作者论坛账号：Light紫星
【2021春节】解题领红包之三
逆向总结本题所用到的工具：
Jadx1.2.0
FrIDA14.2.13
ida7.5.0
Python3.7.2
首先安装apk，打开后是这个界面：
随便输入flag，提示flag格式错误，请重试。拖入jadx，找到关键函数，如下：
package p004cn.pojie52.cm01;import android.os.Bundle;import android.view.View;import android.widget.EditText;import android.widget.Toast;import androidx.appcompat.app.AppCompatActivity;/* renamed from: cn.pojie52.cm01.MainActivity */public class MainActivity extends AppCompatActivity {public native boolean check(String str);static {System.loadLibrary("native-lib");}/* access modifiers changed from: protected */// androidx.activity.ComponentActivity, androidx.core.app.ComponentActivity, androidx.appcompat.app.AppCompatActivity, androidx.fragment.app.FragmentActivitypublic void onCreate(Bundle bundle) {super.onCreate(bundle);setContentView(C0266R.layout.activity_main);final EditText editText = (EditText) findViewById(C0266R.C0268id.flag);findViewById(C0266R.C0268id.check).setOnClickListener(new View.OnClickListener() {/* class p004cn.pojie52.cm01.MainActivity.View$OnClickListenerC02651 */public void onClick(View view) {String trim = editText.getText().toString().trim();if (trim.length() != 30) {Toast.makeText(MainActivity.this, "flag格式错误，请重试", 0).show();} else if (MainActivity.this.check(trim)) {Toast.makeText(MainActivity.this, "恭喜你，验证正确！", 0).show();} else {Toast.makeText(MainActivity.this, "flag错误，再接再厉", 0).show();}}});}}
这里判断了输入的长度是否为30位，然后进入了so验证。
下一步，把so拖入ida，直接定位到关键函数：Java_cn_pojie52_cm01_MainActivity_check
该函数内容如下：
__int64 __fastcall Java_cn_pojie52_cm01_MainActivity_check(_JNIEnv *a1, __int64 a2, __int64 a3){const char *v5; // x21size_t v6; // w0int v7; // w0__int64 v8; // x0_BYTE *v9; // x0int8x16_t v10; // q0int8x16_t v11; // q4int8x16_t v12; // q2int8x16_t v13; // q5int8x16_t v14; // q1int8x16_t v15; // q0__int64 v16; // x8unsigned int v17; // w19_BYTE v19[33]; // [xsp+0h] [xbp-A0h]int v20; // [xsp+21h] [xbp-7Fh]char v21; // [xsp+25h] [xbp-7Bh]char v22; // [xsp+26h] [xbp-7Ah]char v23; // [xsp+27h] [xbp-79h]char v24; // [xsp+28h] [xbp-78h]char dest[16]; // [xsp+38h] [xbp-68h] BYREF__int128 v26; // [xsp+48h] [xbp-58h]__int128 v27; // [xsp+58h] [xbp-48h]__int128 v28; // [xsp+68h] [xbp-38h]__int64 v29; // [xsp+78h] [xbp-28h]v29 = *(_ReadStatusReg(ARM64_SYSREG(3, 3, 13, 0, 2)) + 40);if ( a1->functions->GetStringUTFLength(a1, a3) != 30 )return 0;v5 = a1->functions->GetStringUTFChars(a1, a3, 0LL);v28 = 0u;v27 = 0u;v26 = 0u;*dest = 0u;v6 = strlen(v5);strncpy(dest, v5, v6);a1->functions->ReleaseStringUTFChars(a1, a3, v5);v7 = strlen(dest);sub_B90(dest, v7, "areyousure??????");v8 = strlen(dest);v9 = sub_D90(dest, v8);*v19 = unk_11A1;*&v19[16] = unk_11B1;*&v19[25] = unk_11BA;v10.n128_u64[0] = 0xB2B2B2B2B2B2B2B2LL;v10.n128_u64[1] = 0xB2B2B2B2B2B2B2B2LL;v11.n128_u64[0] = 0xFEFEFEFEFEFEFEFELL;v11.n128_u64[1] = 0xFEFEFEFEFEFEFEFELL;v19[0] = 53;v12 = veorq_s8(vaddq_s8(veorq_s8(vaddq_s8(*&v19[1], v10), xmmword_1130), xmmword_1140), v11);v13.n128_u64[0] = 0x101010101010101LL;v13.n128_u64[1] = 0x101010101010101LL;v14.n128_u64[0] = 0x3E3E3E3E3E3E3E3ELL;v14.n128_u64[1] = 0x3E3E3E3E3E3E3E3ELL;*&v19[1] = vaddq_s8(veorq_s8(vsubq_s8(v13, vorrq_s8(vshrq_n_u8(v12, 7uLL), vshlq_n_s8(v12, 1uLL))), xmmword_1150),v14);v20 = 1782990162;v15 = veorq_s8(vaddq_s8(veorq_s8(vaddq_s8(*&v19[17], v10), xmmword_1160), xmmword_1170), v11);v21 = ((1- ((2 * ((((unk_11C6 - 78) ^ 0xB2) - 117) ^ 0xFE)) | ((((((unk_11C6 - 78) ^ 0xB2) - 117) ^ 0xFE) & 0x80) != 0))) ^ 0x25)+ 62;v16 = 0LL;v22 = ((1- ((2 * ((((unk_11C7 - 78) ^ 0xB1) - 118) ^ 0xFE)) | ((((((unk_11C7 - 78) ^ 0xB1) - 118) ^ 0xFE) & 0x80) != 0))) ^ 0x26)+ 62;*&v19[17] = vaddq_s8(veorq_s8(vsubq_s8(v13, vorrq_s8(vshrq_n_u8(v15, 7uLL), vshlq_n_s8(v15, 1uLL))), xmmword_1180),v14);v23 = ((1- ((2 * ((((unk_11C8 - 78) ^ 0xB0) - 119) ^ 0xFE)) | ((((((unk_11C8 - 78) ^ 0xB0) - 119) ^ 0xFE) & 0x80) != 0))) ^ 0x27)+ 62;v24 = ((1- ((2 * ((((unk_11C9 - 78) ^ 0xBF) - 120) ^ 0xFE)) | ((((((unk_11C9 - 78) ^ 0xBF) - 120) ^ 0xFE) & 0x80) != 0))) ^ 0x28)+ 62;while ( v9[v16] == v19[v16] ){if ( v9[v16] ){if ( ++v16 != 41 )continue;}v17 = 1;goto LABEL_9;}v17 = 0;LABEL_9:free(v9);return v17;}
大概看一下流程，先判断输入是否30位，然后把输入的数据传入sub_B90进行处理，再传入sub_D90处理一次，处理后的结果为v9，最后v9和v19进行比较。所以这里要先看一下sub_b90和sub_d90是干什么的，直接使用frida进行hook调用这两个函数，
frida代码如下：
# -*- coding: UTF-8 -*-import frida, sysjscode = '''function inline_hook() {var so_addr = Module.findBaseAddress("libnative-lib.so");if (so_addr) {console.log("so_addr:", so_addr);var addr_b90 = so_addr.add(0xb90);var sub_b90 = new NativeFunction(addr_b90 , 'int', ['pointer', 'int','pointer']);var arg1 = Memory.allocUtf8String('111111111111111111111111111111');var arg2 = 30;var arg3 = Memory.allocUtf8String('areyousure??????');var ret_b90 = sub_b90(arg1,arg2,arg3);console.log(Memory.readByteArray(arg1,64));var addr_d90 = so_addr.add(0xd90);var sub_d90 = new NativeFunction(addr_d90 , 'pointer', ['pointer', 'int' ]);var arg1 = Memory.allocUtf8String('111111111111111111111111111111');var arg2 = 30;var ret_d90 = sub_d90(arg1,arg2);console.log(Memory.readByteArray(ret_d90,64));}}setImmediate(inline_hook)'''def on_message(message, data):if message['type'] == 'send':print(" {0}".format(message['payload']))else:print(message)pass#print(frida.enumerate_devices())# 查找USB设备并附加到目标进程device = frida.get_remote_device()#pid = device.spawn(["com.live.xctv"])#session = device.attach(pid)session =device.attach('cn.pojie52.cm01') #这里是要注入的apk包名# 在目标进程里创建脚本script = session.create_script(jscode)# 注册消息回调script.on('message', on_message)print(' Start attach')# 加载创建好的javascript脚本script.load()# 读取系统输入sys.stdin.read()
结果如下：
然后我们先进入sub_b90看一下，
sub_b90函数内容如下：
unsigned __int64 __fastcall sub_B90(_BYTE *a1, unsigned int a2, char *s){unsigned __int64 result; // x0unsigned __int64 v7; // x8signed int v8; // w9int v9; // w11int v10; // w9int v11; // w12int v12; // w9signed int v13; // w11__int64 v14; // x8int v15; // w12int v16; // w9int v17; // w13int v18; // w11int v19; // w14__int128 v20[2]; // [xsp+0h] [xbp-140h]__int128 v21[14]; // [xsp+20h] [xbp-120h] BYREF__int64 v22; // [xsp+108h] [xbp-38h]v22 = *(_ReadStatusReg(ARM64_SYSREG(3, 3, 13, 0, 2)) + 40);result = strlen(s);v20[0] = xmmword_11D0;v20[1] = xmmword_11E0;qmemcpy(v21, " !