【Android逆向】rpc调用某安App的X-App-Token签名函数
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1.目标
在学习的过程中，会遇到有些算法比较麻烦，没有办法直接还原。那我们就另辟蹊径，不去分析具体的算法实现。直接使用rpc的方式调用算法函数，本文章以某安App的X-App-Token签名函数为例。
2.操作环境
-
mac系统
-
frida-dexdump：导出加固后dex文件
-
Charles：抓取http接口
-
已Root安卓机：脱壳
-
Python3.8：实现rpc功能
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Jadx：导出dex文件为源码
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Android Studio：静态分析
3.流程
寻找切入点
通过Charles抓包获取到关键词为X-App-Token，这也就是我们的切入点：
image-20220729145855104
静态分析
使用查壳工具发现该apk使用的是360加固，启动App后，使用frida-dexdump的frida-dexdump -FU命令导出dex文件：
image-20220729151122154
由于dex文件较多，不方便查询，使用jadx把多个dex文件导出为源码:
import osfor file in os.listdir(os.curdir):if file.find(".dex") > 0:sh = 'jadx -j 1 -r -d ./ ./' + fileprint(sh)os.system(sh)
将以上的python脚本放到dex同级目录，切换到dex目录，并执行以上脚本，执行完成后会生成sources文件夹，使用Android Studio打开该文件夹，全局搜索X-App-Token:
image-20220729152147309
找到关键函数：
private final String[] m13135() {String str;Locale locale = Locale.getDefault();String valueOf = String.valueOf(Build.VERSION.SDK_INT);String str2 = locale.getLanguage() + '-' + ((Object) locale.getCountry());byte[] bytes = (this.f16167.m13205() + "; ; ; " + this.f16167.m13207() + "; " + ((Object) Build.MANUFACTURER) + "; " + ((Object) Build.BRAND) + "; " + ((Object) Build.MODEL) + "; " + ((Object) Build.DISPLAY) + "; " + ((Object) C4765.m13174().m12851())).getBytes(Charsets.UTF_8);Intrinsics.checkNotNullExpressionValue(bytes, "this as java.lang.String).getBytes(charset)");String encodeToString = Base64.encodeToString(bytes, 0);Intrinsics.checkNotNullExpressionValue(encodeToString, "encodeToString(device.to…eArray(), Base64.DEFAULT)");String sb = new StringBuilder(encodeToString).reverse().toString();Intrinsics.checkNotNullExpressionValue(sb, "StringBuilder(device).reverse().toString()");String replace = new Regex("\\r\\n|\\r|\\n|=").replace(sb, BuildConfig.FLAVOR);String as = AuthUtils.getAS(this.f16166, replace);if (C4765.m13166().m13307()) {str = "1";} else {str = C4765.m13166().m13300() ? "2" : "0";}Intrinsics.checkNotNullExpressionValue(as, "appToken");String r1 = this.f16167.m13203();Intrinsics.checkNotNullExpressionValue(r1, "appMetadata.channel");return new String[]{"User-Agent", this.f16170, "X-Requested-With", "XMLHttpRequest", "X-Sdk-Int", valueOf, "X-Sdk-Locale", str2, "X-App-Id", "com.coolapk.market", "X-App-Token", as, "X-App-Version", this.f16168, "X-App-Code", String.valueOf(this.f16169), "X-Api-Version", "12", "X-App-Device", replace, "X-Dark-Mode", str, "X-App-Channel", r1, "X-App-Mode", this.f16167.m13197().toString(), "X-App-Supported", String.valueOf(this.f16167.m13199())};}
删除无关代码后：
private final String[] m13135() {byte[] bytes = (this.f16167.m13205() + "; ; ; " + this.f16167.m13207() + "; " + ((Object) Build.MANUFACTURER) + "; " + ((Object) Build.BRAND) + "; " + ((Object) Build.MODEL) + "; " + ((Object) Build.DISPLAY) + "; " + ((Object) C4765.m13174().m12851())).getBytes(Charsets.UTF_8);String encodeToString = Base64.encodeToString(bytes, 0);String sb = new StringBuilder(encodeToString).reverse().toString();String replace = new Regex("\\r\\n|\\r|\\n|=").replace(sb, BuildConfig.FLAVOR);String as = AuthUtils.getAS(this.f16166, replace);}
由此可看出，X-App-Token参数由AuthUtils.getAS方法生成，本篇文章的目的是通过rpc直接调用getAS函数，所以不会去具体分析getAS方法的实现。
调用getAS方法的入参有两个，第一个是context，第二个参数即为我们需要拼接的参数。看源码可知，该参数由多个参数拼成，然后再base64。具体查看每一个函数，整理后的结果如下：
this.f16167.m13205()：android_id
this.f16167.m13207()：wifi的mac地址
((Object) Build.MANUFACTURER)：硬件制造商
((Object) Build.MODEL)：系统定制商
((Object) Build.DISPLAY)：显示屏参数
((Object) C4765.m13174().m12851())：这个参数为空，我们就暂且不管
你也可以直接使用命令frida-trace -UF -j '*!*getBytes*':
{onEnter(log, args, state) {log(`String.getBytes=${this.toString()}=`)log(`String.getBytes(${args.map(JSON.stringify).join(', ')})`);},onLeave(log, retval, state) {if (retval !== undefined) {log(`<= ${JSON.stringify(retval)}`);}}
}
获取到结果如下，根据结果反推参数值
e8e69e7384cb09c0; ; ; F4:F5:DB:24:A6:E1; Xiaomi; xiaomi; MI 5X; QL1515-tiffany-build-20171026203938; null
结果
至此，我们的getAS函数的入参已经确定，接下来就是实现RPC
python源码如下：
import frida
from flask import Flask, request
import base64
result = {}def on_message(message, data):if message['type'] == 'send':payload = message['payload']if "###" in payload:global resultarray = payload.split("###")result[array[0]] = array[1]print(message['payload'])elif message['type'] == 'error':print(message['stack'])js_code = '''
rpc.exports = {// 函数名getASsign: function(params){Java.perform(function(){//拿到context上下文var currentApplication = Java.use('android.app.ActivityThread').currentApplication();var context = currentApplication.getApplicationContext();// use 加载的类路径var AuthUtils = Java.use('com.coolapk.market.util.AuthUtils');var sign = AuthUtils.getAS(context, params); // context，paramssend(params+"###"+sign);})}
};
'''process = frida.get_usb_device().attach('酷安')
script = process.create_script(js_code)
script.on('message', on_message)
script.load()app = Flask(__name__)@app.route('/get_sign')
def get_sign():device_id = request.args.get('device_id', '')mac_address = request.args.get('mac_address', '').upper()manufacturer = request.args.get('manufacturer', '')model = request.args.get('model', '')display = request.args.get('display', '')params = f'{device_id}; ; ; {mac_address}; {manufacturer}; {model}; {display}; null'base_en = base64.encodebytes(params.encode('utf-8')).decode('utf-8')base_en = base_en[::-1]base_en = base_en.replace('\n', '')base_en = base_en.replace('\r', '')base_en = base_en.replace('=', '')script.exports.sign(base_en)sign = result[base_en]return signif __name__ == '__main__':app.run()
运行python脚本后，浏览器调用get_sign方法即可获取到X-App-Token
image-20220729190018280
End
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