ClaudiuGeorgiu/Obfuscapk

A black-box obfuscation tool for Android apps.

Obfuscapk is a modular Python tool for obfuscating Android apps without needing their source code, since apktool is used to decompile the original apk file and to build a new application, after applying some obfuscation techniques on the decompiled smali code, resources and manifest. The obfuscated app retains the same functionality as the original one, but the differences under the hood sometimes make the new application very different from the original (e.g., to signature-based antivirus software).

🆕 Android App Bundle support 🆕

Obfuscapk is adding support for Android App Bundles (aab files) by using BundleDecompiler (see #121). In order to use this new feature, download the latest version of BundleDecompiler available from here, save it as BundleDecompiler.jar in a directory included in PATH (e.g., in Ubuntu, /usr/local/bin or /usr/bin) and make sure it has the executable flag set.

NOTE: BundleDecompiler doesn't work on Windows yet, so app bundle obfuscation is not supported by Obfuscapk on Windows platform. Also, app bundle support is still in early development, so if you faced any problems or if you want to help us improve, please see contributing.

❱ Publication

More details about Obfuscapk can be found in the paper "Obfuscapk: An open-source black-box obfuscation tool for Android apps". You can cite the paper as follows:

@article{aonzo2020obfuscapk,
    title = "Obfuscapk: An open-source black-box obfuscation tool for Android apps",
    journal = "SoftwareX",
    volume = "11",
    pages = "100403",
    year = "2020",
    issn = "2352-7110",
    doi = "https://doi.org/10.1016/j.softx.2020.100403",
    url = "https://www.sciencedirect.com/science/article/pii/S2352711019302791",
    author = "Simone Aonzo and Gabriel Claudiu Georgiu and Luca Verderame and Alessio Merlo",
    keywords = "Android, Obfuscation, Program analysis"
}

❱ Demo

❱ Architecture

Obfuscapk is designed to be modular and easy to extend, so it's built using a plugin system. Consequently, every obfuscator is a plugin that inherits from an abstract base class and needs to implement the method obfuscate. When the tool starts processing a new Android application file, it creates an obfuscation object to store all the needed information (e.g., the location of the decompiled smali code) and the internal state of the operations (e.g., the list of already used obfuscators). Then the obfuscation object is passed, as a parameter to the obfuscate method, to all the active plugins/obfuscators (in sequence) to be processed and modified. The list and the order of the active plugins is specified through command line options.

The tool is easily extensible with new obfuscators: it's enough to add the source code implementing the obfuscation technique and the plugin metadata (a <obfuscator-name>.obfuscator file) in the src/obfuscapk/obfuscators directory (take a simple existing obfuscator like Nop as a starting example). The tool will detect automatically the new plugin, so no further configuration is needed (the new plugin will be treated like all the other plugins bundled with the tool).

❱ Installation

There are two ways of getting a working copy of Obfuscapk on your own computer: either by using Docker or by using directly the source code in a Python 3 environment. In both cases, the first thing to do is to get a local copy of this repository, so open up a terminal in the directory where you want to save the project and clone the repository:

$ git clone https://github.com/ClaudiuGeorgiu/Obfuscapk.git

Docker image

Prerequisites

This is the suggested way of installing Obfuscapk, since the only requirement is to have a recent version of Docker installed:

$ docker --version
Docker version 20.10.21, build baeda1f

Official Docker Hub image

The official Obfuscapk Docker image is available on Docker Hub (automatically built from this repository):

$ # Download the Docker image.
$ docker pull claudiugeorgiu/obfuscapk
$ # Give it a shorter name.
$ docker tag claudiugeorgiu/obfuscapk obfuscapk

Install

If you downloaded the official image from Docker Hub, you are ready to use the tool so go ahead and check the usage instructions, otherwise execute the following command in the previously created Obfuscapk/src/ directory (the folder containing the Dockerfile) to build the Docker image:

$ # Make sure to run the command in Obfuscapk/src/ directory.
$ # It will take some time to download and install all the dependencies.
$ docker build -t obfuscapk .

When the Docker image is ready, make a quick test to check that everything was installed correctly:

$ docker run --rm -it obfuscapk --help
usage: python3 -m obfuscapk.cli [-h] -o OBFUSCATOR [-w DIR] [-d OUT_APK_OR_AAB]
...

Obfuscapk is now ready to be used, see the usage instructions for more information.

