The Password Verification Process in .NET

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The Password Verification Process
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One basic step that is relatively simple and is likely to reveal much about how Cryptex goes about its business is to find out how it knows whether or not the user has typed the correct password. This will also be a good indicator of whether or not Cryptex is secure (depending on whether the password or some version of it is actually stored in the archive).
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Catching the Bad Password Message
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The easiest way to go about checking Cryptex s password verification process is to create an archive (Test1.crx from earlier in this chapter would do just fine), and to start Cryptex in a debugger, feeding it with an incorrect password. You would then try to catch the place in the code where Cryptex notifies the user that a bad password has been supplied. This is easy to accomplish because you know from Listing 6.2 that Cryptex uses the printf runtime library function. It is very likely that you ll be able to catch a printf call that contains the bad password message, and trace back from that call to see how Cryptex made the decision to print that message. Start by loading the program in any debugger, preferably a user-mode one such as WinDbg or OllyDbg (I personally picked OllyDbg), and placing a breakpoint on the printf function from MSVCR71.DLL. Notice that unlike the previous reversing session where you relied exclusively on dead listing,
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this time you have a real program to work with, so you can easily perform this reversing session from within a debugger. Before actually launching the program you must also set the launch parameters so that Cryptex knows which archive you re trying to open. Keep in mind that you must type an incorrect password, so that Cryptex generates its incorrect password message. As for which command to have Cryptex perform, it would probably be best to just have Cryptex list the files in the archive, so that nothing is actually written into the archive (though Cryptex is unlikely to change anything when supplied with a bad password anyway). I personally used Cryptex l test1 6666666665, and placed a breakpoint on printf from the MSVCR71.DLL (using the Executable Modules window in OllyDbg and then listing its exports in the Names window). Upon starting the program, three calls to printf were caught. The first contained the Cryptex 1.0 . . . message, the second contained the Listing all file . . . message, and the third contained what you were looking for: the ERROR: Invalid password . . . string. From here, all you must do is jump back to the caller and hopefully locate the logic that decides whether to accept or reject the password that was passed in. Once you hit that third printf, you can use Ctrl+F9 in Olly to go to the RET instruction that will take you directly into the function that made the call to printf. This function is given in Listing 6.3.
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004011C0 004011C1 004011C2 004011C6 004011C8 004011CA 004011CC 004011CD 004011D3 004011D5 004011D9 004011DA 004011DC 004011E1 004011E2 004011E8 004011EA 004011EC 004011ED 004011EE 004011EF 004011F9 004011FB PUSH ECX PUSH ESI MOV ESI,SS:[ESP+C] PUSH 0 ; Origin = FILE_BEGIN PUSH 0 ; pOffsetHi = NULL PUSH 0 ; OffsetLo = 0 PUSH ESI ; hFile CALL DS:[<&KERNEL32.SetFilePointer>] PUSH 0 ; pOverlapped = NULL LEA EAX,SS:[ESP+8] PUSH EAX ; pBytesRead PUSH 28 ; BytesToRead = 28 (40.) PUSH cryptex.00406058 ; Buffer = cryptex.00406058 PUSH ESI ; hFile CALL DS:[<&KERNEL32.ReadFile>] ; ReadFile TEST EAX,EAX JNZ SHORT cryptex.004011EF POP ESI POP ECX RETN CMP DWORD PTR DS:[406058],70597243 JNZ SHORT cryptex.0040123C CMP DWORD PTR DS:[40605C],39586554
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Listing 6.3 Cryptex s header-verification function that reads the Cryptex archive header and checks the supplied password.
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