}
    val unusedBitCount = source.readByte().toInt() and 0xff
    val byteString = source.readByteString(bytesLeft)
    return BitString(byteString, unusedBitCount)
  }

  fun readOctetString(): ByteString {
    if (bytesLeft == -1L || constructed) {
      throw ProtocolException("constructed octet strings not supported for DER")
    }
    return source.readByteString(bytesLeft)
  }