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Table of Contents

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ezAndroid(Rev)

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search patiently

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When I launched the APK file provided as the challenge binary in an emulator, it turned out to be an application that validates a username and password as shown below.

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I first unpacked the APK with apktool and checked the Manifest, which showed that debugging was enabled.

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<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"no\"?><manifest xmlns:android=\"http://schemas.android.com/apk/res/android\" android:compileSdkVersion=\"33\" android:compileSdkVersionCodename=\"13\" package=\"com.wmctf.ezandroid\" platformBuildVersionCode=\"33\" platformBuildVersionName=\"13\">\n    <permission android:name=\"com.wmctf.ezandroid.DYNAMIC_RECEIVER_NOT_EXPORTED_PERMISSION\" android:protectionLevel=\"signature\"/>\n    <uses-permission android:name=\"com.wmctf.ezandroid.DYNAMIC_RECEIVER_NOT_EXPORTED_PERMISSION\"/>\n    <application android:allowBackup=\"true\" android:appComponentFactory=\"androidx.core.app.CoreComponentFactory\" android:dataExtractionRules=\"@xml/data_extraction_rules\" android:debuggable=\"true\" android:fullBackupContent=\"@xml/backup_rules\" android:icon=\"@mipmap/ic_launcher\" android:label=\"@string/app_name\" android:supportsRtl=\"true\" android:theme=\"@style/Theme.EzAndroid\">\n        <activity android:exported=\"true\" android:name=\"com.wmctf.ezandroid.MainActivity\">\n            <intent-filter>\n                <action android:name=\"android.intent.action.MAIN\"/>\n                <category android:name=\"android.intent.category.LAUNCHER\"/>\n            </intent-filter>\n        </activity>\n        <provider android:authorities=\"com.wmctf.ezandroid.androidx-startup\" android:exported=\"false\" android:name=\"androidx.startup.InitializationProvider\">\n            <meta-data android:name=\"androidx.emoji2.text.EmojiCompatInitializer\" android:value=\"androidx.startup\"/>\n            <meta-data android:name=\"androidx.lifecycle.ProcessLifecycleInitializer\" android:value=\"androidx.startup\"/>\n        </provider>\n    </application>\n</manifest>
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Next, I decompiled the smali file for com.wmctf.ezandroid.MainActivity into Java source using smali2java.

