Java Coffee Break Newsletter Volume 1 Issue 04

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Java Coffee Break Newsletter

 · 9 months ago

                Java Coffee Break Newsletter Vol 1, Issue 4 
                             ISSN 1442-3790 

 Welcome to the fourth Java Coffee Break Newsletter. This issue covers 
 decompiler software, which allows software developers to generate 
 source code from compiled Java classes. This article is updated with 
 new information about stronger decompilers, and is based on an earlier 
 article from the JCB site. 

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Java Coffee Break Updates 

 * Java Tutorials 

1. Java Tutorials 
  
   If you're new to the Java programming language, and need a little 
   help, then try some of the free tutorials available from the 
   Java Coffee Break. These aren't your normal tutorials - they don't 
   require extensive Java experience or knowledge. 

   All our Java tutorials can be found at 
   http://www.davidreilly.com/jcb/tutorials.html 

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Decompilers - Friend or Foe? 

 
When a programmer writes software, and releases it to the public, he 
(or she) normally releases a compiled version of the application, that 
users can run on their own machine. Whether it is a commercial 
offering, or a free piece of software, the programmer has put a 
considerable amount of time and effort into producing it. As a general 
view, programmers don't give the source code for their product away. 
Yet few developers realise that every time you release compiled 
software, you are also giving people the opportunity to reconstruct 
the source code! 

Software that examines software 

Decompilers are programs that analyse compiled code, and from this, 
reconstruct the original source code. Decompilation and reverse 
engineering is often prohibited by software license agreements - but 
this won't always stop an unscrupulous competitor, or an enthusiastic 
hacker from analysing your code. Decompilers are freely available for 
a variety of languages and platforms, including Java! Read on, and 
I'll introduce you to the world of decompilation. 

How do they work? 

Decompilers work by analysing the byte code of software, and then 
deduce the code that created it. Most classes also contain additional 
debugging information, which can help the decompiler create a more 
accurate representation of the original source code. This debugging 
information is invisible to normal users, and many programmers don't 
even realise just how much information can be obtained from their 
classes - but there are ways to protect your code. 

Software is available that will strip away debugging information, 
and even change the names of local and member variables inside your 
classes. SourceGuard, for example, will rename your variables to 
meaningless names, which decreases the readability of decompiled 
source. When you protect your code with applications like 
SourceGuard, decompilers have less information on which to base 
their analysis on, and it becomes harder for programmers to 
understand the code they produce. 

The success of decompilation varies upon the amount of protection 
that software developers use, and the decompiler software that one 
uses to decompile. Many decompilers fail to decompile correctly, 
and some will even produce code that won't compile - particularly 
when faced with strong protection from a product like SourceGuard 
which offers a feature called byte-code range modification. BRM 
prevents most software from decompiling methods that have try { } 
catch blocks, and will produce garbled code with most decompilers. 
  
Preventing decompilation is a valuable feature. Of course, such 
protection isn't uncrackable. While there are plenty of free 
decompilers out there, you really get what you pay for. With free 
tools, the code that is produced ranges from complex to unusable 
when protected by a tool that is decompiler resistant. With 
commercial tools, you can get varying degrees of success, and at 
least one tool is capable of breaking the byte-code range 
modification technique of SourceGuard. 

SourceAgain, by Ahpah Software Inc, is capable of decompiling BRM 
protected classes effectively, and produces much more readable 
code than free software like Mocha or DejaVu. SourceAgain is 
available in three versions, a standalone decompiler, a 
professional edition that integrates with Symantec Visual Cafe 
and Microsoft Visual J++, and a Unix version. For those interested  
in using decompilers, a trial of SourceAgain is accessible from 
the web, and it can decompile classes that are accessible from a 
http:// address. 

The full article is available at 
http://www.davidreilly.com/jcb/articles/decompilers_friend_or_foe.html 

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