 0) {
              return null;
          }
  
          final String method = header.substring(0, space);
          if (!prefix.equalsIgnoreCase(method)) {
              return null;
          }
  
          return header.substring(space + 1);
      }