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Old 11-19-2009, 05:07 AM
dotnetresearcher dotnetresearcher is offline
Join Date: Dec 2008
Posts: 11
Default Help with a particular function

I'm about half way through cracking some software, very simple stuff, stripping strong names and nop'ing license checks - which is about my limit.

I've hit a problem with a particular function. I can't use reflector, so I'm in Ildasm. I need to stop an exception being thrown and have the correct object returned. I've attached the ildasm view.

I've tried nop'ing the 7A "throw", which broke it (JIT compiler internal limitation) and I've tried nop'ing the switch statement (i don't fully understand how the jump tables work) but that caused a method access exception.

Help would be appreciated. I've cracked this software before, but they've changed the protection on this version.

Edit: I've managed to get a bit further. I changed:

IL_006c: /* 2B | C8 */ br.s IL_0036


IL_006c: /* 2B | 10 */ br.s IL_007E

Which I'm *fairly* sure directed the code to the happy path. The next exception certainly indicates that that I've got further, but the exception that is being thrown now is a MethodAccessException, which I suspect means I've fouled up somewhere.

Many thanks,

Attached Files
File Type: txt problem.txt (4.1 KB, 8 views)

Last edited by dotnetresearcher : 11-19-2009 at 06:47 AM. Reason: Advancing
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Old 11-19-2009, 06:49 AM
kao kao is offline
Senior Member
Join Date: Sep 2007
Posts: 184

I replaced switch with jumps, rearranged the code and removed useless instructions. You can see the program logic now:
.method private hidebysig instance class xxx.yyy.PdfTranslationSession 
        a(class xxx.yyy.OcrEngine A_0,
          class xxx.yyy.OcrDocument A_1) cil managed
  .maxstack  3
  .locals init (class xxx.yyy.PdfTranslationSession V_0,
           int32 V_1)

  IL_0026:  ldarg.1
  IL_0027:  brtrue.s   IL_004c

  IL_006e:  ldstr      "engine"
  IL_0073:  ldstr      "Translate needs a valid OcrEngine object."
  IL_0078:  newobj     instance void [mscorlib]System.ArgumentNullException::.ctor(string,string)
  IL_007d:  throw

  IL_004c:  ldarg.1
  IL_004d:  ldstr      "pdftranslator"
  IL_0052:  callvirt   instance object xxx.yyy.OcrEngine::GetLicenseFlag(string)
  IL_0057:  brtrue.s   IL_007e

  IL_0036:  ldstr      "PDF Translator license not found."
  IL_003b:  newobj     instance void [xxx.Shared]xxx.Imaging.xxxLicenseException::.ctor(string)
  IL_0040:  throw

  IL_007e:  ldarg.0
  IL_007f:  newobj     instance void xxx.yyy.PdfTranslationSession::.ctor(class xxx.yyy.PdfTranslator)
  IL_0084:  stloc.0
  IL_0085:  ldloc.0
  IL_0086:  ldarg.1
  IL_0087:  ldarg.2
  IL_0088:  callvirt   instance void xxx.yyy.PdfTranslationSession::ConnectToEngine(class xxx.yyy.OcrEngine, class xxx.yyy.OcrDocument)
  IL_008d:  ldarg.0
  IL_008e:  ldarg.2
  IL_008f:  call       instance void xxx.yyy.PdfTranslator::a(class xxx.yyy.OcrDocument)
  IL_0094:  ldloc.0
  IL_0095:  ret
If you decide to patch something, you must make sure that the stack is right and all instructions are valid. Noping out just "throw" instruction is wrong, you should nop also corresponding "ldstr" instructions. Same restriction applies if the patched instruction is of different size than original one!

Easiest patch I can think of is: IL_0009: br.s IL_007e.
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Old 11-19-2009, 06:59 AM
dotnetresearcher dotnetresearcher is offline
Join Date: Dec 2008
Posts: 11

[Please do not reply to yourself, use the Edit button to add to your post]

Hi Kao, thanks for the pointers on nop'ing.

I did pretty much the same thing (albeit further down the method) to jump to the happy path. (see my edit in the original post).

The problem I have is that although this method now seems to complete successfully, when the caller of this method calls the next method, I get a MethodAccessException.

Any suggestions of what this might be?

Thanks again,


Aar, OK sussed it. The next method that's being called is an internal class with InternalsVisibleToAttribute specified in the manifest.

Obviously the strong names are stripped, so what's the best way of tackling this? How do I make the class public? I'd rather not go through the hassle of resigning and patching the manifests if possible.

OK scrub my last

Found how to change class and method visibility within CFF Explorer. God Damn that is a fine tool.

Last edited by Git : 11-19-2009 at 10:32 AM.
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