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public class com.wmctf.ezandroid.MainActivity extends androidx.appcompat.app.AppCompatActivity {\n\t /* .source \"MainActivity.java\" */\n\t /* # static fields */\n\t public static java.lang.String x;\n\t /* # instance fields */\n\t android.widget.Button checkoutButton;\n\t android.widget.Button exitButton;\n\t android.widget.EditText passwordInput;\n\t android.widget.EditText usernameInput;\n\t /* # direct methods */\n\t static com.wmctf.ezandroid.MainActivity ( ) {\n\t\t /* .locals 1 */\n\t\t /* .line 22 */\n\t\t final String v0 = \"ezandroid\"; // const-string v0, \"ezandroid\"\n\t\t java.lang.System .loadLibrary ( v0 );\n\t\t /* .line 23 */\n\t\t return;\n\t } // .end method\n\t public com.wmctf.ezandroid.MainActivity ( ) {\n\t\t /* .locals 0 */\n\t\t /* .line 15 */\n\t\t /* invoke-direct {p0}, Landroidx/appcompat/app/AppCompatActivity;-><init>()V */\n\t\t return;\n\t } // .end method\n\t static void lambda$messageBox$0 ( android.content.DialogInterface p0, Integer p1 ) { //synthethic\n\t\t /* .locals 0 */\n\t\t /* .param p0, \"dialogInterface\" # Landroid/content/DialogInterface; */\n\t\t /* .param p1, \"i\" # I */\n\t\t /* .line 28 */\n\t\t return;\n\t } // .end method\n\t /* # virtual methods */\n\t public void CheckOutClick ( android.view.View p0 ) {\n\t\t /* .locals 6 */\n\t\t /* .param p1, \"view\" # Landroid/view/View; */\n\t\t /* .line 32 */\n\t\t v0 = this.usernameInput;\n\t\t (( android.widget.EditText ) v0 ).getText ( ); // invoke-virtual {v0}, Landroid/widget/EditText;->getText()Landroid/text/Editable;\n\t\t (( java.lang.Object ) v0 ).toString ( ); // invoke-virtual {v0}, Ljava/lang/Object;->toString()Ljava/lang/String;\n\t\t /* .line 33 */\n\t\t /* .local v0, \"username\":Ljava/lang/String; */\n\t\t v1 = this.passwordInput;\n\t\t (( android.widget.EditText ) v1 ).getText ( ); // invoke-virtual {v1}, Landroid/widget/EditText;->getText()Landroid/text/Editable;\n\t\t (( java.lang.Object ) v1 ).toString ( ); // invoke-virtual {v1}, Ljava/lang/Object;->toString()Ljava/lang/String;\n\t\t /* .line 34 */\n\t\t /* .local v1, \"password\":Ljava/lang/String; */\n\t\t final String v2 = \"\"; // const-string v2, \"\"\n\t\t v3 = \t\t (( java.lang.String ) v0 ).equals ( v2 ); // invoke-virtual {v0, v2}, Ljava/lang/String;->equals(Ljava/lang/Object;)Z\n\t\t /* if-nez v3, :cond_3 */\n\t\t v2 = \t\t (( java.lang.String ) v1 ).equals ( v2 ); // invoke-virtual {v1, v2}, Ljava/lang/String;->equals(Ljava/lang/Object;)Z\n\t\t if ( v2 != null) { // if-eqz v2, :cond_0\n\t\t\t /* .line 37 */\n\t\t } // :cond_0\n\t\t v2 = \t\t (( com.wmctf.ezandroid.MainActivity ) p0 ).CheckUsername ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->CheckUsername(Ljava/lang/String;)I\n\t\t final String v3 = \"failed login\"; // const-string v3, \"failed login\"\n\t\t int v4 = 1; // const/4 v4, 0x1\n\t\t /* if-eq v2, v4, :cond_1 */\n\t\t /* .line 38 */\n\t\t (( com.wmctf.ezandroid.MainActivity ) p0 ).messageBox ( v3 ); // invoke-virtual {p0, v3}, Lcom/wmctf/ezandroid/MainActivity;->messageBox(Ljava/lang/String;)V\n\t\t /* .line 40 */\n\t } // :cond_1\n\t /* new-instance v2, Ljava/lang/StringBuilder; */\n\t /* invoke-direct {v2}, Ljava/lang/StringBuilder;-><init>()V */\n\t (( java.lang.StringBuilder ) v2 ).append ( v0 ); // invoke-virtual {v2, v0}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n\t final String v5 = \"123456\"; // const-string v5, \"123456\"\n\t (( java.lang.StringBuilder ) v2 ).append ( v5 ); // invoke-virtual {v2, v5}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n\t (( java.lang.StringBuilder ) v2 ).toString ( ); // invoke-virtual {v2}, Ljava/lang/StringBuilder;->toString()Ljava/lang/String;\n\t /* .line 41 */\n\t v2 = \t (( com.wmctf.ezandroid.MainActivity ) p0 ).check2 ( v1 ); // invoke-virtual {p0, v1}, Lcom/wmctf/ezandroid/MainActivity;->check2(Ljava/lang/String;)I\n\t /* if-eq v2, v4, :cond_2 */\n\t /* .line 42 */\n\t (( com.wmctf.ezandroid.MainActivity ) p0 ).messageBox ( v3 ); // invoke-virtual {p0, v3}, Lcom/wmctf/ezandroid/MainActivity;->messageBox(Ljava/lang/String;)V\n\t /* .line 44 */\n} // :cond_2\n/* new-instance v2, Ljava/lang/StringBuilder; */\n/* invoke-direct {v2}, Ljava/lang/StringBuilder;-><init>()V */\nfinal String v3 = \"WMCTF{\"; // const-string v3, \"WMCTF{\"\n(( java.lang.StringBuilder ) v2 ).append ( v3 ); // invoke-virtual {v2, v3}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n(( java.lang.StringBuilder ) v2 ).append ( v0 ); // invoke-virtual {v2, v0}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n(( java.lang.StringBuilder ) v2 ).append ( v1 ); // invoke-virtual {v2, v1}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n/* const-string/jumbo v3, \"}\" */\n(( java.lang.StringBuilder ) v2 ).append ( v3 ); // invoke-virtual {v2, v3}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;\n(( java.lang.StringBuilder ) v2 ).toString ( ); // invoke-virtual {v2}, Ljava/lang/StringBuilder;->toString()Ljava/lang/String;\n/* .line 45 */\n/* .local v2, \"flag\":Ljava/lang/String; */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).messageBox ( v2 ); // invoke-virtual {p0, v2}, Lcom/wmctf/ezandroid/MainActivity;->messageBox(Ljava/lang/String;)V\n/* .line 35 */\n} // .end local v2 # \"flag\":Ljava/lang/String;\n} // :cond_3\n} // :goto_0\n/* const-string/jumbo v2, \"username or password is empty!