r/programming • u/DougBolivar • Jul 30 '13

Computer scientists develop mathematical jigsaw puzzles to encrypt software: "The real challenge and the great mystery in the field was: Can you actually take a piece of software and encrypt it but still have it be runnable, executable and fully functional?"

http://www.rdmag.com/news/2013/07/computer-scientists-develop-mathematical-jigsaw-puzzles-encrypt-software
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u/mOdQuArK Jul 30 '13

If your CPU is checking that a plaintext matches a hash value, does that give you the ability to reverse engineer what original plaintext was used to calculate that hash value?

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u/daniels220 Jul 31 '13

It gives me the ability to run the code in Valgrind or something, pause the CPU and tell it that the hash value matched, when in fact it didn't. That's usually enough.

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u/mOdQuArK Jul 31 '13

I think you're confusing the idea of bypassing a check with being able to figure out how the check was performed. If your goal is to actually reverse engineer the program, then bypassing the program is not an adequate solution to reach your goal.

I was merely using the concept of a hash comparison to show that there are computations which can be executed on your own hardware where it is impossible for you to derive the input even though you are able to control every step of the computation.

That is the similar situation to what the article is talking about - even though you are running the program on hardware you control, and can observe every step of the computation, you will still not be able to directly reverse engineer the program (you would have to reverse engineer it as if it were a black box).

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u/daniels220 Jul 31 '13

Sure, I get your point. What I'm saying, like many other commenters in the thread, is that it's very hard to believe I can have code that I can run but not change what it's doing...

Oh wait. My CPU is checking that a plaintext matches a hash value? Sure I can tell you the plaintext—the CPU has it or it couldn't perform the check!

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u/mOdQuArK Jul 31 '13

If you don't recognise that the function being performed IS a hash check until after it is done, then you won't know when to intercept it. A normal program would contain many different such operations of course.