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).
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 = "http://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"
}
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).
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.gitThis is the suggested way of installing Obfuscapk, since the only requirement is to have a recent version of Docker installed:
$ docker --version
Docker version 19.03.0, build aeac949The 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 obfuscapkIf 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]
...Obfuscapk is now ready to be used, see the usage instructions for more information.
Make sure to have a recent version of
apktool,
jarsigner
and zipalign installed
and available from the command line:
$ apktool
Apktool v2.4.1 - a tool for reengineering Android apk files
...$ jarsigner
Usage: jarsigner [options] jar-file alias
jarsigner -verify [options] jar-file [alias...]
...$ zipalign
Zip alignment utility
Copyright (C) 2009 The Android Open Source Project
...To install and use apktool you need a recent version of Java, which should also have
jarsigner bundled. zipalign is included in the Android SDK. The location of the
executables can also be specified through the following environment variables:
APKTOOL_PATH, JARSIGNER_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 3.6) installation (along with its package manager pip).
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, e.g., virtualenv.
$ # If not using virtualenv (https://virtualenv.pypa.io/), skip the next 2 lines.
$ virtualenv -p python3 venv
$ source venv/bin/activate
$ # Install Obfuscapk's requirements.
$ python3 -m pip install -r src/requirements.txtAfter 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]
...Obfuscapk is now ready to be used, see the usage instructions for more information.
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] [-i] [-p] [-k VT_API_KEY]
[--keystore-file KEYSTORE_FILE] [--keystore-password KEYSTORE_PASSWORD]
[--key-alias KEY_ALIAS] [--key-password KEY_PASSWORD]
<APK_FILE>There are two mandatory parameters: <APK_FILE>, the path (relative or absolute) to
the apk 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 NewSignature -o NewAlignment). 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 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). 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(s) to be used when communicating with Virus Total. Can be set multiple times to
cycle through the API keys during the requests (e.g.,
-k VALID_VT_KEY_1 -k VALID_VT_KEY_2).
--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.
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 NewSignature -o NewAlignment original.apkWhen 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
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)
NewAlignment obfuscator uses zipalign tool to align the resulting apk file
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, NewSignature and NewAlignment
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.
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, NewSignature, NewAlignment and VirusTotal), but they are implemented
as obfuscators to keep the architecture modular and easy to extend with new
functionality.
Uses reflection to invoke dangerous APIs of the Android Framework. To find out if a method belongs to the Android Framework, Obfuscapk refers to the mapping discovered by Backes et al.
Insert junk code. In this case, the junk code is composed by arithmetic computations and a branch instruction depending on the result of these computations, crafted in such a way that the branch is never taken.
Encrypt asset files.
This technique modifies the control-flow graph without impacting the code semantics: it adds new methods that invoke the original ones. For example, an invocation to the method m1 will be substituted by a new wrapper method m2, that, when invoked, it calls the original method m1.
Change the package name and rename classes (even in the manifest file).
Encrypt constant strings in code.
Remove debug information.
Rename fields.
Given a method, it inserts a
gotoinstruction pointing to the end of the method and anothergotopointing to the instruction after the firstgoto; it modifies the control-flow graph by adding two new nodes.
Encrypt native libs.
It exploits the overloading feature of the Java programming language to assign the same name to different methods but using different arguments. Given an already existing method, this technique creates a new void method with the same name and arguments, but it also adds new random arguments. Then, the body of the new method is filled with random arithmetic instructions.
Rename methods.
Realign the application.
Re-sign the application with a new custom signature.
Insert junk code. Nop, short for no-operation, is a dedicated instruction that does nothing. This technique just inserts random
nopinstructions within every method implementation.
Randomly reorder entries in the manifest file.
Rebuild the application.
This technique analyzes the existing code looking for method invocations of the app, ignoring the calls to the Android framework (see
AdvancedReflection). If it finds an instruction with a suitable method invocation (i.e., no constructor methods, public visibility, enough free registers, etc.) such invocation is redirected to a custom method that will invoke the original method using the Reflection APIs.
This technique consists of changing the order of basic blocks in the code. When a branch instruction is found, the condition is inverted (e.g., branch if lower than, becomes branch if greater or equal than) and the target basic blocks are reordered accordingly. Furthermore, it also randomly re-arranges the code abusing
gotoinstructions.
Encrypt strings in resources (only those called inside code).
Send the original and the obfuscated application to Virus Total. You must provide the VT API key (see
-koption).
Questions, bug reports and pull requests are welcome on GitHub at https://github.com/ClaudiuGeorgiu/Obfuscapk. Make sure to also check FAQ and troubleshooting, since some of the most common questions are already answered there.
You are free to use this code under the MIT License.
This software was developed for research purposes at the Computer Security Lab (CSecLab), hosted at DIBRIS, University of Genoa.