Decrypticon: A Generic Android Simplifier

• Dependencies
• Usage
  • Offline Mode
• Tests
• Real-life Use Case
• Special Thanks

Decrypticon monitors an Android app's execution and then annotates the disassembled codebase with the results of the marked functions' execution. This allows the analyst to go through the annotated codebase and understand:

    * Input:
        * Android APK
        * A bunch of functions to mark
    * Processing:
        * Run the app and observe the marked functions
    * Output:
        * A disassembled codebase that annotates the arguments and returns values of each marked function

Dependencies

• ANDROID_HOME must be in PATH, along with any verison of build_tools. Something like this should exist in your .profile:

  export ANDROID_HOME=$HOME/Library/Android/sdk
  export PATH="$ANDROID_HOME/platform-tools:$PATH"
  export PATH="$ANDROID_HOME/build-tools/27.0.3:$PATH"
  

• jarsigner

• apktool 2.4.+ (if you're running OSX: brew install apktool is good enough)

• Python3 3.7.+

• virtualenv 20.+

Usage

• Make a hooks file that consists of the functions you want to monitor. An example exists here. You can find signture using either apktool or radare2:

  Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;
  Lcom/afjoseph/test/Cryptor;->second(III)Ljava/lang/String;
  Lcom/afjoseph/test/Cryptor;->third(III)Ljava/lang/String;
  

• Run an emulator. I recommend using my own scripts/run_avd.sh

• Run and install frida-server on the test device. I recommend using my own scripts/install_frida_server.rb

• A sample workflow should look like this

  $ cat hooks
  Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;
  Lcom/afjoseph/test/Cryptor;->second(III)Ljava/lang/String;
  Lcom/afjoseph/test/Cryptor;->third(III)Ljava/lang/String;
  
  # Run an emulator
  $ ./scripts/run_avd.sh --android_api_level=28
  
  # Install Frida server on it
  $ ./scripts/install_frida_server
  
  # Initialize virtualenv
  $ virtualenv -p python3 venv
  $ venv/bin/pip3 install -r requirements.txt
  
  # Run Decrypticon
  venv/bin/python3 decrypticon.py \
    --mode online \
    --apk example/test_project/love.apk \
    --hooks hooks \
    --out example/test_project/annotated \
    --focus_pkg com/afjoseph/test
  

The above flow is exactly how the test script looks like.

Offline Mode

Sometimes, you'd want to save the results of the marked functions (which the project identifies as recorded invocations). The --pickle_to flag can pickle (Python term for "serialize") all recorded invocations in a file, which you can replay at any time later.

Let's assume you ran Decrypticon using --mode=online before and used --pickle_to flag to save all recorded invocations in my_tender_pickles file. You can reply those invocations to annotate the codebase again using the following:

venv/bin/python3 decrypticon.py \
  --mode offline \
  --apk example/test_project/love.apk \
  --out example/test_project/annotated \
  --focus_pkg com/afjoseph/test --pickle_from my_tender_pickles

This would take love.apk, annotate it using the recorded invocations in my_tender_pickles, and then write the annotated codebase to example/test_project/annotated.

Tests

Run ./scripts/run_test_suite.rb. This is also a good location to see how the project is supposed to run.

Real-life Use Case

Take the following Java code:

Map<String, String> params = new HashMap<>();
String address_1 = Cryptor.get(30, 20, 100);
String enc_address_1 = Encryptor.Encrypt(address_1);
params.put("address_1", enc_address_1);

String country_1 = Cryptor.get(100, 200, 300);
String enc_country_1 = Encryptor.Encrypt(address_1);
params.put("country_1", enc_country_1);

String token_1 = Cryptor.get(99, 66, 99);
String enc_token_1 = Encryptor.Encrypt(token_1);
params.put("token_1", enc_token_1);

String address_2 = Cryptor.get(55, 22, 32);
String enc_address_2 = Encryptor.Encrypt(address_2);
params.put("address_2", enc_address_2);

String country_2 = Cryptor.get(92, 22, 55);
String enc_country_2 = Encryptor.Encrypt(address_2);
params.put("country_2", enc_country_2);

String token_2 = Cryptor.get(88, 72, 86);
String enc_token_2 = Encryptor.Encrypt(token_2);
params.put("token_2", enc_token_2);

An easier way to write this would be:

params.put("address_1", "neverwhere");

Where neverwhere would be the value of the address, but this makes analysis pretty easy since neverwhere exists verbatim in the source code.

