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What RC4 key value will completely invert $S$ after the initial permutation, so that $S[0] = 255, S[1] = 254, …, S[254] = 1, S[255] = 0$?

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I'll only give hints on this apparent homework. A) If you could achieve your goal with a key of k bytes, with what other key lengths and keys could you achieve the same goal? Now choose a k that you won't regret. B) Each round swaps S[i] and S[j]; what latitude do you have on deciding indexes i and j? C) Pick and place according to your target S[], whatever that is. –  fgrieu Feb 25 at 11:36
    
In case you're wondering why no one is stepping up to give you the answer, well, the whole point of homework is for you to learn. If someone would just give you the answer, well, that might mean that you can hand in your homework, but you won't learn from it. –  poncho Mar 3 at 22:01

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