Bytecode obfuscation

Principles

Java is a language which code is quite intuitive to read. But some also complain that compiled code is as easy to read as source code - or at least it is easy to recover. You will find here a couple of hints and tips about this matter of fact, and how to deal with it if you need to prevent people to exploit code they should not work with.

How to recover Source Code from Bytecode?

The main program for uncompiling code is JAD (JAva Decompiler). It provides following advantages :

• Recover code from Java ByteCode,
• Get clean code for your own programs,
• Remove Comments, Javadoc, Names of local Variables, Names of Parameters,
• Several Graphical interfaces, available on the web site.

How to prevent your Java code to be Reverse-engineered ?

Several actions can be taken for preventing reverse-engineering :

• Code Obfuscation. This is done mainly through variable renaming; see next paragraph for more precisions,
• Suppression of End Of File Characters. This makes the code difficult to parse,
• Use of anonymous classes for handling events. This seems not to be handled by many Decompiler; however, JAD copes pretty well with this.
• File encoding. This implies some overhead for uncyphering at runtime. Several tools are available:: Canner, by Cinnabar Systems, or JLock by JSoft. They are available for evaluation, and the first is proposed currently for Windows Platforms only.

What tools do exists for Obfuscation ?

A lot of tools exist for Java code Obfuscation. You can find extensive lists under following URLs, or simply type 'obfuscator' in your favorite search engine :

• http://directory.google.com/Top/Computers/Programming/Languages/Java/Development_Tools/Obfuscators/
• http://proguard.sourceforge.net/alternatives.html

Among those projects, some are open source project, and therefore more suitable for research - but also for enterprises who wish to control the programs they use (without any warranty):

• Proguard is a shrinker (make code more compact), and optimizer and obfuscator.
• Jode is a decompiler, an optimizer and an obfuscator. It contains facilities for cleaning logging statements,,
• Jarg,
• Javaguard, which is a simple obfuscator, without many documentation,
• CafeBabe, which allows precise view of Bytecode files and single file obfuscation; a good tool for teaching ByteCode Structure, more than a production tool.

Using Proguard

The following section provides a short tutorial for using Proguard.

First, download the code under following url and unzip it.

For this tutorial, we use the genericFrame.jar package, part of a simple demo application .

Go tothe main directory of Proguard. For lauching it, you can use following script with given parameters :

      java -jar lib/proguard.jar @config-genericFrame.pro

config-genericFrame.pro is the option file :

-obfuscationdictionary ./examples/dictionaries/compact.txt
-libraryjars /usr/java/j2sdk1.4.2_10/jre/lib/rt.jar
-injars genericFrame.jar
-outjar genericFrameOut.jar
-dontshrink
-dontoptimize
-keep public class proguard.ProGuard {
public static void main(java.lang.String[]);
}

Remark that the 'keep' option is mandatory, we use this default class for not keep anything out.

The example dictionnary (here compact.txt) is given with the code.

The output is stored in the package 'genericFrameOut.jar'.

You can observe the modifications implied by obfuscation with following commands :

jar xvf genericFrameOut.jar
cd genericFrame/pub/gui/
jad c.class
more c.jad more c.jad

Remark than Strings are kept unmodified. If you want you code to be hard to read, do not forget to remove any debugging and logging comments. Jode has some facilities for making this easier.

Using CafeBabe

CafeBabe is a convenient tool for teaching structure of ByteCode files. You can download it at this URL.

Unzip it and execute following command :
java -classpath CafeBabe.jar org.javalobby.apps.cafebabe.CafeBabe

Have a look at some class from the original genericFrame.jar package.

Then obfuscate it, and compare both - original and modified class :

• with the CafeBabe viewer,
• after decompiling it with JAD.

What conclusion can you draw of it ?

Using Jode

Jode is to be found here.