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I'm trying to understand the KSA algorithm of RC4 which apparently scrambles the array which will later be used by the PRGA to generate the keystream. My question is, does the seed used by the KSA have to be kept secret?

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Yes, the seed to the KSA in RC4 is essentially the secret key for the stream cipher. With the secret key (the only input to the KSA) an adversary can recreate the keystream, and thereby recover any information encrypted under that key.

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    $\begingroup$ Er, isn't there an IV in there somewhere too? In stuff like WEP? Otherwise you'd have to hard persist the state. $\endgroup$ – Paul Uszak Feb 28 '18 at 0:27
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    $\begingroup$ WEP != RC4. WEP concatenates the IV with the key to form the seed for RC4, so knowing the seed is equivalent to knowing both, and again allows you to generate the keystream. $\endgroup$ – puzzlepalace Feb 28 '18 at 0:32

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