\" */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).messageBox ( v2 ); // invoke-virtual {p0, v2}, Lcom/wmctf/ezandroid/MainActivity;->messageBox(Ljava/lang/String;)V\n/* .line 50 */\n} // :goto_1\nreturn;\n} // .end method\npublic native Integer CheckUsername ( java.lang.String p0 ) {\n} // .end method\npublic native Integer check2 ( java.lang.String p0 ) {\n} // .end method\npublic void messageBox ( java.lang.String p0 ) {\n/* .locals 3 */\n/* .param p1, \"title\" # Ljava/lang/String; */\n/* .line 26 */\n/* new-instance v0, Landroid/app/AlertDialog$Builder; */\n/* invoke-direct {v0, p0}, Landroid/app/AlertDialog$Builder;-><init>(Landroid/content/Context;)V */\n/* .line 27 */\n/* .local v0, \"dialog\":Landroid/app/AlertDialog$Builder; */\n(( android.app.AlertDialog$Builder ) v0 ).setMessage ( p1 ); // invoke-virtual {v0, p1}, Landroid/app/AlertDialog$Builder;->setMessage(Ljava/lang/CharSequence;)Landroid/app/AlertDialog$Builder;\n/* .line 28 */\n/* new-instance v1, Lcom/wmctf/ezandroid/MainActivity$$ExternalSyntheticLambda0; */\n/* invoke-direct {v1}, Lcom/wmctf/ezandroid/MainActivity$$ExternalSyntheticLambda0;-><init>()V */\nfinal String v2 = \"OK\"; // const-string v2, \"OK\"\n(( android.app.AlertDialog$Builder ) v0 ).setPositiveButton ( v2, v1 ); // invoke-virtual {v0, v2, v1}, Landroid/app/AlertDialog$Builder;->setPositiveButton(Ljava/lang/CharSequence;Landroid/content/DialogInterface$OnClickListener;)Landroid/app/AlertDialog$Builder;\n/* .line 29 */\n(( android.app.AlertDialog$Builder ) v0 ).show ( ); // invoke-virtual {v0}, Landroid/app/AlertDialog$Builder;->show()Landroid/app/AlertDialog;\n/* .line 30 */\nreturn;\n} // .end method\nprotected void onCreate ( android.os.Bundle p0 ) {\n/* .locals 2 */\n/* .param p1, \"savedInstanceState\" # Landroid/os/Bundle; */\n/* .line 54 */\n/* invoke-super {p0, p1}, Landroidx/appcompat/app/AppCompatActivity;->onCreate(Landroid/os/Bundle;)V */\n/* .line 55 */\n/* const v0, 0x7f0b001c */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).setContentView ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->setContentView(I)V\n/* .line 56 */\n/* const v0, 0x7f0801de */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).findViewById ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->findViewById(I)Landroid/view/View;\n/* check-cast v0, Landroid/widget/EditText; */\nthis.usernameInput = v0;\n/* .line 57 */\n/* const v0, 0x7f08014a */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).findViewById ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->findViewById(I)Landroid/view/View;\n/* check-cast v0, Landroid/widget/EditText; */\nthis.passwordInput = v0;\n/* .line 58 */\n/* const v0, 0x7f08006f */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).findViewById ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->findViewById(I)Landroid/view/View;\n/* check-cast v0, Landroid/widget/Button; */\nthis.checkoutButton = v0;\n/* .line 59 */\n/* const v0, 0x7f0800b3 */\n(( com.wmctf.ezandroid.MainActivity ) p0 ).findViewById ( v0 ); // invoke-virtual {p0, v0}, Lcom/wmctf/ezandroid/MainActivity;->findViewById(I)Landroid/view/View;\n/* check-cast v0, Landroid/widget/Button; */\nthis.exitButton = v0;\n/* .line 60 */\nv0 = this.checkoutButton;\n/* new-instance v1, Lcom/wmctf/ezandroid/MainActivity$1; */\n/* invoke-direct {v1, p0}, Lcom/wmctf/ezandroid/MainActivity$1;-><init>(Lcom/wmctf/ezandroid/MainActivity;)V */\n(( android.widget.Button ) v0 ).setOnClickListener ( v1 ); // invoke-virtual {v0, v1}, Landroid/widget/Button;->setOnClickListener(Landroid/view/View$OnClickListener;)V\n/* .line 66 */\nv0 = this.exitButton;\n/* new-instance v1, Lcom/wmctf/ezandroid/MainActivity$2; */\n/* invoke-direct {v1, p0}, Lcom/wmctf/ezandroid/MainActivity$2;-><init>(Lcom/wmctf/ezandroid/MainActivity;)V */\n(( android.widget.Button ) v0 ).setOnClickListener ( v1 ); // invoke-virtual {v0, v1}, Landroid/widget/Button;->setOnClickListener(Landroid/view/View$OnClickListener;)V\n/* .line 72 */\nreturn;\n} // .end method
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First, notice that the native library functions CheckUsername and check2 are declared.