From source

Prerequisites

Make sure to have a recent version of apktool, apksigner and zipalign installed and available from the command line:

$ apktool
Apktool v2.7.0 - a tool for reengineering Android apk files
...

$ apksigner
Usage:  apksigner <command> [options]
        apksigner --version
        apksigner --help
...

$ zipalign
Zip alignment utility
Copyright (C) 2009 The Android Open Source Project
...

To support app bundles obfuscation you also need BundleDecompiler, so download the latest available version from here, save it as BundleDecompiler.jar in a directory included in PATH (e.g., in Ubuntu, /usr/local/bin or /usr/bin) and make sure it has the executable flag set.

To use BundleDecompiler and apktool you also need a recent version of Java. zipalign and apksigner are included in the Android SDK. The location of the executables can also be specified through the following environment variables: APKTOOL_PATH, BUNDLE_DECOMPILER_PATH, APKSIGNER_PATH and ZIPALIGN_PATH (e.g., in Ubuntu, run export APKTOOL_PATH=/custom/location/apktool before running Obfuscapk in the same terminal).

Apart from the above tools, the only requirement of this project is a working Python 3 (at least 3.7) installation (along with its package manager pip).

Install

Run the following commands in the main directory of the project (Obfuscapk/) to install the needed dependencies:

$ # Make sure to run the commands in Obfuscapk/ directory.

$ # The usage of a virtual environment is highly recommended.
$ python3 -m venv venv
$ source venv/bin/activate

$ # Install Obfuscapk's requirements.
$ python3 -m pip install -r src/requirements.txt

After the requirements are installed, make a quick test to check that everything works correctly:

$ cd src/
$ # The following command has to be executed always from Obfuscapk/src/ directory
$ # or by adding Obfuscapk/src/ directory to PYTHONPATH environment variable.
$ python3 -m obfuscapk.cli --help
usage: python3 -m obfuscapk.cli [-h] -o OBFUSCATOR [-w DIR] [-d OUT_APK_OR_AAB]
...

Obfuscapk is now ready to be used, see the usage instructions for more information.

❱ Usage

From now on, Obfuscapk will be considered as an executable available as obfuscapk, so you need to adapt the commands according to how you installed the tool:

• Docker image: a local directory containing the application to obfuscate has to be mounted to /workdir in the container (e.g., the current directory "${PWD}"), so the command:

  $ obfuscapk [params...]

  becomes:

  $ docker run --rm -it -u $(id -u):$(id -g) -v "${PWD}":"/workdir" obfuscapk [params...]

• From source: every instruction has to be executed from the Obfuscapk/src/ directory (or by adding Obfuscapk/src/ directory to PYTHONPATH environment variable) and the command:

  $ obfuscapk [params...]

  becomes:

  $ python3 -m obfuscapk.cli [params...]

Let's start by looking at the help message:

$ obfuscapk --help
obfuscapk [-h] -o OBFUSCATOR [-w DIR] [-d OUT_APK_OR_AAB] [-i] [-p] [-k VT_API_KEY]
          [--keystore-file KEYSTORE_FILE] [--keystore-password KEYSTORE_PASSWORD]
          [--key-alias KEY_ALIAS] [--key-password KEY_PASSWORD] [--use-aapt2]
          <APK_OR_BUNDLE_FILE>

There are two mandatory parameters: <APK_OR_BUNDLE_FILE>, the path (relative or absolute) to the apk or app bundle file to obfuscate and the list with the names of the obfuscation techniques to apply (specified with a -o option that can be used multiple times, e.g., -o Rebuild -o NewAlignment -o NewSignature). The other optional arguments are as follows:

• -w DIR is used to set the working directory where to save the intermediate files (generated by apktool). If not specified, a directory named obfuscation_working_dir is created in the same directory as the input application. This can be useful for debugging purposes, but if it's not needed it can be set to a temporary directory (e.g., -w /tmp/).

• -d OUT_APK_OR_AAB is used to set the path of the destination file: the apk file generated by the obfuscation process (e.g., -d /home/user/Desktop/obfuscated.apk or -d /home/user/Desktop/obfuscated.aab). If not specified, the final obfuscated file will be saved inside the working directory. Note: existing files will be overwritten without any warning.

• -i is a flag for ignoring known third party libraries during the obfuscation process, to use fewer resources, to increase performances and to reduce the risk of errors. The list of libraries to ignore is adapted from LiteRadar project.

• -p is a flag for showing progress bars during the obfuscation operations. When using the tool in batch operations/automatic builds it's convenient to have progress bars disabled, otherwise this flag should be enabled to see the obfuscation progress.

