code: func (bigEndian) ReadUintN(b io.ByteReader, length uint8) (uint64, error) {
  	var res uint64
  	for i := uint8(0); i < length; i++ {
  		bt, err := b.ReadByte()
  		if err != nil {
  			return 0, err
  		}
  		res ^= uint64(bt) << ((length - 1 - i) * 8)
  	}
  	return res, nil
  }
docs: // ReadUintN reads N bytes