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When you click the app’s check button, the entered username is passed to CheckUsername and the password to check2, so we can see that the validation happens there.

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Using Ghidra and Android Studio, I managed to identify where the checks happen, but unfortunately I could not fully trace the processing inside the native library functions, so I gave up at that point.

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From here on, I continue solving the challenge while referring to the official writeup.

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Reference: wm-team/WMCTF-2023

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Identifying the offsets of native library functions

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To analyze the binary, I first needed to identify the offsets of the CheckUsername and check2 functions inside the native library.

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In this binary, the functions were not registered in the export table of the native library.

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So when I investigated it on my own, I set breakpoints on several likely-looking functions I found in Ghidra and used the Android Studio debugger to identify the correct ones.

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According to the writeup, native libraries like this, whose functions are not registered in the export table, are implemented using a dynamic registration(?) mechanism via the JNI_onload function, and hooking the RegisterNatives method with Frida seems to be an effective way to identify function addresses cleanly.

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Reference: java - What does the registerNatives() method do? - Stack Overflow

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So, while recalling the content of Five techniques I learned so I won’t lose to Android app analysis challenges, I loaded the challenge APK into Android Studio, launched the emulator, and prepared a Frida hook.

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First, run the following commands in order to start the Frida server on the emulator.

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adb root\nadb.exe install ezAndroid.apk\nren frida-server-16.1.3-android-x86 frida-server\nadb push frida-server /data/local/tmp\nadb shell chmod +x /data/local/tmp/frida-server\nadb shell /data/local/tmp/frida-server
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Next, save the following JavaScript code as payload.js to hook RegisterNatives.

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function hook_RegisterNatives() {\n    var symbols = Process.getModuleByName('libart.so').enumerateSymbols();\n    var RegisterNatives_addr = null;\n    for (let i = 0; i < symbols.length; i++) {\n        var symbol = symbols[i];\n        if (symbol.name.indexOf(\"RegisterNatives\") != -1 && symbol.name.indexOf(\"CheckJNI\") == -1) {\n            RegisterNatives_addr = symbol.address;\n        }\n    }\n    console.log(\"RegisterNatives_addr: \", RegisterNatives_addr);\n    Interceptor.attach(RegisterNatives_addr, {\n        onEnter: function (args) {\n            var env = Java.vm.tryGetEnv();\n            var className = env.getClassName(args[1]);\n            var methodCount = args[3].toInt32();\n            for (let i = 0; i < methodCount; i++) {\n                var methodName = args[2].add(Process.pointerSize * 3 * i).add(Process.pointerSize * 0).readPointer().readCString();\n                var signature = args[2].add(Process.pointerSize * 3 * i).add(Process.pointerSize * 1).readPointer().readCString();\n                var fnPtr =\n                    args[2].add(Process.pointerSize * 3 * i).add(Process.pointerSize * 2).readPointer();\n                var module = Process.findModuleByAddress(fnPtr);\n                console.log(className, methodName, signature, fnPtr, module.name, fnPtr.sub(module.base));\n            }\n\n        }, onLeave: function (retval) {\n        }\n    })\n}\n\nhook_RegisterNatives();
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When I tried the Frida hook with the payload.js above, I found that the offset of CheckUsername was 0x28c0 and that of check2 was 0x2d20.

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frida.exe -U -l payload.js -f com.wmctf.ezandroid\n>\ncom.wmctf.ezandroid.MainActivity CheckUsername (Ljava/lang/String;)I 0xbc3ed8c0 libezandroid.so 0x28c0\ncom.wmctf.ezandroid.MainActivity check2 (Ljava/lang/String;)I 0xbc3edd20 libezandroid.so 0x2d20
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When I inspected the stack at the call site, I found that the third argument, v11, was the correct username with the string 123456 appended.

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Since the password string and its length are both involved, maybe it is doing some kind of XOR or similar processing.

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I let the sub_7B0 function execute once, and the value in the memory region where the password had been stored changed into what looked like an encrypted byte sequence.

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The decompiled result of sub_7B0 is shown below.