A common obfuscation scheme is to rely on layers of abstraction to "hide" the value of neverwhere. Cryptor.get() could look like this (this is a hypothetical function. The code is not compilable):

public final class Cryptor {
  private static char[] arr = new char[]{'\ucad9', '\ue9a1', '\u1a1c', '\u00a9', '\u591c', '\u9e7e', '\u751c', '\u9cc9', '\u1191', '\ua7e5', '\ucd9e', '\ueca5', '\u1119', '\ucae5', '\u591e', '\u9a5c', '\u5cc0', '\u791a', '\u1ea1', '\u55d5', '\uccca' '\u70d1', '\u9ec1', '\ucc97', '\ua5ac', '\uc1ae', '\ue191', '\u177a', '\ucd1c', '\u5c51', '\u99ce', '\ueea9', '\u95d1', '\ucca9', '\u5199', '\uc711', '\u9daa', '\uac9e', '\uc9c7', '\u5e50', '\uc571', 'e', '\ue915', '\u51c1', '\uc7e5', '&', '\uaeee', '\uc0e0', '\u5e59', '\u7c99', '\u05ec', '\u510c', '\ucaac', '\ud9cc', '\ueaaa', '\u101a', '\ua75c', '\u9d05'};
  privage static int field_99 = 0;
  privage static int field_91 = 2;
  privage static int field_92 = 4;

  private static String get(int var0, int var1, int var2) {
    while(var5 < var8) {
      var10000 = field_91 + 1;
      field_92 = var10000 % 128;
      if (var10000 % 2 == 0) {
      }

      var4[var5] = (char)((int)((long)arr[var9 + var5] ^ (long)var5 * field_90 ^ (long)var7));
      ++var5;
    }

    int var10000 = 2 % 2;
    char var7 = var0;
    int var8 = var1;
    int var9 = var2;
    char[] var4 = new char[var1];
    int var5 = 0;
    var10000 = field_92 + 99;
    field_91 = var10000 % 128;
    switch(var10000 % 2 != 0 ? 66 : 35) {
      case 35:
      default:
        var10000 = 2 % 2;
        break;
      case 66:
        var10000 = 5 * 3;
    }

    String var12 = new String(var4);
    int var10001 = field_91 + 49;
    field_92 = var10001 % 128;
    switch(var10001 % 2 == 0 ? 28 : 47) {
      case 28:
      default:
        try {
          var10001 = ((Object[])null).length;
          return var12;
        } catch (Throwable var11) {
          throw var11;
        }
      case 47:
        return var12;
    }
  }
}

In that case, there is no easy way of understanding what is the output of Cryptor.get(). An easy way of handling this would be to execute Cryptor.get() and monitor its value. That is what Decrypticon does, plus annotate the disassembled codebase with the args and return value. The annotated smali code will have a bunch of >>> DECRYPTICON directives on the marked functions that reveal the execution flow:

After executing Decrypticon

>>> DECRYPTICON:: func(30, 20, 100) = neverwhere
    invoke-static {v1, v2, v3}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

...

>>> DECRYPTICON:: func(100, 200, 300) = usa
    invoke-static {v3, v4, v5}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

...

>>> DECRYPTICON:: func(99, 66, 99) = 12341234
    invoke-static {v6, v5, v6}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

...

>>> DECRYPTICON:: func(55, 22, 32) = baldurs_gate
    invoke-static {v9, v8, v7}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

...

>>> DECRYPTICON:: func(92, 22, 55) = temeria
    invoke-static {v11, v8, v9}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

...

>>> DECRYPTICON:: func(88, 72, 86) = abcdabcd
    invoke-static {v11, v12, v13}, Lcom/afjoseph/test/Cryptor;->get(III)Ljava/lang/String;

Special Thanks

I'd like to thank the following projects and their contributors. They were a major part of this project:

• Frida: Decrypticon relies on it.
• Radare2: 99% of the analysis done here was made using r2
• dex-oracle and Simplify: Mr. Fenton's work was the reason I made this project: I encountered some obfuscated malware and ran it through both with good results. There was still quite a bit of manual work I needed to do, though. After checking both codebases, the problem I found was that dex-oracle was too simplistic since it relied on heuristics and specific argument positioning, which the malware didn't follow. simplify went into an endless loop trying to execute the marked instructions. After checking the GH issues on both, I realized that they were made with different ideas in mind. I thought that a solution in the middle must exist, to which decrypticon was my humble answer.
• My company, Adjust, for giving me the time and space to work on such a project.
• To all the future contributors who will help me make decrypticon better. Please reach out to me through Twitter (@MalwareCheese) or in the Issues section in this repo.