code: func MD5Sum(txt string) (sum string) {
  	h := md5.New()
  	buf := bufio.NewWriterSize(h, 128)
  	buf.WriteString(txt)
  	buf.Flush()
  	sign := make([]byte, hex.EncodedLen(h.Size()))
  	hex.Encode(sign, h.Sum(nil))
  	sum = string(bytes.ToUpper(sign))
  	return
  }
docs: // MD5Sum 计算 32 位长度的 MD5 sum