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Elf32_Dyn **__cdecl sub_7B0(int a1, int a2, int a3)\n{\n  int v3; // esi\n  int v4; // edx\n  Elf32_Dyn **result; // eax\n  int v6; // [esp+30h] [ebp-7Ch]\n  int v7; // [esp+34h] [ebp-78h]\n  int v8; // [esp+38h] [ebp-74h]\n  int v9; // [esp+50h] [ebp-5Ch] BYREF\n  int v10; // [esp+54h] [ebp-58h]\n  _DWORD v11[21]; // [esp+58h] [ebp-54h] BYREF\n\n  v10 = -970393338;\n  strlen(a3);\n  v9 = -1522528529;\n  sub_C40(&v9, a3);\n  v8 = 0;\n  v6 = -1028578145;\n  do\n  {\n    while ( 1 )\n    {\n      while ( 1 )\n      {\n        while ( v6 >= -304130802 )\n        {\n          if ( v6 < 703599849 )\n          {\n            if ( v6 < 90447858 )\n            {\n              if ( v6 == -304130802 )\n              {\n                v4 = v10 - 1292562961;\n                if ( v7 < 10 )\n                  v4 = v10 + 1060841196;\n                v6 = v4;\n              }\n            }\n            else if ( v6 == 90447858 )\n            {\n              v9 = v10 + 1871195796;\n              sub_15A0(&v9, v11);\n              v9 = v10 + 41419369;\n              sub_1840(&v9, v11);\n              v9 = v10 + 475928445;\n              sub_1B90(&v9, v11);\n              v9 = v10 - 878836549;\n              sub_12E0(&v9, v11, v7);\n              v6 = v10 + 154390384;\n            }\n          }\n          else if ( v6 < 2032010997 )\n          {\n            if ( v6 == 703599849 )\n            {\n              v8 += 16;\n              v6 = v10 - 58184807;\n            }\n          }\n          else if ( v6 == 2032010997 )\n          {\n            v9 = v10 + 1871195796;\n            sub_15A0(&v9, v11);\n            v9 = v10 + 41419369;\n            sub_1840(&v9, v11);\n            v9 = v10 - 878836549;\n            sub_12E0(&v9, v11, 10);\n            v9 = v10 + 1852563524;\n            sub_2140(&v9, v11, v8 + a1);\n            v6 = v10 + 1673993187;\n          }\n        }\n        if ( v6 >= -582053114 )\n          break;\n        if ( v6 < -816002954 )\n        {\n          if ( v6 == -1028578145 )\n          {\n            v3 = v10 + 388340224;\n            if ( v8 < a2 )\n              v3 = v10 + 643763145;\n            v6 = v3;\n          }\n        }\n        else if ( v6 == -816002954 )\n        {\n          ++v7;\n          v6 = v10 + 666262536;\n        }\n      }\n      if ( v6 < -326630193 )\n        break;\n      if ( v6 == -326630193 )\n      {\n        v9 = v10 + 478257682;\n        sub_1030(&v9, v8 + a1, v11);\n        v9 = v10 - 878836549;\n        sub_12E0(&v9, v11, 0);\n        v7 = 1;\n        v6 = v10 + 666262536;\n      }\n    }\n  }\n  while ( v6 != -582053114 );\n  result = &off_8FBC;\n  if ( _stack_chk_guard != v11[16] )\n    return (Elf32_Dyn **)sub_C40(a1, a2);\n  return result;\n}
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As usual, I still had no idea what was going on here at my current skill level.

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However, after using ghidra-findcrypt for labeling, I confirmed that data that appears to be an AES S-box is defined as shown below.

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\n \n )\n print(\"0x%02X\"%new_contrary_sbox[i],end=\",\")\n

To be honest, I still do not understand at all what the S2 array generated by the above process is used for, but I assume it is a standard AES decryption pattern, so I will study it properly another day.

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Using this S-box, the username + 123456 string passed as the key, and the hard-coded encrypted byte sequence, I ran the following solver.