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmno", 80);v21[5] = xmmword_1240;v21[6] = xmmword_1250;v21[7] = xmmword_1260;v21[8] = xmmword_1270;v21[9] = xmmword_1280;v7 = 0LL;v8 = 0;v21[10] = xmmword_1290;v21[11] = xmmword_12A0;v21[12] = xmmword_12B0;v21[13] = xmmword_12C0;do{v9 = *(v20 + v7);v10 = v8 + v9 + s[v7 - v7 / result * result];v11 = v10 + 255;if ( v10 >= 0 )v11 = v10;v8 = v10 - (v11 & 0xFFFFFF00);*(v20 + v7++) = *(v20 + v8);*(v20 + v8) = v9;}while ( v7 != 256 );if ( a2 ){v12 = 0;v13 = 0;v14 = a2;do{v15 = v12 + 1;if ( v12 + 1 >= 0 )v16 = v12 + 1;elsev16 = v12 + 256;v12 = v15 - (v16 & 0xFFFFFF00);v17 = *(v20 + v12);v18 = v13 + v17;v19 = v18 + 255;if ( v18 >= 0 )v19 = v18;v13 = v18 - (v19 & 0xFFFFFF00);--v14;*(v20 + v12) = *(v20 + v13);*(v20 + v13) = v17;*a1++ ^= *(v20 + (*(v20 + v12) + v17));}while ( v14 );}return result;}
大概分析一下sub_b90，是根据传入的第三个参数s把v20进行了一个初始化，然后再把参数a1和v20进行了异或运算，主要看这个异或运算，先设想一下，如果是把a1进行了异或，那么得到的结果和a1之前的数据再异或就可以计算出异或的key，这里我们把它叫做xorkey，那么先看一下我们传入的参数，是30个1，也就是30个0x31 ，然后看结果，第一位是0xe0，0x31^0xe0 = 209，然后把参数改为30个2，即0x32，得出首位的结果是0xe3，0xe3^0x32结果也是209，证明我们的思路是正确的，然后依次求出所有的xorkey，
最后计算出的结果为：
xorkey = [209, 90, 6, 144, 68, 230, 199, 229, 222, 40, 247, 242, 102, 145, 200, 133, 66, 223, 249, 224, 130, 1, 43, 59, 56, 99, 55, 189, 46, 77]
接下来看sub_d90，咋一看返回值，全是字母，看起来有点像base64，于是用base64编码30个1进行测试，发现结果吻合，于是可以断定sub_d90是base64函数。
接下来，就可以写出通过v9求输入参数的函数：
import base64xorkey = [209, 90, 6, 144, 68, 230, 199, 229, 222, 40, 247, 242, 102, 145, 200, 133, 66, 223, 249, 224, 130, 1, 43, 59, 56, 99, 55, 189, 46, 77]def sub_B90(data,l):ret = []for i in range(l):ret.append(((data[i]))^xorkey[i])s=''for i in ret:s+=chr(i)print(s)return retdef resv(data):data =base64.b64decode(data)t = sub_B90(data,len(data))return(t)
调用resv即可计算出输入的参数。
这个时候我们发现，还有一个v19是我们不知道的，如果找到v19然后代入resv就能求出本题的结果！
根据ida的注释，我们知道v19是xsp+0h，而dest是xsp+38h，而dest又作为参数传入了sub_b90，这里我直接hooksub_b90，得到xsp，然后再在v19初始化结束之后输出xsp的值，即可得到v19，
这里的hook代码如下：
# -*- coding: UTF-8 -*-import frida, sysjscode = '''var destAddr = ''; //定位xsp地址function inline_hook() {var so_addr = Module.findBaseAddress("libnative-lib.so");if (so_addr) {console.log("so_addr:", so_addr);var addr_b90 = so_addr.add(0xB90);var sub_b90 = new NativeFunction(addr_b90 , 'int', ['pointer', 'int', 'pointer']);Interceptor.attach(sub_b90, {onEnter: function(args){destAddr = args[0];console.log('onEnter B90');},//在hook函数之后执行的语句onLeave:function(retval){console.log('onLeave B90');}});var addr_b2c = so_addr.add(0xb2c);console.log("The addr_b2c:", addr_b2c);Java.perform(function() {Interceptor.attach(addr_b2c, {onEnter: function(args) {console.log("addr_b2c OnEnter :", Memory.readByteArray(destAddr.sub(0x38),64) );}})})}}setImmediate(inline_hook)'''def on_message(message, data):if message['type'] == 'send':print(" {0}".format(message['payload']))else:print(message)pass#print(frida.enumerate_devices())# 查找USB设备并附加到目标进程device = frida.get_remote_device()#pid = device.spawn(["com.live.xctv"])#session = device.attach(pid)session =device.attach('cn.pojie52.cm01') #这里是要注入的apk包名# 在目标进程里创建脚本script = session.create_script(jscode)# 注册消息回调script.on('message', on_message)print(' Start attach')# 加载创建好的javascript脚本script.load()# 读取系统输入sys.stdin.read()
随便输入一个30位的注册码，得到的结果如下：
看来这个字符串就是我们要的了。把这个字符串代入函数resv，即可求出本题的flag：
输入52pojieHappyChineseNewYear2021到输入框，点击验证按钮，提示成功！本题分析结束。
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本文始发于微信公众号（吾爱破解论坛）：【2021春节】安卓中级题逆向总结
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