• -k VT_API_KEY is needed only when using VirusTotal obfuscator, to set the API key to be used when communicating with Virus Total.

• --keystore-file KEYSTORE_FILE, --keystore-password KEYSTORE_PASSWORD, --key-alias KEY_ALIAS and --key-password KEY_PASSWORD can be used to specify a custom keystore (needed for the apk signing). If --keystore-file is used, --keystore-password and --key-alias must be specified too, while --key-password is needed only if the chosen key has a different password from the keystore password. By default (when --keystore-file is not specified), a keystore bundled with Obfuscapk is used for the signing operations.

• --ignore-packages-file IGNORE_PACKAGES_FILE is a path to a file which includes package names to be ignored. All the classes inside those packages will not be obfuscated when this option is used. The file should have one package name per line as shown in the example below:

  com.mycompany.dontobfuscate
  com.mycompany.ignore
  ...
  

• --use-aapt2 is a flag for using aapt2 option when rebuilding an app with apktool.

Let's consider now a simple working example to see how Obfuscapk works:

$ # original.apk is a valid Android apk file.
$ obfuscapk -o RandomManifest -o Rebuild -o NewAlignment -o NewSignature original.apk

When running the above command, this is what happens behind the scenes:

• since no working directory was specified, a new working directory (obfuscation_working_dir) is created in the same location as original.apk (this can be useful to inspect the smali files/manifest/resources in case of errors)

• some checks are performed to make sure that all the needed files/executables are available and ready to be used

• the actual obfuscation process begins: the specified obfuscators are executed (in order) one by one until there's no obfuscator left or until an error is encountered

  • when running the first obfuscator, original.apk is decompiled with apktool and the results are stored into the working directory

  • since the first obfuscator is RandomManifest, the entries in the decompiled Android manifest are reordered randomly (without breaking the xml structures)

  • Rebuild obfuscator simply rebuilds the application (now with the modified manifest) using apktool, and since no output file was specified, the resulting apk file is saved in the working directory created before

  • NewAlignment obfuscator uses zipalign tool to align the resulting apk file

  • NewSignature obfuscator signs the newly created apk file with a custom certificate contained in a keystore bundled with Obfuscapk (though a different keystore can be specified with the --keystore-file parameter)

• when all the obfuscators have been executed without errors, the resulting obfuscated apk file can be found in obfuscation_working_dir/original_obfuscated.apk, signed, aligned and ready to be installed into a device/emulator

As seen in the previous example, Rebuild, NewAlignment and NewSignature obfuscators are always needed to complete an obfuscation operation, to build the final obfuscated apk. They are not actual obfuscation techniques, but they are needed in the build process and so they are included in the list of obfuscators to keep the overall architecture modular.

Not working as expected? See FAQ and troubleshooting.

❱ Obfuscators

The obfuscators included in Obfuscapk can be divided into different categories, depending on the operations they perform:

• Trivial: as the name suggests, this category includes simple operations (that do not modify much the original application), like signing the apk file with a new signature.

• Rename: operations that change the names of the used identifiers (classes, fields, methods).

• Encryption: packaging encrypted code/resources and decrypting them during the app execution. When Obfuscapk starts, it automatically generates a random secret key (32 characters long, using ASCII letters and digits) that will be used for encryption.

• Code: all the operations that involve the modification of the decompiled source code.

• Resources: operations on the resource files (like modifying the manifest).

• Other

The obfuscators currently bundled with Obfuscapk are briefly presented below (in alphabetical order). Please refer to the source code of the project for more details.

NOTE: not all the obfuscators below correspond to real obfuscation techniques (e.g., Rebuild, NewAlignment, NewSignature and VirusTotal), but they are implemented as obfuscators to keep the architecture modular and easy to extend with new functionality.

❱ Contributing

Questions, bug reports and pull requests are welcome on GitHub at https://github.com/ClaudiuGeorgiu/Obfuscapk (see contributing). Make sure to also check FAQ and troubleshooting, since some of the most common questions are already answered there.

❱ License

You are free to use this code under the MIT License.

❱ Credits

This software was developed for research purposes at the Computer Security Lab (CSecLab), hosted at DIBRIS, University of Genoa.

❱ Team

• Simone Aonzo - Research Assistant
• Gabriel Claudiu Georgiu - Core Developer
• Luca Verderame - Postdoctoral Researcher
• Alessio Merlo - Faculty Member