Programmer corner
Cracking an encrypted dll scheme: Virtual Turntables 1.5
by Johnny+X
31 July 1998
here
Courtesy of Reverser's page of reverse engineering
reverser's comments
Yeah! After so many years we finally begin to see programmers doing something correctly! This kind of 'concealed double schemes' are a good idea (that we have proposed so many times!):
"Shareware programmers should deal with these problems implementing random checks (say once every four or five days or once every ten runs of their program) that trigger - not immediately! - a COMPLETELY UNRELATED "real" protection scheme if the code has been patched or simply enough and VERY effective, if softice and wdasm are on the same harddisk as the target :-)
Best choice is - as usual - an auto-crippling scheme and an automated email once on line (this scares the hell out of a newbie :-)
No strings in this part of the protection: everything should be build dynamically ONLY IF THE SCHEME HAS BEEN TRIGGERED. Crackers that have found strings in the 'smoke' protection part will judge the shareware author a moron and won't usually (unless they zen the code) seek dynamically created strings elsewhere.".
Of course Virtual Turntables' programmer as Johnny+X writes, "has been very sneaky in that all the non-limiting stuff (nags, REGISTER! menus etc.) is in the main executable, where anyone can bypass them if they feel like it... but the real guts of the protection (the time limit) is hidden away inside the encrypted InitSound.dat... Is this to lull us into a false sense of security, to throw us off the track?"
This is a very good and interesting essay for intermediate crackers and protectors alike.
Title
Cracking Carrot Innovations' Virtual Turntables 1.5
Written by Johnny+X on July 25rd, 1998.
Rating
( )Beginner (x)Intermediate ( )Advanced ( )Expert
An understanding of how to create a dead listing of a program is required, as well as a basic understanding of procedure calling.
Introduction
Well, this is my first essay, and my first successful attempt at cracking something a little more complicated than a "good consumer/naughty cracker" routine. Although it might seem simple to begin with, there's more to it than meets the eye ;) Some may call this essay unfocused; I prefer to refer to it as 'comprehensive' ;)
Tools Required
Windows Disassembler v8.x
Hex editor (I use HIEW)
FileMon
Cocktail of your choice, to be sipped as desired.
I recommend Diesel (half a pint of cider, some blackcurrent juice, then half a pint of beer, then add cranberry-flavoured vodka to taste, if so desired); a brute force drink for brute force cracking ;)
Program History
I don't have any earlier versions, so I can't compare it to its previous incarnations.
T
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Ok, first things first; fire up VTT and let's see what we're up against. Ok, there's a nag screen *click* and a splash screen (although technically I suppose it's a nag, since you have to click to go on) *click* and an ugly REGISTER! menu too... *20 minutes later* ...and a time limit as well. *sigh* It seems the author doesn't want us to enjoy the unregistered version... ;) Oh well, let's get on with it then. Click REGISTER! and you can see that there are 3 things to enter; your name, your registration code, and your registration key. Let's try fiddling it so it accepts anything. Type anything into the boxes and click ok. Surprise surpise, our serial isn't correct ;) Note the message ("Incorrect registration code..."), then load vtt.exe into w32dasm. Once it's disassembled, search through the strings for "Incorrect registration code..." Jump to it in the program, and you'll see the following:
* Reference To: Devices.?CheckRegistered@@YAHXZ, 
Ord:0026h
                                  |
:00408E6C E8BB960000              call 0041252C
:00408E71 83F801                  cmp eax, 00000001
:00408E74 6A00                    push 00000000
:00408E76 6A00                    push 00000000
:00408E78 7407                    je 00408E81

* StringData Ref from Data Obj ->"Incorrect registration code... "
                                        ->" Did you type it in EXACTLY as "
                                        ->"it appears?"
                                  |
:00408E7A 6844C84100              push 0041C844
:00408E7F EB05                    jmp 00408E86

* Referenced by a Jump at Address:00408E78(C)
|

* Possible StringData Ref from Data Obj ->"Registration complete!  Thank "
                                        ->"you for purchasing VTT.  Please "
                                        ->"restart VTT in order to activate "
                                        ->"the new version"
                                  |
:00408E81 6890C84100              push 0041C890

There's a tasty-looking reference up there (can you spot it? ;), but we'll leave that for now; as you can see, all we need to do to make it accept anything we throw at it is to change it from je 408E81 to jmp 408E81 (EB07, as you should know ;) So, make the change (at offset 8278h) and try registering VTT again. Success! All we have to do is restart it and we'll have a fully-registered version, yes? Ah, no. If only things could be that easy. There's obviously a check elsewhere; maybe it's hidden in that CheckRegistered we saw above? That's in devices.dll, so load that in and take a look. Um, problem: CheckRegistered seems to refer to some outside code, but where is it? Searching through the other dll's reveals nothing; maybe there's a dll hidden in windows\system? Time to fire up FileMon and have a look.

