Processing the Username in .NET

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004029C9 004029D1 004029D9 004029E1 004029E9 6D 64 78 6C 3E 65 69 61 20 0A 3E 67 64 6E 00 20 69 65 75 3C 74 63 6D 31 20 69 62 36 68 6D 65 2D 65 61 72 me> <16digit he xadecima l number >..
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So, you ve obviously reached the bad parameters message display code. There is no need to examine this code you should just get into the good parameters code sequence and see what it does. Looks like you re close!
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Jumping to 402AC4, you will see that it s not that simple. There s quite a bit of code still left to go. The code first performs some kind of numeric processing sequence on the username string. The sequence computes a modulo 48 on each character, and that modulo is used for performing a left shift on the character. One interesting detail about this left shift is that it is implemented in a dedicated, somewhat complicated function. Here s the listing for the shifting function:
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00401681 00401684 00401686 00401689 0040168B 0040168E 00401690 00401691 00401693 00401695 00401698 0040169A 0040169B 0040169D 0040169F CMP CL,40 JNB SHORT Defender.0040169B CMP CL,20 JNB SHORT Defender.00401691 SHLD EDX,EAX,CL SHL EAX,CL RETN MOV EDX,EAX XOR EAX,EAX AND CL,1F SHL EDX,CL RETN XOR EAX,EAX XOR EDX,EDX RETN
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This code appears to be a 64-bit left-shifting logic. CL contains the number of bits to shift, and EDX:EAX contains the number being shifted. In the case of a full-blown 64-bit left shift, the function uses the SHLD instruction. The SHLD instruction is not exactly a 64-bit shifting instruction, because it doesn t shift the bits in EAX; it only uses EAX as a source of bits to shift into EDX. That s why the function also needs to use a regular SHL on EAX in case it s shifting less than 32 bits to the left.
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After the 64-bit left-shifting function returns, you get into the following code:
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00402B1C 00402B22 00402B28 00402B2A 00402B30 ADD MOV ADC MOV MOV EAX,DWORD ECX,DWORD ECX,EDX DWORD PTR DWORD PTR PTR SS:[EBP-190] PTR SS:[EBP-18C] SS:[EBP-190],EAX SS:[EBP-18C],ECX
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Figure 11.16 shows what this sequence does in mathematical notation. Essentially, Defender is preparing a 64-bit integer that uniquely represents the username string by taking each character and adding it at a unique bit position in the 64-bit integer. The function proceeds to perform a similar, but slightly less complicated conversion on the serial number. Here, it just takes the 16 hexadecimal digits and directly converts them into a 64-bit integer. Once it has that integer it calls into 401EBC, pushing both 64-bit integers into the stack. At this point, you re hoping to find some kind of verification logic in 401EBC that you can easily understand. If so, you ll have cracked Defender!
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Of course, 401EBC is also encrypted, but there s something different about this sequence. Instead of having a hard-coded decryption key for the XOR operation or read it from a global variable, this function is calling into another function (at 401D18) to obtain the key. Once 401D18 returns, the function stores its return value at [EBP-1C] where it is used during the decryption process.
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Figure 11.16 Equation used by Defender to convert username string to a 64-bit value.
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Let s step into this function at 401D18 to determine how it produces the decryption key. As soon as you enter this function, you realize that you have a bit of a problem: It is also encrypted. Of course, the question now is where does the decryption key for this function come from There are two code sequences that appear to be relevant. When the function starts, it performs the following:
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00401D1F 00401D22 00401D29 MOV EAX,DWORD PTR SS:[EBP+8] IMUL EAX,DWORD PTR DS:[406020] MOV DWORD PTR SS:[EBP-10],EAX
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This sequence takes the low-order word of the name integer that was produced earlier and multiplies it with a global variable at [406020]. If you go back to the function that obtained the volume serial number, you will see that it was stored at [406020]. So, Defender is multiplying the low part of the name integer with the volume serial number, and storing the result in [EBP10]. The next sequence that appears related is part of the decryption loop:
00401D7B 00401D7E 00401D81 00401D83 00401D86 MOV MOV SUB MOV XOR EAX,DWORD ECX,DWORD ECX,EAX EAX,DWORD ECX,DWORD PTR SS:[EBP+10] PTR SS:[EBP-10] PTR SS:[EBP-28] PTR DS:[EAX]
This sequence subtracts the parameter at [EBP+10] from the result of the previous multiplication, and XORs that value against the encrypted function! Essentially Defender is doing Key = (NameInt * VolumeSerial) LOWPART(SerialNumber). Smells like trouble! Let the decryption routine complete the decryption, and try to step into the decrypted code. Here s what the beginning of the decrypted code looks like (this is quite random your milage may vary).
00401E32 00401E33 00401E34 00401E37 00401E3D 00401E3E PUSHFD AAS ADD BYTE PTR DS:[EDI],-22 AND DH,BYTE PTR DS:[EAX+B84CCD0] LODS BYTE PTR DS:[ESI] INS DWORD PTR ES:[EDI],DX
It is quite easy to see that this is meaningless junk. It looks like the decryption failed. But still, it looks like Defender is going to try to execute this code! What happens now really depends on which debugger you re dealing with, but Defender doesn t just go away. Instead it prints its lovely Sorry . . . Bad Key. message. It looks like the top-level exception handler installed earlier is the one generating this message. Defender is just crashing because of the bad code in the function you just studied, and the exception handler is printing the message.