29 Apr 2015

How to learn Reverse Engineering

Every other days or two a new guy appears at Tuts4You asking "I want to learn Reverse Engineering, where do I start?".


There are lots of suggestions, eg. in this Reddit thread. One of the most common recommendation is to with Lena151's tutorials. And there's a good reason for that - these tutorials give a good overview of most common tasks, most common tools and provide "instant gratification". But do they actually teach you Reverse Engineering? I don't think so.

Why Lena151's tutorials are bad

Apparently I'm not the only one who thinks so:

I have been thinking about why this happens. Thinking back to myself, I started learning reverse engineering by reading the Lena151 tutorials. I thought they were awesome until Daeken told me that was an awful approach to learn reverse engineering.

At first I didn’t understand why they were so bad. After all, Lena’s tutorials had taught me how to crack my first software.

And that's exactly the problem. You managed to crack your first software. Instant gratification! But what did you actually learn? Run some common tools, find the "bad boy jump" and patch it? Wow! You must be so l33t!

In reality, these tutorials have produced entire generation of wanna-be-crackers who can only use ready-made tools, but are actually unable to think for themselves. For every problem they need a video tutorial. For every small obstacle they create a new topic asking for help.

And that's only half of the problem.

Video tutorials made by beginners are even worse

Albert Einstein once said:

The more I learn, the more I realize how much I don't know.

Beginners who watched Lena151's series don't realize that. They cracked their first program and they consider themselves to be reversers. And what's even worse, they try to spread their "knowledge" by making an incredible amount of incredibly crappy video "tutorials" to be watched by next generation of wannabes.

I've actually watched a 15 minute movie titled "How to unpack CryptoObfuscator". You know how? You drag-and-drop the file on de4dot. Yes, that simple. Yes, someone made 15 minute movie to teach you that.

So, what's the alternative?

ReverseWithMe blog suggests to learn:

  1. x86 assembly (electronics and wires in the car analogy)
  2. How operating systems work and how they manage memory (the engine of the car)
  3. The compiling process from C-code to assembly (this is equivalent to knowing how a car-fabric is assembling a car)
  4. The life of a binary (equivalent to everything that happens in the car from the key-switch to the off-switch)

I don't think this is the correct approach either.

It's like learning a foreign language by reading a dictionary. Start with an "a", and once you finish with "z", you'll know all the words. Maybe. But you won't be able to make a proper sentence, let alone speak or understand a native speaker.

To put it into context: I've been reversing .NET executables for 10 years now. I've written unpackers for pretty much every .NET protection there is. And yet I still don't know IL assembly "by heart". Why? Because I don't need to. What's the mnemonic for "branch-if-equal"? Is it be, beq or bre? Does it pop one or two arguments from stack? I don't know. If I'll ever need that, the answer is one Google search away.

Yes, to be a great reverser, you will need to master most of those items. But you don't need to know all that at the start of your journey.

Gimme a solution, goddamit!

Well, start with Lena151's tutorials. Yes, I said they are not good, but that's the best there is. And if you follow few extra advices, you'll do just fine:

  • Learn to think for yourself. That's the most important part. Don't just blindly follow tutorials, try to understand why it works and how it works.
  • Learn to search. Most questions have already been answered, you just need to find the answer. Make sure Google is your friend!
  • Learn your tools. You don't need to know every single option and feature of them. Most people use ~10% of all Microsoft Excel features. Power Users use around 20%. It's the same with RE tools. If you've mastered 10% of Olly or IDA features, you're good to go.
  • And last but not least - have fun! Nothing kills your productivity faster than boredom. If the problem is too hard, let it go, try something else and come back to it later.
24 Apr 2015

Sniffing correct serial in .NET crackmes


In this tutorial I'll show you a generic way how to break most of the crackmes written in VB.NET. It uses the fact that most crackmes made by beginners will calculate correct serial and do a simple comparison "if enteredSerial = correctSerial then"...

To break such a crackme, you only need to find this comparison and sniff the correct serial. This is a very common approach in x86 world but in .NET world it's not that popular yet.

As for my target, I'm using "RDG Simple Crackme .NET v4 2015"

GetProcAddress in .NET

In x86 world you can use GetProcAddress function to get address of any API function from any DLL. Can we do something similar in managed environment like .NET? It turns out that we can, but it's a little bit harder.

So, for example, to get address of Assembly.Load(byte[]) you need to do:

This works well with static classes and static methods. How about non-static methods like RijndaelManaged.CreateDecryptor(byte[], byte[])?

That's doable as well, like this:

To make this reference almost complete - here's how to get address of .ctor:

There are a few gotchas, however..

