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It looks like you ve run into a bit of a problem. You simply don t have the key that is needed in order to decrypt the success path in Defender. It looks like Defender is using the username and serial number information to generate this key, and the user must type the correct information in order to unlock the code. Of course, closely observing the code that computes the key used in the decryption reveals that there isn t just a single username/serial number pair that will unlock the code. The way this algorithm works there could probably be a valid serial number for any username typed. The only question is what should the difference be between the VolumeSerial * NameLowPart and the low part of the serial number It is likely that once you find out that difference, you will have successfully cracked Defender, but how can you do that
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Brute-Forcing Your Way through Defender
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It looks like there is no quick way to get that decryption key. There s no evidence to suggest that this decryption key is available anywhere in Defender.EXE; it probably isn t. Because the difference you re looking for is only 32 bits long, there is one option that is available to you: brute-forcing. Brute-forcing means that you let the computer go through all possible keys until it finds one that properly decrypts the code. Because this is a 32-bit key there are only 4,294,967,296 possible options. To you this may sound like a whole lot, but it s a piece of cake for your PC. To find that key, you re going to have to create a little brute-forcer program that takes the encrypted data from the program and tries to decrypt it using every key, from 0 to 4,294,967,296, until it gets back valid data from the decryption process. The question that arises is: What constitutes valid data The answer is that there s no real way to know what is valid and what isn t. You could theoretically try to run each decrypted block and see if it works, but that s extremely complicated to implement, and it would be difficult to create a process that would actually perform this task reliably. What you need is to find a token a long-enough sequence that you know is going to be in the encrypted block. This will allow you to recognize when you ve actually found the correct key. If the token is too generic, you will get thousands or even millions of hits, and you ll have no idea which is the correct key. In this particular function, you don t need an incredibly long token because it s a relatively short function. It s likely that 4 bytes will be enough if you can find 4 bytes that are definitely going to be a part of the decrypted code. You could look for something that s likely to be in the code such as those repeated calls to NtDelayExecution, but there s one thing that might be a bit easier. Remember that funny variable in the first function that was set to one and then immediately checked for a zero value You later found that the
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encrypted code contained code that sets it back to zero and jumps back to that address. If you go back to look at every encrypted function you ve gone over, they all have this same mechanism. It appears to be a generic mechanism that reencrypts the function before it returns. The local variable is apparently required to tell the prologue code whether the function is currently being encrypted or decrypted. Here are those two lines from 401D18, the function you re trying to decrypt.
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00401D49 00401D50 00401D54 MOV DWORD PTR SS:[EBP-4],1 CMP DWORD PTR SS:[EBP-4],0 JE SHORT Defender.00401DBF
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As usual, a local variable is being set to 1, and then checked for a zero value. If I m right about this, the decrypted code should contain an instruction just like the first one in the preceding sequence, except that the value being loaded is 0, not 1. Let s examine the code bytes for this instruction and determine exactly what you re looking for.
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00401D49 C745 FC 01000000 MOV DWORD PTR SS:[EBP-4],1
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Here s the OllyDbg output that includes the instruction s code bytes. It looks like this is a 7-byte sequence should be more than enough to find the key. All you have to do is modify the 01 byte to 00, to create the following sequence:
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