前言
将脱壳后的dex文件在jadx打开，由于脱壳后会产生多个dex，因此先用脚本合并一下比较方便。当然最新版jadx支持增加文件，也可以直接添加。
import os, syspath = r'xxxx'# 文件夹目录files = os.listdir(path)out_path =r'xxxxx'#路径s = []for file in files: # 遍历文件夹if file.find("dex") > 0: ## 查找dex 文件sh = f'jadx -j 1 -r -d {out_path} {path}\{file}'print(sh)os.system(sh)`
同时使用抓包软件开始对app进行抓包，对于移动端的app抓包，证书的安装一直是一个很繁琐的问题，在这里推荐一款Magisk的模块Move Certificates,可以很方便的将用户证书移动到系统层，实现https的抓包。
这里主要的功能侧重点在bbsapi.domo.cn和class.domo.cn上，因为该app主要的功能点就是在这两个地址下。
查看一下数据包的内容，可以看到path中是包含sign值的
/newapi/live/square/live-count?noncestr=84140992&sign=47e714f977952ad75eb6eeb3165083dae93502b3×tamp=1627008983764<
目标就是要找到这个sign值的生成算法。首先到jad中进行代码定位，为了降低逆向难度，可以选择从noncestr入手，进行搜索可能会简单一点，当然这只能是经验之谈，一切还是要以实际分析情况为准。
可以看一下如果搜索sign，会有多少结果。
继续看noncestr，看到搜索到了14个相关代码，根据类名来看，选择一个最最有可能的先看一下。
可以看到是有getSign函数在这个类下面的，采用直接hook这个函数也不是很现实，由于jadx的问题，是无法显示该函数下的代码，这个时候，可以采用直接hook该函数的系统加密函数的方法，来查看是否是直接采用了比如MD5,SHA1，Base64等等常规加密的方法，对于此，直接hook后查看输入和输出。
编写frida脚本进行测试一下。
update:appSignKey=ac6190d7dfaa77df726f0a82244d3eda68675ccd4e95de802f5042e91d15edc7bae3026d8f0fb2a8287446bb289563970264&noncestr=12520893×tamp=1627023153533digest:e2931f94fdf5cbad61d95bf23b192841a12f1a8c
可以看到上传数据的构成是appSignKey＋noncestr和timestamp构成。多次抓包发现appSignKey是一个固定值（只是class.domo.cn下的固定值，该app不同的网址采用不同的Key值）并且结果跟抓包数据完全一致。这时候主要的问题就是找到使用的默认算法。可以看到第一个调用的函数就是java.security.MessageDigest.digest(Native Method)，该类提供了消息摘要算法，如 MD5 或 SHA，的功能。因此可以模拟测试一下，看一下采用的是MD5还是SHA算法。
测试发现采用的是SHA1算法。noncestr就是随机8位数字组成，每次可以采用上一次所生成的，或者是自己生成即可。noncestr算法还原。
Random random=new Random();for(int i=0;i<8;i++){str.append(random.nextInt(10));}int num=Integer.parseInt(str.toString());System.out.println(num);
此外，对于bbsapi.domo.cn，它的update是
appSignKey=Wj8BI3VUZ6BuojAkqzBM3HWHNHv08xdZEtaksbRg6snnuLsvivwa8IvR6PvQ76H0IQQsqkIsa5OKJtg6QcBMfCblMMywgZaA8co&noncestr=63606047×tamp=1627023164959采用了不同的key值来进行拼接。
总结
该app的签名算法还原后即为，key+noncestr+timestamp。之后进行SHA1加密，生成sign。最后附上frida hook代码
base64DecodeChars = new Array((-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), (-1), 62, (-1), (-1), (-1), 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, (-1), (-1), (-1), (-1), (-1), (-1), (-1), 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, (-1), (-1), (-1), (-1), (-1), (-1), 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, (-1), (-1), (-1), (-1), (-1));var stringToBase64 = function (e) {var r, a, c, h, o, t;for (c = e.length, a = 0, r = ''; a < c;) {if (h = 255 & e.charCodeAt(a++), a == c) {r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4),r += '==';break}if (o = e.charCodeAt(a++), a == c) {r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4 | (240 & o) >> 4),r += base64EncodeChars.charAt((15 & o) << 2),r += '=';break}t = e.charCodeAt(a++),r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4 | (240 & o) >> 4),r += base64EncodeChars.charAt((15 & o) << 2 | (192 & t) >> 6),r += base64EncodeChars.charAt(63 & t)}return r}var base64ToString = function (e) {var r, a, c, h, o, t, d;for (t = e.length, o = 0, d = ''; o < t;) {dor = base64DecodeChars[255 & e.charCodeAt(o++)];while (o < t && r == -1);if (r == -1)break;doa = base64DecodeChars[255 & e.charCodeAt(o++)];while (o < t && a == -1);if (a == -1)break;d += String.fromCharCode(r << 2 | (48 & a) >> 4);do {if (c = 255 & e.charCodeAt(o++), 61 == c)return d;c = base64DecodeChars[c]} while (o < t && c == -1);if (c == -1)break;d += String.fromCharCode((15 & a) << 4 | (60 & c) >> 2);do {if (h = 255 & e.charCodeAt(o++), 61 == h)return d;h = base64DecodeChars[h]} while (o < t && h == -1);if (h == -1)break;d += String.fromCharCode((3 & c) << 6 | h)}return d}var hexToBytes = function (str) {var pos = 0;var len = str.length;if (len % 2 != 0) {return null;}len /= 2;var hexA = new Array();for (var i = 0; i < len; i++) {var s = str.substr(pos, 2);var v = parseInt(s, 16);hexA.push(v);pos += 2;}return hexA;}var bytesToHex = function (arr) {var str = '';var k, j;for (var i = 0; i < arr.length; i++) {k = arr[i];j = k;if (k < 0) {j = k + 256;}if (j < 16) {str += "0";}str += j.toString(16);}return str;}var stringToHex = function (str) {var val = "";for (var i = 0; i < str.length; i++) {if (val == "")val = str.charCodeAt(i).toString(16);elseval += str.charCodeAt(i).toString(16);}return val}var stringToBytes = function (str) {var ch, st, re = [];for (var i = 0; i < str.length; i++) {ch = str.charCodeAt(i);st = [];do {st.push(ch & 0xFF);ch = ch >> 8;}while (ch);re = re.concat(st.reverse());}return re;}var bytesToString = function (arr) {var str = '';arr = new Uint8Array(arr);for (var i in arr) {str += String.fromCharCode(arr[i]);}return str;}var bytesToBase64 = function (e) {var r, a, c, h, o, t;for (c = e.length, a = 0, r = ''; a < c;) {if (h = 255 & e[a++], a == c) {r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4),r += '==';break}if (o = e[a++], a == c) {r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4 | (240 & o) >> 4),r += base64EncodeChars.charAt((15 & o) << 2),r += '=';break}t = e[a++],r += base64EncodeChars.charAt(h >> 2),r += base64EncodeChars.charAt((3 & h) << 4 | (240 & o) >> 4),r += base64EncodeChars.charAt((15 & o) << 2 | (192 & t) >> 6),r += base64EncodeChars.charAt(63 & t)}return r}var base64ToBytes = function (e) {var r, a, c, h, o, t, d;for (t = e.length, o = 0, d = []; o < t;) {dor = base64DecodeChars[255 & e.charCodeAt(o++)];while (o < t && r == -1);if (r == -1)break;doa = base64DecodeChars[255 & e.charCodeAt(o++)];while (o < t && a == -1);if (a == -1)break;d.push(r << 2 | (48 & a) >> 4);do {if (c = 255 & e.charCodeAt(o++), 61 == c)return d;c = base64DecodeChars[c]} while (o < t && c == -1);if (c == -1)break;d.push((15 & a) << 4 | (60 & c) >> 2);do {if (h = 255 & e.charCodeAt(o++), 61 == h)return d;h = base64DecodeChars[h]} while (o < t && h == -1);if (h == -1)break;d.push((3 & c) << 6 | h)}return d}Java.perform(function () {function showStacks() {console.log(Java.use("android.util.Log").getStackTraceString(Java.use("java.lang.Exception").$new()));}var secretKeySpec = Java.use('javax.crypto.spec.SecretKeySpec');secretKeySpec.$init.overload('[B', 'java.lang.String').implementation = function (a, b) {showStacks();var result = this.$init(a, b);console.log("======================================");console.log("算法名：" + b + "|Dec密钥:" + bytesToString(a));console.log("算法名：" + b + "|Hex密钥:" + bytesToHex(a));return result;}var mac = Java.use('javax.crypto.Mac');mac.getInstance.overload('java.lang.String').implementation = function (a) {showStacks();var result = this.getInstance(a);console.log("======================================");console.log("算法名：" + a);return result;}mac.update.overload('[B').implementation = function (a) {showStacks();this.update(a);console.log("======================================");console.log("update:" + bytesToString(a))}mac.update.overload('[B', 'int', 'int').implementation = function (a, b, c) {showStacks();this.update(a, b, c)console.log("======================================");console.log("update:" + bytesToString(a) + "|" + b + "|" + c);}mac.doFinal.overload().implementation = function () {showStacks();var result = this.doFinal();console.log("======================================");console.log("doFinal结果:" + bytesToHex(result));console.log("doFinal结果:" + bytesToBase64(result));return result;}mac.doFinal.overload('[B').implementation = function (a) {showStacks();var result = this.doFinal(a);console.log("======================================");console.log("doFinal参数:" + bytesToString(a));console.log("doFinal结果:" + bytesToHex(result));console.log("doFinal结果:" + bytesToBase64(result));return result;}var md = Java.use('java.security.MessageDigest');md.getInstance.overload('java.lang.String', 'java.lang.String').implementation = function (a, b) {showStacks();console.log("======================================");console.log("算法名：" + a);return