code: @Nonnull
      public String createValidityInfoKeyName( @Nonnull final String origKey ) {
          if ( origKey == null ) {
              throw new IllegalArgumentException( "The sessionId must not be null." );
          }
          return "validity:" + _storageKeyFormat.format(origKey);
      }
docs: Creates the name/key that can be used for storing the encoded session validity information.
  @param origKey the session id (or validity info key) for that a key shall be created.
  @return a String.