The Password Transformation Algorithm in .NET

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The Password Transformation Algorithm
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The easiest way to locate the algorithm that transforms the plaintext password into this 16-byte sequence is to place a memory breakpoint on the global variable
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that stores the currently typed password. This is the variable at 00405038 against which the header data was compared in Listing 6.3. In OllyDbg, a memory breakpoint can be set by opening the address (00405038) in the Dump window, right-clicking the address, and selecting Breakpoint Hardware, On write Dword. Keep in mind that you must restart the program before you do this because at the point where the bad password message is being printed this variable has already been initialized. Restart the program, place a hardware breakpoint on 00405038, and let the program run (with the same set of command-line parameters). The debugger breaks somewhere inside RSAENH.DLL, the Microsoft Enhanced Cryptographic Provider. Why is the Microsoft Enhanced Cryptographic Provider writing into a global variable from Cryptex.exe Probably because Cryptex.EXE had supplied the address of that global variable. Let s look at the stack and try to trace back and find the call made from Cryptex to the encryption engine. In tracing back through the stack in the Stack Window, you can see that we are currently running inside the CryptGetHashParam API, which was called from a function inside Cryptex. Listing 6.4 shows the code for this function.
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00402280 00402286 00402289 0040228C 0040228D 0040228F 00402291 00402296 00402297 0040229D 0040229F 004022A1 004022A5 004022A8 004022AA 004022AC 004022AD 004022AE 004022B4 004022B6 004022B9 004022BB 004022BC 004022C2 004022C4 004022C7 MOV ECX,DS:[405048] SUB ESP,8 LEA EAX,SS:[ESP] PUSH EAX PUSH 0 PUSH 0 PUSH 8003 PUSH ECX CALL DS:[<&ADVAPI32.CryptCreateHash>] TEST EAX,EAX JE SHORT cryptex.004022C2 MOV EDX,SS:[ESP+C] MOV EAX,SS:[ESP] PUSH 0 PUSH 14 PUSH EDX PUSH EAX CALL DS:[<&ADVAPI32.CryptHashData>] TEST EAX,EAX MOV ECX,SS:[ESP] JNZ SHORT cryptex.004022C8 PUSH ECX CALL DS:[<&ADVAPI32.CryptDestroyHash>] XOR EAX,EAX ADD ESP,8 RETN
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Listing 6.4 Function in Cryptex that calls into the cryptographic service provider the 16byte password-identifier value is written from within this function. (continued)
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6
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004022C8 004022CC 004022CD 004022CF 004022D3 004022D4 004022D5 004022D7 004022D8 004022E0 004022E6 004022EA 004022EB 004022ED 004022F3 004022F5 004022F6 004022F9
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MOV EAX,SS:[ESP+10] PUSH ESI PUSH 0 LEA EDX,SS:[ESP+C] PUSH EDX PUSH EAX PUSH 2 PUSH ECX MOV DWORD PTR SS:[ESP+1C],10 CALL DS:[<&ADVAPI32.CryptGetHashParam>] MOV EDX,SS:[ESP+4] PUSH EDX MOV ESI,EAX CALL DS:[<&ADVAPI32.CryptDestroyHash>] MOV EAX,ESI POP ESI ADD ESP,8 RETN
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Listing 6.4 (continued)
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Deciphering the code in Listing 6.4 is not going to be easy unless you do some reading and figure out what all of these hash APIs are about. For this purpose, you can easily go to http://msdn.microsoft.com and lookup the functions CryptCreateHash, CryptHashData, and so on. A hash is defined in MSDN as A fixed-sized result obtained by applying a mathematical function (the hashing algorithm) to an arbitrary amount of data. The CryptCreateHash function initiates the hashing of a stream of data, the CryptHashData function adds data to a specified hash object, while the CryptGetHashParam retrieves data that governs the operations of a hash object. With this (very basic) understanding, let s analyze the function in Listing 6.4 and try to determine what it does. The code starts out by creating a hash object in the CryptCreateHash call. Notice the second parameter in this call; This is how the hashing algorithm is selected. In this case, the algorithm parameter is hard-coded to 0x8003. Finding out what 0x8003 stands for is probably easiest if you look for a popular hashing algorithm identifier such as CALG_MD2 and find it in the Crypto header file, WinCrypt.H. It turns out that these identifiers are made out of several identifiers, one specifying the algorithm class (ALG_CLASS_HASH), another specifying the algorithm type (ALG_TYPE_ANY), and finally one that specifies the exact algorithm type (ALG_SID_MD2). If you calculate what 0x8003 stands for, you can see that the actual algorithm is ALG_SID_MD5.
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