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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
Jadx：导出dex文件为源码
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：
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：
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 os
for file in os.listdir(os.curdir):
if file.find(".dex") > 0:
sh = 'jadx -j 1 -r -d ./ ./' + file
print(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:
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 result
array = payload.split("###")
result[array[0]] = array[1]
print(message['payload'])
elif message['type'] == 'error':
print(message['stack'])
js_code = '''
rpc.exports = {
// 函数名getAS
sign: 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，params
send(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 sign
if __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
End
阅读此文档的过程中遇到任何问题，请关注公众号【移动端Android和iOS开发技术分享】或加QQ群【309580013】
