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this is from an IT Security exercise which my friend and me cannot solve.

Given

  • 2 AES-CTR stream-cipher encrypted files
  • Same known initalizing vector
  • Same (unkown) key
  • Same 128bit Counter initialized with 0
  • Files are .png so there is known plain text in both files (the file header)

Our approach was to use the known equivalence of

$E(A) \oplus E(B) = A \oplus B$

From there the only option we see is to brute force $A$ and $B$ for the known $\oplus$ product.

Is there a smarter way to get $A$, $B$? Thanks for any hints!

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  • $\begingroup$ Possible duplicate of Why must IV/key-pairs not be reused in CTR mode? That is two-time pad. $\endgroup$ – kelalaka Nov 29 '19 at 17:12
  • $\begingroup$ I agree, because there aren't any other mathematical equivalences to work with, so you are left guessing the bit values within $A$ and $B$ (not sure if you'd call that brute force though). How to do that depends on the type of message - in this case PNG pixels. That you have the header doesn't make it any easier. $\endgroup$ – Maarten Bodewes Nov 29 '19 at 17:40
  • $\begingroup$ Comment below if you have a generic method of getting the original values within PNG files that are XOR'ed together. Happy to reopen for that. $\endgroup$ – Maarten Bodewes Nov 29 '19 at 17:41

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