Nope, no new dll's. But something interesting does happen; the file InitSound.dat and a temporary file are alternately read from and written to. Look at InitSound.dat in a hex editor; it's jumbled crap. Now look at the temporary file; it's program code! A dll in fact. Obviously, InitSound.dat contains the serial validation routine, and is encrypted to keep it away from prying eyes (i.e. us :), then decrypted at run-time. Of course, this also means that we can't modify it without somehow re-encrypting it. Doesn't it? ;)

Of course not! After all, we're resourceful crackers :) What we need to do is to replace InitSound.dat with a decrypted version (which we can then crack), then bypassing the decryption code so that InitSound.dat is written straight to the temp file without decryption. So, copy the temp file to your VTT dir, then quit VTT. Rename InitSound.dat to InitSound.bak, then rename the temp file to Initsound.dat. Also, notice the file lengths of the original InitSound.dat and the unencrypted version; the original is a byte longer. This may (and indeed, it does) cause problems, so put an extra byte on the end of it (00 will do).

Now to find the decryption code. There's probably a logical way of doing this; for me, it was mostly a process of trial & error. Reload vtt.exe into W32dsm and press ctrl+l; this will allow us to trace through the program, rather like SoftIce does. Click 'Load' (since we don't want to supply any command line parameters) and you should be presented with the debugging windows. On the left we can look at the various registers, set breakpoints, etc. while on the right we can see the executed code, as well as the program execution controls. Now we need to set a breakpoint in the appropriate place. Scroll through the 'Active DLLs' box in the left-hand window so you see devices.dll. Now double-click it, then click Yes. The dead listing of devices.dll is loaded into the main W32dsm window. We want a breakpoint where the manipulation of the temporary file begins, so search through the string references for 'tmp' (or look through the imported functions for KERNEL32.GetTempFileNameA), then press F2 to set a breakpoint (a yellow square should appear where the breakpoint is set). Now press F9 to begin program execution.

After a few seconds the program should stop running, and we'll be at the breakpoint. Now single-step through using F8 (step over -- the equivalent of F10 in SoftIce). You can speed things up by identifying loops and setting a breakpoint just after them, then pressing F9 to quickly skip through the loop (for example, there's a loop which terminates at 10005C13, so when you get there move down a little and set a breakpoint at 10005C15, then press F9. Remember that code you've already executed has its address highlighted red). Make a note of all calls to internal routines; we'll be investigating these later. Continue until you get to 10005CB8, where the temp file is closed (and therefore finished with. You can check this by looking by dir'ing the windows\temp directory; the file should have a file length of 13824 bytes -- if it was still being written to its length would appear to be 0). We now know that the encryption lies in the calls we have noted. These are call 10001000 and call 100019d0. Let's look at 10001000 first. Terminate the process (or you can press F9 to continue running it, and verify that it doesn't work). Look through the code for any calls. There's one at 10001036; call 10001050. Follow that, and look through the code that follows. This routine screamed "decrypter!" at me. I can't explain why; maybe I'm beginning to 'feel' the code? Anyway, let's remove it. Overwrite E815000000 (call 10001050) at 10001036 (offset 436h) with EB00EB0090 (two 'blank' jumps -- i.e. jumps to the next line along, and a NOP) and run VTT again. Allow yourself a little smile when you see that beloved nag appear :) The decryption routine has now been bypassed completely. You can check this yourself by typing:
fc /b initso~1.dat c:\windows\temp\tmpXXXX.tmp
at a DOS prompt (where tmpXXXX.tmp is the name of the newly-created temp file). Apart from the fact that one is longer than the other (remember we had to add an extra byte?), they're perfectly identical. Now all we have to do is crack InitSound.dat and we're done!

Ok, load InitSound.dat into W32dsm. Remember CheckRegistered from before? Look in the exported functions and you'll see it, so double-click and we're there.

