weixin api message encryption and decryption

weixin_crypt.go

package main

import (
	"bytes"
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha1"
	"encoding/base64"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"encoding/xml"
	"fmt"
	"io"
	mrand "math/rand"
	"sort"
	"strings"
	"time"
)


// weixinEncrypt encrypts plaintext and signs ciphertext
func weixinEncrypt(plaintext, key, appid, token string) (ciphertext, signature, nonce, ts string, err error) {
	var (
		buf bytes.Buffer
		r   = rand.Reader
	)
	// pin randomness, helps debugging
	// r = strings.NewReader("aaaaaaaaaaaaaaaa")
	// nonce = "n123"
	// ts = "ts123"
	io.CopyN(&buf, r, 16)                                        // 16 bytes random string
	binary.Write(&buf, binary.BigEndian, uint32(len(plaintext))) // 4  bytes of length of text plaintext, in big-endian (network byte order in TCP/IP stack means big-endian)
	buf.WriteString(plaintext)                                   // text msg
	buf.WriteString(appid)                                       // appid
	npad := 32 - buf.Len()%32                                    // 0 < npad <= 31
	buf.Write(bytes.Repeat([]byte{byte(npad)}, npad))
	bkey, err := base64.StdEncoding.DecodeString(key + "=")
	if err != nil {
		return
	}
	cipherBlock, err := aes.NewCipher(bkey)
	if err != nil {
		return
	}
	encrypter := cipher.NewCBCEncrypter(cipherBlock, bkey[:16])
	data := make([]byte, buf.Len())
	copy(data, buf.Bytes())
	encrypter.CryptBlocks(data, data)
	ciphertext = base64.StdEncoding.EncodeToString(data)
	if ts == "" {
		ts = fmt.Sprint(time.Now().UnixMilli())
	}
	if nonce == "" {
		nonce = fmt.Sprint(mrand.Int63())
	}
	signature = weixinSign(ciphertext, token, nonce, ts)
	return
}

// weixinDecrypt decrypts ciphertext
func weixinDecrypt(ciphertext, key string) (plaintext string, err error) {
	bkey, err := base64.StdEncoding.DecodeString(key + "=")
	cipherBlock, err := aes.NewCipher(bkey)
	if err != nil {
		return
	}
	cipherbytes, err := base64.StdEncoding.DecodeString(ciphertext)
	if err != nil {
		return
	}
	decryptor := cipher.NewCBCDecrypter(cipherBlock, bkey[:16])
	data := make([]byte, len(cipherbytes))
	copy(data, cipherbytes)
	decryptor.CryptBlocks(data, data)
	n := binary.BigEndian.Uint32(data[16:20])
	plaintext = string(data[20 : 20+n]) // drop 16 + 4 at head and appid + npad at tail
	return
}

// weixinSign signs ciphertext
func weixinSign(ciphertext, token, nonce, ts string) (signature string) {
	sigInputs := []string{
		token,
		ts,
		nonce,
		ciphertext,
	}
	sort.Strings(sigInputs)
	sha1sum := sha1.Sum([]byte(strings.Join(sigInputs, "")))
	signature = hex.EncodeToString(sha1sum[:])
	return
}

// weixinVerify verifies signature
func weixinVerify(ciphertext, signature, token, nonce, ts string) (verified bool) {
	verified = weixinSign(ciphertext, token, nonce, ts) == signature
	return
}

// # References
// [1]: https://developers.weixin.qq.com/doc/aispeech/confapi/thirdkefu/flow.html  "第三方客服系统接入 - 接入和数据运转流程"
// [2]: https://developers.weixin.qq.com/doc/oplatform/Third-party_Platforms/2.0/api/Before_Develop/Message_encryption_and_decryption.html   "消息加解密说明"
// [3]: https://res.wx.qq.com/op_res/-serEQ6xSDVIjfoOHcX78T1JAYX-pM_fghzfiNYoD8uHVd3fOeC0PC_pvlg4-kmP   "示例代码下载 -- too out-dated, fk"
// [4]: https://developers.weixin.qq.com/doc/aispeech/platform/example/kefu.html "智能对话 第三方客服系统接入"