package main

import (
    "bytes"
    "crypto/cipher"
    "crypto/des"
    "encoding/base64"
    "encoding/hex"
    "fmt"
    "strings"
)

const (
    KEY = "FRNX"
    IV  = "23456789"
)

func main() {
    miwen := "7379533330307A63724F554661783369495A653253393533775377426948704A"
    str := decode(miwen)
    str
    fmt.Println(str)
}

func genKey(key string) string {
    if len(key) < 32 {
        key += strings.Repeat("0", 32)
    }
    return key[:32]
}

func decode(miwen string) string {
    s, _ := hex.DecodeString(miwen)
    x, _ := base64.StdEncoding.DecodeString(string(s))
    k, _ := base64.StdEncoding.DecodeString(genKey(KEY))
    str, _ := tripleDesDecrypt(x, k)
    return string(str)
}

// 3DES解密
func tripleDesDecrypt(crypted, key []byte) ([]byte, error) {
    block, err := des.NewTripleDESCipher(key)
    if err != nil {
        fmt.Println(err.Error())
        return nil, err
    }
    blockMode := cipher.NewCBCDecrypter(block, []byte(IV))
    origData := make([]byte, len(crypted))
    // origData := crypted
    blockMode.CryptBlocks(origData, crypted)
    origData = PKCS5UnPadding(origData)
    // origData = ZeroUnPadding(origData)
    return origData, nil
}

// 3DES加密
func TripleDesEncrypt(origData, key []byte) ([]byte, error) {
    block, err := des.NewTripleDESCipher(key)
    if err != nil {
        return nil, err
    }
    origData = PKCS5Padding(origData, block.BlockSize())
    // origData = ZeroPadding(origData, block.BlockSize())
    blockMode := cipher.NewCBCEncrypter(block, []byte(IV))
    crypted := make([]byte, len(origData))
    blockMode.CryptBlocks(crypted, origData)
    return crypted, nil
}

func PKCS5Padding(ciphertext []byte, blockSize int) []byte {
    padding := blockSize - len(ciphertext)%blockSize
    padtext := bytes.Repeat([]byte{byte(padding)}, padding)
    return append(ciphertext, padtext...)
}

func PKCS5UnPadding(origData []byte) []byte {
    length := len(origData)
    // 去掉最後一個位元組 unpadding 次
    unpadding := int(origData[length-1])
    return origData[:(length - unpadding)]
}