# Attacking Double DES (OFB Mode) given the initialization vector and not much else? [closed]

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?

## closed as off-topic by Squeamish Ossifrage, kelalaka, AleksanderRas, Maarten Bodewes♦Oct 17 at 11:52

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• Is this a CTF or homework? See also Wikipedia about weak keys. – kelalaka Sep 14 at 4:04
• @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. – PolarBITS Sep 14 at 4:06