上图为设计的模型，解密的秘钥放在云函数中，默认腾讯服务器是可以信任的，解密是通过云函数searchCodeInfo。

下面为云函数代码

// 云函数入口文件
const cloud = require('wx-server-sdk')
var request = require('request')

cloud.init()

//放入MD5密码库js源代码
/* 
 * A JavaScript implementation of the RSA Data Security, Inc. MD5 Message 
 * Digest Algorithm, as defined in RFC 1321. 
 * Version 1.1 Copyright (C) Paul Johnston 1999 - 2002. 
 * Code also contributed by Greg Holt 
 * See http://pajhome.org.uk/site/legal.html for details. 
 */


/* 
 * Add integers, wrapping at 2^32. This uses 16-bit operations internally 
 * to work around bugs in some JS interpreters. 
 */
function safe_add(x, y) {
    var lsw = (x & 0xFFFF) + (y & 0xFFFF)
    var msw = (x >> 16) + (y >> 16) + (lsw >> 16)
    return (msw << 16) | (lsw & 0xFFFF)
}

/* 
 * Bitwise rotate a 32-bit number to the left. 
 */
function rol(num, cnt) {
    return (num << cnt) | (num >>> (32 - cnt))
}

/* 
 * These functions implement the four basic operations the algorithm uses. 
 */
function cmn(q, a, b, x, s, t) {
    return safe_add(rol(safe_add(safe_add(a, q), safe_add(x, t)), s), b)
}

function ff(a, b, c, d, x, s, t) {
    return cmn((b & c) | ((~b) & d), a, b, x, s, t)
}

function gg(a, b, c, d, x, s, t) {
    return cmn((b & d) | (c & (~d)), a, b, x, s, t)
}

function hh(a, b, c, d, x, s, t) {
    return cmn(b ^ c ^ d, a, b, x, s, t)
}

function ii(a, b, c, d, x, s, t) {
    return cmn(c ^ (b | (~d)), a, b, x, s, t)
}

/* 
 * Calculate the MD5 of an array of little-endian words, producing an array 
 * of little-endian words. 
 */
function coreMD5(x) {
    var a = 1732584193
    var b = -271733879
    var c = -1732584194
    var d = 271733878

    for (var i = 0; i < x.length; i += 16) {
        var olda = a
        var oldb = b
        var oldc = c
        var oldd = d

        a = ff(a, b, c, d, x[i + 0], 7, -680876936)
        d = ff(d, a, b, c, x[i + 1], 12, -389564586)
        c = ff(c, d, a, b, x[i + 2], 17, 606105819)
        b = ff(b, c, d, a, x[i + 3], 22, -1044525330)
        a = ff(a, b, c, d, x[i + 4], 7, -176418897)
        d = ff(d, a, b, c, x[i + 5], 12, 1200080426)
        c = ff(c, d, a, b, x[i + 6], 17, -1473231341)
        b = ff(b, c, d, a, x[i + 7], 22, -45705983)
        a = ff(a, b, c, d, x[i + 8], 7, 1770035416)
        d = ff(d, a, b, c, x[i + 9], 12, -1958414417)
        c = ff(c, d, a, b, x[i + 10], 17, -42063)
        b = ff(b, c, d, a, x[i + 11], 22, -1990404162)
        a = ff(a, b, c, d, x[i + 12], 7, 1804603682)
        d = ff(d, a, b, c, x[i + 13], 12, -40341101)
        c = ff(c, d, a, b, x[i + 14], 17, -1502002290)
        b = ff(b, c, d, a, x[i + 15], 22, 1236535329)

        a = gg(a, b, c, d, x[i + 1], 5, -165796510)
        d = gg(d, a, b, c, x[i + 6], 9, -1069501632)
        c = gg(c, d, a, b, x[i + 11], 14, 643717713)
        b = gg(b, c, d, a, x[i + 0], 20, -373897302)
        a = gg(a, b, c, d, x[i + 5], 5, -701558691)
        d = gg(d, a, b, c, x[i + 10], 9, 38016083)
        c = gg(c, d, a, b, x[i + 15], 14, -660478335)
        b = gg(b, c, d, a, x[i + 4], 20, -405537848)
        a = gg(a, b, c, d, x[i + 9], 5, 568446438)
        d = gg(d, a, b, c, x[i + 14], 9, -1019803690)
        c = gg(c, d, a, b, x[i + 3], 14, -187363961)
        b = gg(b, c, d, a, x[i + 8], 20, 1163531501)
        a = gg(a, b, c, d, x[i + 13], 5, -1444681467)
        d = gg(d, a, b, c, x[i + 2], 9, -51403784)
        c = gg(c, d, a, b, x[i + 7], 14, 1735328473)
        b = gg(b, c, d, a, x[i + 12], 20, -1926607734)