  • In case your target type is located in assembly that's not NGEN'ed yet, I suggest that you use ngen and install the assembly in cache. That can prevent certain problems later.
  • Addresses of functions are obviously different in .NET 2.0 and 4.0. You must compile for correct framework version and target the correct .NET assembly.
  • Addresses of functions are different for x86 and x64 framework versions, too. Make sure your assembly is compiled correctly.

Sniffing string compare

Suprisingly, string comparison in VisualBasic.NET and other .NET languages is different. It's caused by Option Compare statement present in Visual Basic language. So, if the crackme is made in VB.NET, you need to examine Operators.CompareString(string,string,bool) function. For crackmes made in other languages, you'll need to examine string.Equals(string) or some other variation of this method.

So, using the code I mentioned above, I learned that address of Operators.CompareString(string,string,bool) on my PC is 599F1D30. Now I need to sniff data passed to this function.

There are several possible approaches. You can try using VisualStudio & Reflector plugin as SpoonStudio tried, you can try using ILSpy and it's debugger plugin, or you can inject DLL into crackme process, as suggested by noth!ng - but I prefer to use OllyDbg.

Load crackme in OllyDbg, make sure that all the anti-anti-debug plugins are working, all the exceptions ignored, put a breakpoint on 599F1D30 and hope for the best.

Nope. Operators.CompareString is called literally thousands of times. So, we need to do something smarter.

For example, we can use conditional logging breakpoints in Olly. Those breakpoints are quite slow, but it's still faster than to write some sort of hooking DLL and inject it into crackme. So, we need to set 2 logging breakpoints - one for each string compared. Here is first one:
Place second breakpoint at the next instruction (59CD1D31) and log string at edx+8.

Run the crackme, enter some fake but easily recognizable serial and few minutes later we have the answer:
My entered serial was "1234567890123456789012345678901234567890" and it's being compared to "C49476D583364356253377056314435396D456F44796C7A55746431564433544". Hmm, could that be the correct serial for my nickname? wink Yes, it is!

Final notes

This was quite nice crackme and I only showed the simplest way to beat it. When you start looking into it, you'll find some nice anti-debug tricks, some nice anti-patching tricks and pretty nicely obfuscated code.

But that's a matter for another story. Have fun!

16 Apr 2015

About e-governments

Two days ago Google released Chrome 42. It's the answer to life, the universe and everything. And among other things, it disables all NPAPI plugins by default, finally putting that Java nightmare into it's grave. Good riddance!

But what about other NPAPI plugins? Like, you know, the ones used for electronic documents, digital signatures and other e-government thingies?

Well, here are 2 ways how government agencies approach the same problem:
e-governments compared

Welcome to the 21st century. If you're Estonian, that is..

08 Apr 2015

Catch me when you can


Exception filters have been part of ECMA-335 specification since the very beginning. I'm guessing, they were added because Visual Basic used them extensively and therefore Visual Basic.NET had to support them as well. They look something like this:

Until now C# supported try/catch but did not have support for filters. That's going to change in C# 6.0/VS2015.

How does it work

In early versions of VS2015 the syntax was "catch-if", as you can see in the initial announcement. In the latest VS2015 CTP builds, they changed syntax to "catch-when", and there's a good reason for it.

So, how does it work and what does it mean for reversers?

It's a compiler-level feature

As I mentioned before, .NET Framework has supported exception filters since the very beginning. So, this feature works even in .NET 2.0 - if you decide to target .NET 2.0 Framework in VS2015 project settings. Not that you really want to do that..

It's very useful for debugging

catch-when is implemented as an IL exception filter. So, when an exception is thrown, exception filters are processed before the stack is unwound. This means that filter method has created an error report that included the current stack trace, it would show the frame in which the exception occurred. Sounds complicated? It isn't.

Let's implement exception filtering in the "old" way:

and this is how the stack looks when we get to filter(ex):
You can't see much here. All the context is gone, you must rely on exception stack trace and message. That's what we've always done, right? smile

If we write it in a "new" way, the code looks like this:

and stack trace will give us full context of exception:
Much better, isn't it? You can see which method threw the exception, on which line, you have access to local variables and everything else. Yummy! smile

Decompiler support for exception filters is crappy

They say, a picture is worth thousand words.. In a very simple example, Reflector gets the code structure right, just filter conditions are missing:
ILSpy handles it slightly worse, filters are messed up and unreadable. Filter code is gone, too:
And the latest JustDecompile just throws an exception:

Have fun with it!

Here is a small keygen-me for you to play with: https://www.mediafire.com/?k5b9vy0p9dfgb97

The difficulty is 2/10, you should be able to solve it in 30 minutes or so. The entire protection is designed to show you try-catch-when feature, so avoid patching - you can't learn anything by nopping-out few instructions. wink