Breaking Protections in .NET

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Congratulations! You ve just cracked Defender! This is quite impressive, considering that Defender is quite a complex protection technology, even compared to top-dollar commercial protection systems. If you don t fully understand every step of the process you just undertook, fear not. You should probably practice on reversing Defender a little bit and quickly go over this chapter again. You can take comfort in the fact that once you get to the point where you can easily crack Defender, you are a world-class cracker. Again, I urge you to only use this knowledge in good ways, not for stealing. Be a good cracker, not a greedy cracker.
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Protection Technologies in Defender
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Let s try and summarize the protection technologies you ve encountered in Defender and attempt to evaluate their effectiveness. This can also be seen as a good executive summary of Defender for those who aren t in the mood for 50 pages of disassembled code. First of all, it s important to understand that Defender is a relatively powerful protection compared to many commercial protection technologies, but it could definitely be improved. In fact, I intentionally limited its level of protection to make it practical to crack within the confines of this book. Were it not for these constraints, cracking would have taken a lot longer.
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Localized Function-Level Encryption
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Like many copy protection and executable packing technologies, Defender stores most of its key code in an encrypted form. This is a good design because it at least prevents crackers from elegantly loading the program in a disassembler such as IDA Pro and easily analyzing the entire program. From a livedebugging perspective encryption is good because it prevents or makes it more difficult to set breakpoints on the code. Of course, most protection schemes just encrypt the entire program using a single key that is readily available somewhere in the program. This makes it exceedingly easy to write an unpacker program that automatically decrypts the entire program and creates a new, decrypted version of the program. The beauty of Defender s encryption approach is that it makes it much more difficult to create automatic unpackers because the decryption key for each encrypted code block is obtained at runtime.
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Relatively Strong Cipher Block Chaining
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Defender uses a fairly solid, yet simple encryption algorithm called Cipher Block Chaining (CBC) (see Applied Cryptography, Second Edition by Bruce Schneier [Schneier2]). The idea is to simply XOR each plaintext block with the
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previous, encrypted block, and then to XOR the result with the key. This algorithm is quite secure and should not be compared to a simple XOR algorithm, which is highly vulnerable. In a simple XOR algorithm, the key is fairly easily retrievable as soon as you determine its length. All you have to do is find bytes that you know are encrypted within your encrypted block and XOR them with the encrypted data. The result is the key (assuming that you have at least as many bytes as the length of the key). Of course, as I ve demonstrated, a CBC is vulnerable to brute-force attacks, but for this it would be enough to just increase the key length to 64-bits or above. The real problem in copy protection technologies is that eventually the key must be available to the program, and without special hardware it is impossible to hide the key from cracker s eyes.
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Defender reencrypts each function before that function returns to the caller. This creates an (admittedly minor) inconvenience to crackers because they never get to the point where they have the entire program decrypted in memory (which is a perfect time to dump the entire decrypted program to a file and then conveniently reverse it from there).
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