VBReFormer: CrackMe Sample #1

Now that VBReFormer is a well advanced decompiler for Visual Basic application, I was searching for some unsolved crackmes in order to made sample of decompiling for learning purpose.

The website Crackmes.de contains an impressive number of crackmes applications, a perfect source of samples.

For the first sample of CrackMe solving with VBReFormer Professional I decided to take “Step 2” from yudi (more informations).

I will show you, step by step, how it’s simple to solve the yudi’s Step 2 using VBReFormer Professional.

  • Running the application:

yudi's Step 2 (CrackMe) / Screenshot 1

We can see that a serial is generated using the name of the user.

How the serial is generated? See the following step.

  • Now we just open the “Step 2.exe” file with VBReFormer Professional and getting the following result:

VBReFormer VB decompiler screenshot 1

  • We will now take a look to the first method loaded on Visual Basic application.

VBReFormer VB decompiler screenshot 2

We can see on this capture that the “Label4” visibility is set to False (not visible) at the beginning of the application.

Take a look to that control in the resource editor of VBReFormer and you will agree that it’s the control that show the message “Registered user!”

VBReFormer VB decompiler screenshot 3

We now need to know where the “Label4” control visibility is set to true, and what does the “Timer1” control.

  • The analysis of the Timer1 control is interesting but not very useful for the following of this tutorial.

VBReFormer VB decompiler screenshot 4

We can see here that the “Timer1_Timer” function is called every second by “Timer1” control in order to check that no debuggers, and if one is running, to close it.

We can note that it also close any MessageBox windows.

  • Now we are looking for the code under the “Try” button which check if the key match with the name.

That “Try” button is the “Command1” button in VBReFormer:

VBReFormer VB decompiler screenshot 5

Then just look to the Command1_Click() function in order to see the algorithm of key checking:

clip_image013

VBReFormer VB decompiler screenshot 6

The algorithm seems a little complicated for newbie, but complete and without any syntax and source code error from VBReFormer.

That’s a great thing for us; we will be able to test the application into the Visual Basic IDE later (to make a key generator for example).

By analyzing the code we can see the following:


 

This part of code is showing us that the key is generated from the Name, but also with the Date and the Time !

That’s meaning it’s almost impossible to generate a key that does not expire the following second.

  • In order to made the Key Generator, save the project with VBReFormer, and open it with Visual Basic 6.

When it’s opened into the Visual Basic IDE, remove the debugger watching functions and just keep the following:

o Command1_Click

o Command2_Click

Now remove the following conditions block from Command1_Click function:


 

These block are showing an alert when the “Name” field and when the “Key” field are empty, but it’s not usefull for a keygen.

At the end of the Command1_Click function we can see the serial check condition:


 

That code is checking that the serial (stored in var_pv19 variable) generated from the name with the algorithm is the same than the one entered in the “Serial” field (Text2.Text).

To show the generated serial, we just need to replace that condition block by the following line of code:

VBReFormer VB decompiler screenshot 7

You must also remove the following line of code which remove the content of the both fields:


 

After all change and simplifications, we have the following keygen code:


 

  • We now have to test our keygen:

yudi's Step 2 (CrackMe) / Screenshot 2

  • The first window is the windows of our KeyGen created from the original crackme, and the second window is the one of the original Crackme, with the key from the KeyGen.

The result is that our keygen work perfectly!

Just note that the use of date and time make your key valid for only 1 minute after having generated it.

Is it possible to bypass that limitation?

Yes it is ! In fact, to get the “Registered user!” message you even don’t need a key generator. By reading the code you can see that the operator used to perform a comparison between the both string key is the “Like” operator.

The “like” operator allows to comparate a string and a pattern…

Then you just can set “*” into the serial field and you will have a key which will be valid at anytime, with any name:

yudi's Step 2 (CrackMe) / Screenshot 3

 

Source code of the key generator can be downloaded here:

http://www.decompiler-vb.net/documentation/crackmes/step_2.zip

Enjoy it !