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I'm given a Plaintext/Ciphertext pair, a Ciphertext to break, and the IV. One thing of note is that the same IV is used for both DES Encryptions, so $$C = E_{K_2}(E_{K_1}(P, IV), IV)$$ I have limited resources, so brute-forcing a Meet-in-the-Middle attack of complexity $2^{56}$ is unfeasible. What other options do I have for attacking OFB Mode?

EDIT: It has now been revealed to me that both $K_1$ and $K_2$ are weak keys. Hopefully this gives enough info to crack it, however I'm unfamiliar with how to approach weak keys, what do I do with them?

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  • $\begingroup$ Is this a CTF or homework? See also Wikipedia about weak keys. $\endgroup$ – kelalaka Sep 14 at 4:04
  • $\begingroup$ @kelalaka CTF, but aimed at beginners. I'm just trying to learn, really. I think the "weak key" hint will probably let me solve it on my own, though. $\endgroup$ – PolarBITS Sep 14 at 4:06
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Read up on weak keys on Wikipedia.

The weak keys are the ones that result in the left and right key halves at the output of the Permuted Choice function being either all 1's or all 0's. So you can test all possibilities for weak keys to break the OFB mode.

What is more, if weak keys are used DES becomes idempotent, thus double encryption gives the plaintext back. This, however, would normally only work in ECB mode.

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