* have reached a limit, this returns [END_OF_DATA].
   */
  internal fun readHeader(): DerHeader {
    require(peekedHeader == null)

    // We've hit a local limit.
    if (byteCount == limit) return END_OF_DATA

    // We've exhausted the source stream.
    if (limit == -1L && source.exhausted()) return END_OF_DATA

    // Read the tag.