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#include <stdio.h>\n#include <stdlib.h>\n#include <string.h>\n\n\nstatic const int S[16][16] = { \n0x29,0x40,0x57,0x6e,0x85,0x9c,0xb3,0xca,\n0xe1,0xf8,0x0f,0x26,0x3d,0x54,0x6b,0x82,\n0x99,0xb0,0xc7,0xde,0xf5,0x0c,0x23,0x3a,\n0x51,0x68,0x7f,0x96,0xad,0xc4,0xdb,0xf2,\n0x09,0x20,0x37,0x4e,0x65,0x7c,0x93,0xaa,\n0xc1,0xd8,0xef,0x06,0x1d,0x34,0x4b,0x62,\n0x79,0x90,0xa7,0xbe,0xd5,0xec,0x03,0x1a,\n0x31,0x48,0x5f,0x76,0x8d,0xa4,0xbb,0xd2,\n0xe9,0x00,0x17,0x2e,0x45,0x5c,0x73,0x8a,\n0xa1,0xb8,0xcf,0xe6,0xfd,0x14,0x2b,0x42,\n0x59,0x70,0x87,0x9e,0xb5,0xcc,0xe3,0xfa,\n0x11,0x28,0x3f,0x56,0x6d,0x84,0x9b,0xb2,\n0xc9,0xe0,0xf7,0x0e,0x25,0x3c,0x53,0x6a,\n0x81,0x98,0xaf,0xc6,0xdd,0xf4,0x0b,0x22,\n0x39,0x50,0x67,0x7e,0x95,0xac,0xc3,0xda,\n0xf1,0x08,0x1f,0x36,0x4d,0x64,0x7b,0x92,\n0xa9,0xc0,0xd7,0xee,0x05,0x1c,0x33,0x4a,\n0x61,0x78,0x8f,0xa6,0xbd,0xd4,0xeb,0x02,\n0x19,0x30,0x47,0x5e,0x75,0x8c,0xa3,0xba,\n0xd1,0xe8,0xff,0x16,0x2d,0x44,0x5b,0x72,\n0x89,0xa0,0xb7,0xce,0xe5,0xfc,0x13,0x2a,\n0x41,0x58,0x6f,0x86,0x9d,0xb4,0xcb,0xe2,\n0xf9,0x10,0x27,0x3e,0x55,0x6c,0x83,0x9a,\n0xb1,0xc8,0xdf,0xf6,0x0d,0x24,0x3b,0x52,\n0x69,0x80,0x97,0xae,0xc5,0xdc,0xf3,0x0a,\n0x21,0x38,0x4f,0x66,0x7d,0x94,0xab,0xc2,\n0xd9,0xf0,0x07,0x1e,0x35,0x4c,0x63,0x7a,\n0x91,0xa8,0xbf,0xd6,0xed,0x04,0x1b,0x32,\n0x49,0x60,0x77,0x8e,0xa5,0xbc,0xd3,0xea,\n0x01,0x18,0x2f,0x46,0x5d,0x74,0x8b,0xa2,\n0xb9,0xd0,0xe7,0xfe,0x15,0x2c,0x43,0x5a,\n0x71,0x88,0x9f,0xb6,0xcd,0xe4,0xfb,0x12\n};\n\n\nstatic const int S2[16][16] = {\n0x41,0xE8,0x8F,0x36,0xDD,0x84,0x2B,0xD2,\n0x79,0x20,0xC7,0x6E,0x15,0xBC,0x63,0x0A,\n0xB1,0x58,0xFF,0xA6,0x4D,0xF4,0x9B,0x42,\n0xE9,0x90,0x37,0xDE,0x85,0x2C,0xD3,0x7A,\n0x21,0xC8,0x6F,0x16,0xBD,0x64,0x0B,0xB2,\n0x59,0x00,0xA7,0x4E,0xF5,0x9C,0x43,0xEA,\n0x91,0x38,0xDF,0x86,0x2D,0xD4,0x7B,0x22,\n0xC9,0x70,0x17,0xBE,0x65,0x0C,0xB3,0x5A,\n0x01,0xA8,0x4F,0xF6,0x9D,0x44,0xEB,0x92,\n0x39,0xE0,0x87,0x2E,0xD5,0x7C,0x23,0xCA,\n0x71,0x18,0xBF,0x66,0x0D,0xB4,0x5B,0x02,\n0xA9,0x50,0xF7,0x9E,0x45,0xEC,0x93,0x3A,\n0xE1,0x88,0x2F,0xD6,0x7D,0x24,0xCB,0x72,\n0x19,0xC0,0x67,0x0E,0xB5,0x5C,0x03,0xAA,\n0x51,0xF8,0x9F,0x46,0xED,0x94,0x3B,0xE2,\n0x89,0x30,0xD7,0x7E,0x25,0xCC,0x73,0x1A,\n0xC1,0x68,0x0F,0xB6,0x5D,0x04,0xAB,0x52,\n0xF9,0xA0,0x47,0xEE,0x95,0x3C,0xE3,0x8A,\n0x31,0xD8,0x7F,0x26,0xCD,0x74,0x1B,0xC2,\n0x69,0x10,0xB7,0x5E,0x05,0xAC,0x53,0xFA,\n0xA1,0x48,0xEF,0x96,0x3D,0xE4,0x8B,0x32,\n0xD9,0x80,0x27,0xCE,0x75,0x1C,0xC3,0x6A,\n0x11,0xB8,0x5F,0x06,0xAD,0x54,0xFB,0xA2,\n0x49,0xF0,0x97,0x3E,0xE5,0x8C,0x33,0xDA,\n0x81,0x28,0xCF,0x76,0x1D,0xC4,0x6B,0x12,\n0xB9,0x60,0x07,0xAE,0x55,0xFC,0xA3,0x4A,\n0xF1,0x98,0x3F,0xE6,0x8D,0x34,0xDB,0x82,\n0x29,0xD0,0x77,0x1E,0xC5,0x6C,0x13,0xBA,\n0x61,0x08,0xAF,0x56,0xFD,0xA4,0x4B,0xF2,\n0x99,0x40,0xE7,0x8E,0x35,0xDC,0x83,0x2A,\n0xD1,0x78,0x1F,0xC6,0x6D,0x14,0xBB,0x62,\n0x09,0xB0,0x57,0xFE,0xA5,0x4C,0xF3,0x9A\n};\n\n\nstatic int getLeft4Bit(int num) {\n    int left = num & 0x000000f0;\n    return left >> 4;\n}\n\n\nstatic int getRight4Bit(int num) {\n    return num & 0x0000000f;\n}\n\n\nstatic int getNumFromSBox(int index) {\n    int row = getLeft4Bit(index);\n    int col = getRight4Bit(index);\n    return S[row][col];\n}\n\n\nstatic int getIntFromChar(char c) {\n    int result = (int)c;\n    return result & 0x000000ff;\n}\n\n\nstatic void convertToIntArray(char* str, int pa[4][4]) {\n    int k = 0;\n    int i, j;\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++) {\n            pa[j][i] = getIntFromChar(str[k]);\n            k++;\n        }\n}\n\n\nstatic int getWordFromStr(char* str) {\n    int one, two, three, four;\n    one = getIntFromChar(str[0]);\n    one = one << 24;\n    two = getIntFromChar(str[1]);\n    two = two << 16;\n    three = getIntFromChar(str[2]);\n    three = three << 8;\n    four = getIntFromChar(str[3]);\n    return one | two | three | four;\n}\n\n\nstatic void splitIntToArray(int num, int array[4]) {\n    int one, two, three;\n    one = num >> 24;\n    array[0] = one & 0x000000ff;\n    two = num >> 16;\n    array[1] = two & 0x000000ff;\n    three = num >> 8;\n    array[2] = three & 0x000000ff;\n    array[3] = num & 0x000000ff;\n}\n\n\nstatic void leftLoop4int(int array[4], int step) {\n    int temp[4];\n    int i;\n    int index;\n    for (i = 0; i < 4; i++)\n        temp[i] = array[i];\n\n    index = step % 4 == 0 ? 0 : step % 4;\n    for (i = 0; i < 4; i++) {\n        array[i] = temp[index];\n        index++;\n        index = index % 4;\n    }\n}\n\n\nstatic int mergeArrayToInt(int array[4]) {\n    int one = array[0] << 24;\n    int two = array[1] << 16;\n    int three = array[2] << 8;\n    int four = array[3];\n    return one | two | three | four;\n}\n\n\nstatic const int Rcon[10] = { 0x01000000, 0x02000000,\n    0x04000000, 0x08000000,\n    0x10000000, 0x20000000,\n    0x40000000, 0x80000000,\n    0x1b000000, 0x36000000 };\n\n\nstatic int T(int num, int round) {\n    int numArray[4];\n    int i;\n    int result;\n    splitIntToArray(num, numArray);\n    leftLoop4int(numArray, 1);\n\n    for (i = 0; i < 4; i++)\n        numArray[i] = getNumFromSBox(numArray[i]);\n\n    result = mergeArrayToInt(numArray);\n    return result ^ Rcon[round];\n}\n\n\nstatic int w[44];\n\n\nstatic void extendKey(char* key) {\n    int i, j;\n    for (i = 0; i < 4; i++)\n        w[i] = getWordFromStr(key + i * 4);\n\n    for (i = 4, j = 0; i < 44; i++) {\n        if (i % 4 == 0) {\n            w[i] = w[i - 4] ^ T(w[i - 1], j);\n            j++;\n        }\n        else {\n            w[i] = w[i - 4] ^ w[i - 1];\n        }\n    }\n\n}\n\n\nstatic void addRoundKey(int array[4][4], int round) {\n    int warray[4];\n    int i, j;\n    for (i = 0; i < 4; i++) {\n\n        splitIntToArray(w[round * 4 + i], warray);\n\n        for (j = 0; j < 4; j++) {\n            array[j][i] = array[j][i] ^ warray[j];\n        }\n    }\n}\n\n\nstatic int GFMul2(int s) {\n    int result = s << 1;\n    int a7 = result & 0x00000100;\n\n    if (a7 != 0) {\n        result = result & 0x000000ff;\n        result = result ^ 0x1b;\n    }\n\n    return result;\n}\n\nstatic int GFMul3(int s) {\n    return GFMul2(s) ^ s;\n}\n\nstatic int GFMul4(int s) {\n    return GFMul2(GFMul2(s));\n}\n\nstatic int GFMul8(int s) {\n    return GFMul2(GFMul4(s));\n}\n\nstatic int GFMul9(int s) {\n    return GFMul8(s) ^ s;\n}\n\nstatic int GFMul11(int s) {\n    return GFMul9(s) ^ GFMul2(s);\n}\n\nstatic int GFMul12(int s) {\n    return GFMul8(s) ^ GFMul4(s);\n}\n\nstatic int GFMul13(int s) {\n    return GFMul12(s) ^ s;\n}\n\nstatic int GFMul14(int s) {\n    return GFMul12(s) ^ GFMul2(s);\n}\n\n\nstatic int GFMul(int n, int s) {\n    int result;\n\n    if (n == 1)\n        result = s;\n    else if (n == 2)\n        result = GFMul2(s);\n    else if (n == 3)\n        result = GFMul3(s);\n    else if (n == 0x9)\n        result = GFMul9(s);\n    else if (n == 0xb)//11\n        result = GFMul11(s);\n    else if (n == 0xd)//13\n        result = GFMul13(s);\n    else if (n == 0xe)//14\n        result = GFMul14(s);\n\n    return result;\n}\n\n\nstatic void convertArrayToStr(int array[4][4], char* str) {\n    int i, j;\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++)\n            *str++ = (char)array[j][i];\n}\n\n\nstatic int getNumFromS1Box(int index) {\n    int row = getLeft4Bit(index);\n    int col = getRight4Bit(index);\n    return S2[row][col];\n}\n\n\nstatic void deSubBytes(int array[4][4]) {\n    int i, j;\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++)\n            array[i][j] = getNumFromS1Box(array[i][j]);\n}\n\n\nstatic void rightLoop4int(int array[4], int step) {\n    int temp[4];\n    int i;\n    int index;\n    for (i = 0; i < 4; i++)\n        temp[i] = array[i];\n\n    index = step % 4 == 0 ? 