Software_Cracking_Intro_Ollydbg

In this reverse engineering tutorial, I will take you through cracking session of A-One Video to Audio convertor. Here we will go step by step debugging session of A-one Video to Audio convertor using OllyDbg to discover & finally zero it down on its registration protection mechanism.
Disclaimer
This tutorial is intended for educational purposes only and the author or the publisher or this site can not be held liable for any kind of damages done whatsoever to your machine, or damages caused by some other,creative application of this tutorial. In any case you disagree with the above statement, please stop here.
Requirements
A-one Video to Audio convertor (Download from yaomingsoft.com)  OllyDBG  Time & Patience
Registration Step
Download & install A-one Video to Audio convertor. Now as you can see, its a trial version & once you try to register it, it gives an error <obviously>, & we need to find ways against it.
Starting Reversing with OllyDbg
Now, to being with, fire up OllyDBG & load the A-one Video to Audio convertor EXE file in it.
Now in the main window of Ollydbg and search for text "Registration code is error" [This is the exact error message shown when you enter wrong serial number] To search you can Right click and select following action as shown in the screenshot below
CPU window -> Search for -> All Referenced Text Strings
Once OllyDbg find it, double click on it & navigate to the memory address as shown in the screenshot below
Now, once you reach the intended memory address, you can navigate a bit up to see the "register successful" string as shown below.
Navigating a bit above will get a simple logic which calls a specific function & then the function returns a result which is compared with EAX register
CMP EAX,1 & then jumps to 407A0F JNZ SHORT 00407A0F
which is exactly the "registration failed" condition [refer to screenshot above].
The whole scenario means that if value of EAX is anything less than or grater than one, the program will be a trial version & will not accept any invalid serial key. Now you can put a break point above the function call by pressing F2 & run the program. Then enter the serial again, the program will break here & we can then navigate inside the function by pressing F7
You will get into function code. Add the breakpoint there by pressing F2 & restart the program again by pressing Ctrl + F9 Run it again & you will find that it will break it at 00406B40 (where you had put the last breakpoint) Next we will execute code step by step by pressing F8.  After bit of tracing we arrive at following instructions [refer to screenshot below]
JNZ Video2Au.00406C4A which jumps below to POP EDI
and further down we find that the value of EAX is XORed to 0 as shown below.
So in order to insert a precise value into EAX, we will modify the assembly instructions as shown below [You can just double click on the instruction to modify it]
XOR EAX,EAX & changing it to MOV AL,1
This new instruction will set the accumulator's value to 1 because
EAX - 32 Bit reg <extended> AX - 16 Bit reg pair AH / AL - 8 Bit regs
where AL will represent the lower value, & setting it to one will set the accumulator to a precise value of 1, hence setting value of EAX to 1, which will lead to program being registered :) Now once you have done it, right click the code and perform below action
copy to executable-> selection.
In the next window, right click again, save the file.
Now that you have a cracked the registration protection of this software, launch the software and type any serial number. This should work like charm !
ALL CREDITS TO THE ORIGINAL AUTHOR ON  http://securityxploded.com
Conclusion
This reversing tutorial demonstrates basic reverse engineering concepts using OllyDbg on how to find out and break the protection of A-One Video Converter.  Hope you have enjoyed it doing as much as I did !