code: func (h *ExtendedHeader) writeShortHeader(b *bytes.Buffer, v protocol.VersionNumber) error {
  	typeByte := 0x40 | uint8(h.PacketNumberLen-1)
  	typeByte |= byte(h.KeyPhase << 2)
  
  	b.WriteByte(typeByte)
  	b.Write(h.DestConnectionID.Bytes())
  	return h.writePacketNumber(b)
  }
docs: // TODO: add support for the key phase