This time, set a breakpoint just before the jump we blanked out. Run through with F9 'til it hits the breakpoint, then single-step until you hit the end of the routine. Ouch! Obviously the call has to be made. But how necessary is it? Put the call back at 10001036, then move to the beginning of the routine at 10001050. Scroll down, past all the code, to the end of it; notice how it ends with ret 8? Let's replicate that. Overwrite the 515355 (push ecx, push ebx, push ebp) at the beginning of the routine (offset 450h) with C20800 (ret 8), and repeat the breakpoint/run process.

Success! This time there are no problems at the end. Now press F9 -- it crashes, so we're not finished yet. Look at the next call (call 100019D0). No decryption here, but there's another call (call 10001590). Follow that, and you'll find yourself staring at another routine that screams "decrypter!", so we'll negate it in the same way we negated the previous one; overwrite the 8B5144 (mov edx, dword ptr [ecx+44]) at 10001590 (offset 990h) with C20800 (ret 8) and try running again. Allow yourself a little smile as you see the familiar nag pop up. The decryption has been totally bypassed, leaving us with an identical copy of Initsound.dat in the temporary file. To check, type:
fc /b InitSound.dat \windows\temp\tmpXXXX.tmp
at a DOS prompt (where tmpXXXX.tmp is the name of the newly-created temp file). Now all we have to do is crack InitSound.dat and we're done! :)

Remember CheckRegistered? Let's take a look at it. Load InitSound.dat into W32dsm and take a look:
Exported fn(): DllCheckRegistered - 
Ord:0001h
:10001A80 E81B020000              call 10001CA0
:10001A85 8B0D80530010            mov ecx, dword ptr [10005380]
:10001A8B 33C0                    xor eax, eax
:10001A8D 85C9                    test ecx, ecx
:10001A8F 0F95C0                  setne al
:10001A92 C3                      ret
Call 10001CA0 is the serial checking routine. The next line moves the contents of dword ptr [10005380] to ecx. Further down ecx is checked and al set accordingly (1 if registered, otherwise 0).

Follow call 10001CA0. You are now at the beginning of the serial checking routine. Since we know dword ptr [10005380] is 1 if the serial is valid, we can look for that. Search for [10005380] until you arrive at 10001E52:
:10001E52 C7058053001001000000    mov dword ptr [10005380], 1
For some reason (my knowledge of assembly is somewhat lacking, could someone perhaps explain it? :) you can't just move this to the beginning and put a ret after it, but you can put a jump to it at the beginning and put a ret after it. So, you'd change the 6AFF6851240 (push FFFFFFFF and part of push 10002451) at 10001CA0 (offset 10A0h) to E9AD010000 (jmp 10001E52), then put a ret after the mov dword ptr [10005380], 1 at 10001E52 (i.e. change the 8D at offset 125C to C3). And there you have a cracked VTT!
Johny-X said that he could not patch the beginning of  
DllCheckRegistered. 

We are dealing with a DLL and DLL's get relocated by  windows 
during loadtime. This means simply that operands which are adresses 
will get added the image base of the DLL (start of the DLL in 
memory). So, when you patch and overwrite an instruction with an 
address as operand, the new code you put in the file will look 
totally different once loaded in memory by windows. 

If you check that function you will see it has 2 instructions at 
the beginning which have an address as operand:
       :10001A80 E81B020000              call 10001CA0
       :10001A85 8B0D80530010            mov ecx, dword ptr [10005380]
Again, if you patch these lines with other code, it will change 
once loaded to mem. 

However the above is not true for 32 bit windows exe files. They get 
almost always loaded to the same place in mem and hence relocation 
isnt necessary (new compilers dont even put reloc info in exe's by 
default.) So one should be carefull with patching over instructions 
with address (when dealing with dos or DLL files). If there is 
no other way, one could remove appropiate antries from the 
relocation table.

Razzi

Ob duh
I wont even bother explaining you that you should BUY this program if you intend to use it for a longer period than the allowed one. Should you want to STEAL this software instead, you don't need to crack its protection scheme at all: you'll find it on most Warez sites, complete and already regged, farewell.
Final Notes
Send feedback to Johnny_X_@hotmail.com, especially if you know why you can't just put the mov at the beginning of the serial check! :) Also, the programmer has been very sneaky in that all the non-limiting stuff (nags, REGISTER! menus etc.) is in the main executable, where anyone can bypass them if they feel like it... but the real guts of the protection (the time limit) is hidden away inside the encrypted InitSound.dat... Is this to lull us into a false sense of security, to throw us off the track? Remember, don't by distracted by the periphery of a protection system... head straight for the meat! :)
J+X
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