        a = hh(a, b, c, d, x[i + 5], 4, -378558)
        d = hh(d, a, b, c, x[i + 8], 11, -2022574463)
        c = hh(c, d, a, b, x[i + 11], 16, 1839030562)
        b = hh(b, c, d, a, x[i + 14], 23, -35309556)
        a = hh(a, b, c, d, x[i + 1], 4, -1530992060)
        d = hh(d, a, b, c, x[i + 4], 11, 1272893353)
        c = hh(c, d, a, b, x[i + 7], 16, -155497632)
        b = hh(b, c, d, a, x[i + 10], 23, -1094730640)
        a = hh(a, b, c, d, x[i + 13], 4, 681279174)
        d = hh(d, a, b, c, x[i + 0], 11, -358537222)
        c = hh(c, d, a, b, x[i + 3], 16, -722521979)
        b = hh(b, c, d, a, x[i + 6], 23, 76029189)
        a = hh(a, b, c, d, x[i + 9], 4, -640364487)
        d = hh(d, a, b, c, x[i + 12], 11, -421815835)
        c = hh(c, d, a, b, x[i + 15], 16, 530742520)
        b = hh(b, c, d, a, x[i + 2], 23, -995338651)

        a = ii(a, b, c, d, x[i + 0], 6, -198630844)
        d = ii(d, a, b, c, x[i + 7], 10, 1126891415)
        c = ii(c, d, a, b, x[i + 14], 15, -1416354905)
        b = ii(b, c, d, a, x[i + 5], 21, -57434055)
        a = ii(a, b, c, d, x[i + 12], 6, 1700485571)
        d = ii(d, a, b, c, x[i + 3], 10, -1894986606)
        c = ii(c, d, a, b, x[i + 10], 15, -1051523)
        b = ii(b, c, d, a, x[i + 1], 21, -2054922799)
        a = ii(a, b, c, d, x[i + 8], 6, 1873313359)
        d = ii(d, a, b, c, x[i + 15], 10, -30611744)
        c = ii(c, d, a, b, x[i + 6], 15, -1560198380)
        b = ii(b, c, d, a, x[i + 13], 21, 1309151649)
        a = ii(a, b, c, d, x[i + 4], 6, -145523070)
        d = ii(d, a, b, c, x[i + 11], 10, -1120210379)
        c = ii(c, d, a, b, x[i + 2], 15, 718787259)
        b = ii(b, c, d, a, x[i + 9], 21, -343485551)

        a = safe_add(a, olda)
        b = safe_add(b, oldb)
        c = safe_add(c, oldc)
        d = safe_add(d, oldd)
    }
    return [a, b, c, d]
}

/* 
 * Convert an array of little-endian words to a hex string. 
 */
function binl2hex(binarray) {
    var hex_tab = "0123456789abcdef"
    var str = ""
    for (var i = 0; i < binarray.length * 4; i++) {
        str += hex_tab.charAt((binarray[i >> 2] >> ((i % 4) * 8 + 4)) & 0xF) +
            hex_tab.charAt((binarray[i >> 2] >> ((i % 4) * 8)) & 0xF)
    }
    return str
}

/* 
 * Convert an array of little-endian words to a base64 encoded string. 
 */
function binl2b64(binarray) {
    var tab = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
    var str = ""
    for (var i = 0; i < binarray.length * 32; i += 6) {
        str += tab.charAt(((binarray[i >> 5] << (i % 32)) & 0x3F) |
            ((binarray[i >> 5 + 1] >> (32 - i % 32)) & 0x3F))
    }
    return str
}