0 : step % 4;\n    index = 3 - index;\n    for (i = 3; i >= 0; i--) {\n        array[i] = temp[index];\n        index--;\n        index = index == -1 ? 3 : index;\n    }\n}\n\n\nstatic void deShiftRows(int array[4][4]) {\n    int rowTwo[4], rowThree[4], rowFour[4];\n    int i;\n    for (i = 0; i < 4; i++) {\n        rowTwo[i] = array[1][i];\n        rowThree[i] = array[2][i];\n        rowFour[i] = array[3][i];\n    }\n\n    rightLoop4int(rowTwo, 1);\n    rightLoop4int(rowThree, 2);\n    rightLoop4int(rowFour, 3);\n\n    for (i = 0; i < 4; i++) {\n        array[1][i] = rowTwo[i];\n        array[2][i] = rowThree[i];\n        array[3][i] = rowFour[i];\n    }\n}\n\n\nstatic const int deColM[4][4] = { 0xe, 0xb, 0xd, 0x9,\n    0x9, 0xe, 0xb, 0xd,\n    0xd, 0x9, 0xe, 0xb,\n    0xb, 0xd, 0x9, 0xe };\n\n\nstatic void deMixColumns(int array[4][4]) {\n    int tempArray[4][4];\n    int i, j;\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++)\n            tempArray[i][j] = array[i][j];\n\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++) {\n            array[i][j] = GFMul(deColM[i][0], tempArray[0][j]) ^ GFMul(deColM[i][1], tempArray[1][j])\n                ^ GFMul(deColM[i][2], tempArray[2][j]) ^ GFMul(deColM[i][3], tempArray[3][j]);\n        }\n}\n\n\nstatic void addRoundTowArray(int aArray[4][4], int bArray[4][4]) {\n    int i, j;\n    for (i = 0; i < 4; i++)\n        for (j = 0; j < 4; j++)\n            aArray[i][j] = aArray[i][j] ^ bArray[i][j];\n}\n\n\nstatic void getArrayFrom4W(int i, int array[4][4]) {\n    int index, j;\n    int colOne[4], colTwo[4], colThree[4], colFour[4];\n    index = i * 4;\n    splitIntToArray(w[index], colOne);\n    splitIntToArray(w[index + 1], colTwo);\n    splitIntToArray(w[index + 2], colThree);\n    splitIntToArray(w[index + 3], colFour);\n\n    for (j = 0; j < 4; j++) {\n        array[j][0] = colOne[j];\n        array[j][1] = colTwo[j];\n        array[j][2] = colThree[j];\n        array[j][3] = colFour[j];\n    }\n\n}\n\n\nvoid deAes(char* c, int clen, char* key) {\n\n    int cArray[4][4];\n    int keylen, k;\n    keylen = strlen(key);\n    if (clen == 0 || clen % 16 != 0) {\n        printf(\"Clen: %d\\n\", clen);\n        exit(0);\n    }\n\n    extendKey(key);\n\n    for (k = 0; k < clen; k += 16) {\n        int i;\n        int wArray[4][4];\n\n        convertToIntArray(c + k, cArray);\n        addRoundKey(cArray, 10);\n\n        for (i = 9; i >= 1; i--) {\n            deSubBytes(cArray);\n            deShiftRows(cArray);\n            deMixColumns(cArray);\n            getArrayFrom4W(i, wArray);\n            deMixColumns(wArray);\n\n            addRoundTowArray(cArray, wArray);\n        }\n        deSubBytes(cArray);\n        deShiftRows(cArray);\n        addRoundKey(cArray, 0);\n        convertArrayToStr(cArray, c + k);\n\n    }\n}\n\n\nint main() {\n    // 暗号化後のバイト列\n    char encodebuffer[] = { 0x2B, 0xC8, 0x20, 0x8B, 0x5C, 0xD, 0xA7, 0x9B, 0x2A, 0x51, 0x3A, 0xD2, 0x71, 0x71, 0xCA, 0x50 };\n    \n    char* key = (char*)\"Re_1s_eaSy123456\";\n\n    deAes(encodebuffer, 16, key);\n    printf(\"%s\", encodebuffer);\n}
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By running the above and obtaining the correct password, I was able to recover the correct flag as shown below.

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