Slashdot Mirror
Decompiling Java
If you are interested in Decompiling Java, then this book tell you exactly how to do that. There's no fluff and every chapter counts. I can safely concur that Fiachra's observations are indeed correct. You'd better be prepared for some serious hard core details, but then that's what you'd paid for. It is really great to read a book that doesn't end each chapter with a few links to the real material because the author couldn't be bothered to write it up.
So what do you get? As a battle-hardened Java coder of not a few years programming, I wanted to find out about the gory details of bytecodes and how to get at them. Now it's a subject I always knew I should know about, but never took the time to read up on it. Decompiling Java puts all that knowledge into one place.
Here's a quick run-through of the chapters so you know what you're getting:
Ch.1 IntroductionDecompilation isn't just another coding tool - there are other, real world issues like ending up in jail to think about. Godfrey proposes a sort of code-of-honour for decompilers. This book could so easily have been positioned for the fr33ky kod3r skript kiddie market, and I'm glad that the author and publishers took a mature and sensible approach to the subject. I have had to decompile purchased code because of bugs and I'm glad that someone took the time to think about an ethical framework for doing this.
Ch.2 Ghost in the Machine
A good and solid introduction to the JVM and the classfile format. If you're in the market for this book, you probably already know most of this, but a refresher course is always good. For me, it definitely sorted out a lot on internal hand-waving on the subject. Just remember kids, the only thing to fear is fear itself - it's only binary data after all.
Ch.3 Tools of the Trade
Although the author builds his only decompiler later in the book, it nice to get a chapter devoted to the existing toolset and the Java decompiler scene.
Ch. 4 Protecting your Source
For the honest developer, knowing how to decompile code is more about protecting your own source code than breaking someone else's (who wants to read other people's smelly code anyway!). This chapter is one of the most directly practical. I had always assumed that obfuscation was a magic fix that I could apply if necessary. In reality, good obfuscation is just like good encryption (that is, uncommon, difficult to verify, and still subject to lateral attacks). Even compiled bytecode has relatively low entropy, so the value of obfuscation must be considered carefully.
Ch.5 Decompiler Design
This is were it starts getting a wee bit technical. Decompilation, as you can imagine, is a bit of a black art, and there are many ways of doing it. Some of them involve scary maths and some involve scary coding and the rest both. But that's why you don't meet many people who can write decompilers. Godfrey does a great job of taking you on a practical run through this fog of decompilers. At the end of this chapter you will be able to decide for yourself what approach is best suited to your problem domain. Again, this material can be challenging but it's like boot camp: You just gotta.
Ch.6 Decompiler Implementation
If the previous chapter hurt your brain and scared you silly then this chapter will have you weeping for joy. The author takes a practical, effective, and most importantly, understandable approach to actually implementing a compiler. Now, as he freely admits, his design may encounter difficulties with edge effects and infrequently used idioms, but it will take you to the point where you can solve them yourself. I really had to smile at how simple and effective the approach taken here is - instead of the expected multiple passes and mind bending parse tree manipulation, we have a single-pass, source-generating decompiler for Java. You won't follow it all first time, but it does work and you can verify it for yourself. Like I said at the start, you don't get that empty feeling from this book, and this chapter is pretty much why. I bought a book about decompiling Java, and now I can.
Ch.7 Case Studies
This chapter addresses the "why" of decompiling, returning again to the moral questions raised at the start. It's more food for thought than prescriptive preaching though, which again is refreshing. I have admit to dipping into this chapter while reading the rest of the book - the human interest angle always works a treat!
Of course, no book is perfect. What I think could have helped a bit overall would have been a introductory chapter to bytecode. But it's not a great loss and bytecode is actually pretty simple once you get your head around it. Still it might have lessened the learning curve somewhat.
Decompiling Java is a great addition to that section of your bookshelf dedicated to serious books that will be around for a while. The JVM specification and Java bytecode are not going to change that much, so this book is something you'll be able to use for a long time. Personally the best thing about this book for me was that it took me to the next level. Not many books can do this. As a working coder, I pretty much put things like decompilation into the "too hard, just for academics, and I could never grok it", category. It's great when a book comes along that can can you out of that comfort zone.
You can purchase Decompiling Java from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, carefully read the book review guidelines, then visit the submission page.
I've read both and I have to say Covert Java is slightly more in-depth, but perhaps more for people more familiar with Java.
When anger rises, think of the consequences.
Confucius (551 BC - 479 BC)
I have yet to try it on byte-code produced by non-Java languages, but I'd be interested to see the results...
(It sucks that it's no longer free. The version I've got I installed through Debian, for goodness sake, years ago. Does anyone know any free alternatives that work as well?)
You've misunderstood. Java obfuscation is an automated process done with a third-party tool that rearranges Java BYTECODE, not source code. The idea being that someone that tries to decompile the BYTECODE will get a bunch of spaghetti. It doesn't take any extra time or energy by the developer, just 5 seconds to run the tool on your .class files.
There are 2 kinds of people in this world. Those that can keep their train of thought,
Sun has put the Java bytecode specification online for free..
Reverse engineering in Java is as simple as the compile process itself. Besides there are already free tools available so why bother??
I believe encryption is probably not worth the effort for most people, it's not much of a hurdle to overcome. Ultimately all custom class loader execution paths will lead to a ClassLoader.defineClass call which can either be intercepted by creating a modified system class loader or by creating a JVMPI agent which listens for JVMPI_EVENT_CLASS_LOAD_HOOK events. If you can prevent these methods from being employed then you're probably operating in a secure environment where encryption would be overkill.
This approach to "security" in Java is so trivially easy to circumvent that its worthless.
/ 01-qa-0509-jcrypt.html
There are a number of papers and articles detailing why this type of approach to "IP security" is so misguided. One such article is here: http://www.javaworld.com/javaworld/javaqa/2003-05
The crux is that at some point in time, you have to deliver the encrypted class to the JVM in an unencrypted format. Intercepting this delivery is incredibly easy (no expert knowledge required, the details for doing so are detailed in the article above), at which time someone can just write the unecrypted class file out to disk (or wherever they wish). Voila! All your IP are belong to us.