this.getInstance(a, b);}md.getInstance.overload('java.lang.String').implementation = function (a) {showStacks();console.log("======================================");console.log("算法名：" + a);return this.getInstance(a);}md.update.overload('[B').implementation = function (a) {showStacks();console.log("======================================");console.log("update:" + bytesToString(a))return this.update(a);}md.update.overload('[B', 'int', 'int').implementation = function (a, b, c) {showStacks();console.log("======================================");console.log("update:" + bytesToString(a) + "|" + b + "|" + c);return this.update(a, b, c);}md.digest.overload().implementation = function () {showStacks();console.log("======================================");var result = this.digest();console.log("digest结果:" + bytesToHex(result));console.log("digest结果:" + bytesToBase64(result));return result;}md.digest.overload('[B').implementation = function (a) {showStacks();console.log("======================================");console.log("digest参数:" + bytesToString(a));var result = this.digest(a);console.log("digest结果:" + bytesToHex(result));console.log("digest结果:" + bytesToBase64(result));return result;}var ivParameterSpec = Java.use('javax.crypto.spec.IvParameterSpec');ivParameterSpec.$init.overload('[B').implementation = function (a) {showStacks();var result = this.$init(a);console.log("======================================");console.log("iv向量:" + bytesToString(a));console.log("iv向量:" + bytesToHex(a));return result;}var cipher = Java.use('javax.crypto.Cipher');cipher.getInstance.overload('java.lang.String').implementation = function (a) {showStacks();var result = this.getInstance(a);console.log("======================================");console.log("模式填充:" + a);return result;}cipher.update.overload('[B').implementation = function (a) {showStacks();var result = this.update(a);console.log("======================================");console.log("update:" + bytesToString(a));return result;}cipher.update.overload('[B', 'int', 'int').implementation = function (a, b, c) {showStacks();var result = this.update(a, b, c);console.log("======================================");console.log("update:" + bytesToString(a) + "|" + b + "|" + c);return result;}cipher.doFinal.overload().implementation = function () {showStacks();var result = this.doFinal();console.log("======================================");console.log("doFinal结果:" + bytesToHex(result));console.log("doFinal结果:" + bytesToBase64(result));return result;}cipher.doFinal.overload('[B').implementation = function (a) {showStacks();var result = this.doFinal(a);console.log("======================================");console.log("doFinal参数:" + bytesToString(a));console.log("doFinal结果:" + bytesToHex(result));console.log("doFinal结果:" + bytesToBase64(result));return result;}var x509EncodedKeySpec = Java.use('java.security.spec.X509EncodedKeySpec');x509EncodedKeySpec.$init.overload('[B').implementation = function (a) {showStacks();var result = this.$init(a);console.log("======================================");console.log("RSA密钥:" + bytesToBase64(a));return result;}var rSAPublicKeySpec = Java.use('java.security.spec.RSAPublicKeySpec');rSAPublicKeySpec.$init.overload('java.math.BigInteger', 'java.math.BigInteger').implementation = function (a, b) {showStacks();var result = this.$init(a, b);console.log("======================================");//console.log("RSA密钥:" + bytesToBase64(a));console.log("RSA密钥N:" + a.toString(16));console.log("RSA密钥E:" + b.toString(16));return result;}});
算法还原之后，下一篇将进行完结，打造bp插件，实现无缝衔接测试。
【火线短视频精选】
【周度激励】2021.7.26 ～ 2021.8.1公告
【相关精选文章】
火线Zone是[火线安全平台]运营的封闭式社区，社区成员必须在[火线安全平台]提交有效漏洞才能申请免费加入,符合要求的白帽子可联系[火线小助手]加入。
我们不希望出现劣币驱逐良币的结果，我们不希望一个技术社区变成一个水区！
欢迎具备分享精神的白帽子加入火线Zone，共建一个有技术氛围的优质社区！
本文始发于微信公众号（火线Zone）：XX客户端APP签名分析之算法分析篇
- 左青龙
- 微信扫一扫
-
- 右白虎
- 微信扫一扫
-
评论