- Protecting the Ideas Behind Your Code
- Obfuscation As a Protection of Intellectual Property
- Transformations Performed by Obfuscators
- Knowing the Best Obfuscators
- Potential Problems and Common Solutions
- Using Zelix KlassMaster to Obfuscate a Chat Application
- Cracking Obfuscated Code
- Quick Quiz
- In Brief
This chapter is from the book
This chapter is from the book
This chapter is from the book
Using Zelix KlassMaster to Obfuscate a Chat Application
Even though each obfuscator has its own format of configuring the transformations, they all support a common set of features. The Chat application does not contain state-of-the-art algorithms or patent-pending inventions, but it is dear to our hearts so we are going to use Zelix KlassMaster to protect it from the prying eyes of hackers and thieves.
First, we obtain a copy of Zelix KlassMaster and install it on a local machine. Remember that we refer to the Chat application's home directory as CovertJava. Next, we copy ZKM.jar from KlassMaster's installation directory to our project lib directory so we can script against it. The easiest way to create the obfuscation script is with KlassMaster's GUI. Using the command
java -jar ZKM.jar
from the lib directory, we run the GUI. Then, in the initial helper dialog box that appears, we select the Set Classpath option. We now select the runtime libraries of the JDK we're using and, in the Open Classes dialog box that appears next, we select CovertJava/lib/chat.jar. After that, KlassMaster should load all the classes of the Chat application and we should be able to view the internal structure of the bytecode. The screen should look similar to Figure 3.1.
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Figure
3.1 Chat classes loaded into the KlassMaster GUI.
While working with the GUI, you can easily see just how flexible KlassMaster is. You can manually change the names of classes, methods, and fields; modify the visibility of classes or methods; make methods final; change text strings; and do other cool stuff. KlassMaster attempts to propagate the changes throughout the loaded code, so if other classes refer to a method and you change its name, the referring classes are updated to reflect the change. After making all your changes, you can save the classes as is or trim and obfuscate them first. Classes loaded into the GUI environment can be further modified after the obfuscation, even though I can't think of a reason why someone would need to do so. For details of KlassMaster's features and how to use it, please refer to its user manual.
A well-written Java application provides scripts to build it, so let's integrate obfuscation into our build script. We start by using KlassMaster's GUI to create the obfuscation script. Then, we update it manually to make it more flexible. It is entirely possible to write the script manually or copy and modify a sample script. We run the GUI and select ZKM Script Helper from the Tools menu. Then, we do the following:
Read the instructions on the Introductory Page and click Next.
On the Classpath Statement page, select rt.jar and click Next.
On the Open Statement page, navigate to CovertJava/distrib/chat.jar and click > to select it for opening. We only need one file because all our application classes are packaged in it. Click Next.
On the TrimExclude Statement page, the default exclusions are preset to exclude the cases where obfuscation is likely to result in an error. For example, renaming methods of an EJB implementation class makes it unusable, so EJBs are excluded by default.
On the Trim Statement page, select the Delete Source File Attributes check box and the Delete Deprecated Attributes check box to get rid of the debug information; then click Next.
In the Don't Change Main Class Name combo box on the Exclude Statement page, select covertjava.chat.ChatApplication to preserve its name. This keeps JAR manifest entries valid and enables users to continue invoking the chat using a human-readable name.
On the Obfuscate Statement page, select Aggressive in the Obfuscate Control Flow combo box. Then select Aggressive in the Encrypt String Literals combo box, and select Scramble in the Line Number Tables combo box. This ensures adequate protection for the code but enables us to translate stack traces later. Make sure that Produce a Change Log File is checked and click Next.
On the SaveAll Statement page, navigate to CovertJava/distrib and create a subdirectory called obfuscated. Select the newly created directory for output and click Next.
The next page should show the script text and allow us to save it to a directory. Save it as obfuscate_script.txt in the CovertJava/build directory and exit the GUI.
The resulting script should look similar to Listing 3.4.
Listing 3.4 Obfuscation Script Generated by the GUI
/******************************************************************************/
/* Generated by Zelix KlassMaster 4.1.1 ZKM Script Helper 2003.08.13 17:03:43 */
/******************************************************************************/
classpath "c:\java\jdk1.4\jre\lib\rt.jar"
"c:\java\jdk1.4\jre\lib\sunrsasign.jar"
"c:\java\jdk1.4\jre\lib\jsse.jar"
"c:\java\jdk1.4\jre\lib\jce.jar"
"c:\java\jdk1.4\jre\lib\charsets.jar";
open "C:\Projects\CovertJava\distrib\chat.jar";
trim deleteSourceFileAttributes=true
deleteDeprecatedAttributes=true
deleteUnknownAttributes=false;
exclude covertjava.chat.^ChatApplication^ public static main(java.lang.String[]);
obfuscate changeLogFileIn=""
changeLogFileOut="ChangeLog.txt"
obfuscateFlow=aggressive
encryptStringLiterals=aggressive
lineNumbers=scramble;
saveAll archiveCompression=all "C:\Projects\CovertJava\distrib\obfuscated";
A good idea would be to replace the absolute file paths with the relative ones, so that instead of opening C:\Projects\CovertJava\distrib\chat.jar, the script opens distrib\chat.jar. Finally, we will integrate obfuscation into the build process by declaring a custom task and adding a target that calls it. KlassMaster is written in Java and can be called from any build script. Conveniently, it provides a wrapper class for Ant integration, so all we have to do is add the following to Chat's build.xml:
<!-- Define a task that will execute Zelix KlassMaster to obfuscate classes -->
<taskdef name="obfuscate" classname="ZKMTask" classpath="${basedir}/lib/ZKM.jar"/>
...
<!-- Define a target that produces obfuscated version of Chat -->
<target name="obfuscate" depends="release">
<obfuscate scriptFileName="${basedir}/build/obfuscate_script.txt"
logFileName="${basedir}/build/obfuscate_log.txt"
trimLogFileName="${basedir}/build/obfuscate_trim_log.txt"
defaultExcludeFileName="${basedir}/build/obfuscate_defaultExclude.txt"
defaultTrimExcludeFileName="${basedir}/build/obfuscate_defaultTrimExclude.txt"
defaultDirectoryName="${basedir}"
/>
</target>
We can now run Ant on obfuscate target. If the build is successful, a new file (chat.jar) is created in CovertJava/distrib/obfuscated. This file contains the obfuscated version of Chat that can still be invoked using the java -jar chat.jar command. Take a few moments to look inside that JAR and try decompiling some of the classes.
Before we close the subject of using KlassMaster, I'd like to give a few more examples of script file syntax for excluding classes and class members from obfuscation. The format shown in Table 3.2 can be used for statements of obfuscation script that take in names as parameters. ZKM script language supports wildcards, such as * (any sequence of characters) and ? (any one character), and boolean operations, such as || (or) and ! (not). For a detailed explanation and full syntax, please refer to KlassMaster documentation.
Table 3.2 Commonly Used Name Patterns for KlassMaster
|
Syntax |
What It Matches |
|
package1.package2. |
Package names package1 and package2. Other package names and children of package2 are not matched. |
|
*. |
All package names in the application. |
|
Class1 |
The name of the class Class1. |
|
package1.Class1 |
The name of Class1 in package package1 but not package1's name. |
|
package1.^Class1 |
The names of Class1 and package1. |
|
package1.^Class1^ method1() |
The names of package1, Class1, and method1 with no parameters. |
|
package1.^Class1^ method1(*) |
The names of package1, Class1, and all overloaded versions of method1. |