/* 
 * Convert an 8-bit character string to a sequence of 16-word blocks, stored 
 * as an array, and append appropriate padding for MD4/5 calculation. 
 * If any of the characters are >255, the high byte is silently ignored. 
 */
function str2binl(str) {
    var nblk = ((str.length + 8) >> 6) + 1 // number of 16-word blocks  
    var blks = new Array(nblk * 16)
    for (var i = 0; i < nblk * 16; i++) blks[i] = 0
    for (var i = 0; i < str.length; i++)
        blks[i >> 2] |= (str.charCodeAt(i) & 0xFF) << ((i % 4) * 8)
    blks[i >> 2] |= 0x80 << ((i % 4) * 8)
    blks[nblk * 16 - 2] = str.length * 8
    return blks
}

/* 
 * Convert a wide-character string to a sequence of 16-word blocks, stored as 
 * an array, and append appropriate padding for MD4/5 calculation. 
 */
function strw2binl(str) {
    var nblk = ((str.length + 4) >> 5) + 1 // number of 16-word blocks  
    var blks = new Array(nblk * 16)
    for (var i = 0; i < nblk * 16; i++) blks[i] = 0
    for (var i = 0; i < str.length; i++)
        blks[i >> 1] |= str.charCodeAt(i) << ((i % 2) * 16)
    blks[i >> 1] |= 0x80 << ((i % 2) * 16)
    blks[nblk * 16 - 2] = str.length * 16
    return blks
}

/* 
 * External interface 
 */
function hexMD5(str) {
    return binl2hex(coreMD5(str2binl(str)))
}

function hexMD5w(str) {
    return binl2hex(coreMD5(strw2binl(str)))
}

function b64MD5(str) {
    return binl2b64(coreMD5(str2binl(str)))
}

function b64MD5w(str) {
    return binl2b64(coreMD5(strw2binl(str)))
}
/* Backward compatibility */
function calcMD5(str) {
    return binl2hex(coreMD5(str2binl(str)))
}

var base64Pad = '=';
var toBase64Table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
function toBase64(data) {
    var result = '';
    var length = data.length;
    var i;
    // Convert every three bytes to 4 ascii characters.                                                                                                                                                 
    for (i = 0; i < (length - 2); i += 3) {
        result += toBase64Table[data.charCodeAt(i) >> 2];
        result += toBase64Table[((data.charCodeAt(i) & 0x03) << 4) + (data.charCodeAt(i + 1) >> 4)];
        result += toBase64Table[((data.charCodeAt(i + 1) & 0x0f) << 2) + (data.charCodeAt(i + 2) >> 6)];
        result += toBase64Table[data.charCodeAt(i + 2) & 0x3f];
    }
    // Convert the remaining 1 or 2 bytes, pad out to 4 characters.                                                                                                                                    
    if (length % 3) {
        i = length - (length % 3);
        result += toBase64Table[data.charCodeAt(i) >> 2];
        if ((length % 3) == 2) {
            result += toBase64Table[((data.charCodeAt(i) & 0x03) << 4) + (data.charCodeAt(i + 1) >> 4)];
            result += toBase64Table[(data.charCodeAt(i + 1) & 0x0f) << 2];
            result += base64Pad;
        } else {
            result += toBase64Table[(data.charCodeAt(i) & 0x03) << 4];
            result += base64Pad + base64Pad;
        }
    }
    return result;
}

function signature(str) {
    var rtnStr = hexMD5(str);
    return toBase64(rtnStr);
}

// 云函数入口函数
exports.main = async (event, context) => {
    var data = {
        'billCode': event.code,
        'size': 100
    }
    console.log('event:',event)
    var reqInfo = JSON.stringify(data) + '秘钥';
    console.log('reqInfo:', reqInfo)
    console.log('signature(reqInfo):', signature(reqInfo))
    return new Promise((resolve, reject) => {
        request({
            url: '服务器地址',  
            method: "POST",
            json: true,
            headers: {
                'Content-Type': 'application/json;charset=UTF-8',
                'openId': 'uaaa',
                'openSecure': 'uaaa',
                'signature': signature(reqInfo)
            },
            body: data
        }, function (error, response, body) {
            resultInfo = response;
            console.log('error:',error)
            console.log('body:',body)
            console.log('response.statusCode:',response.statusCode)
            if (!error && response.statusCode == 200) {
                try {
                    console.log('body:', body)
                    resolve(body)
                    console.log('resolve(body)', resolve(body))
                } catch (e) {
                    reject()
                }
            }
        })
    })
}