# RC4 : Keystream reconstruction?

First up: it's a step in some challenge found on the Internet, where we get an application and a file. The application must be feed with that file and a password (the password is the flag for the challenge).

So, after reversing the application, I found that the algorithm used is RC4 with a small change in the "S" array, which was not initialized to the identity but initialized to $S[i]= 0xFF -i$ .

The information I get from the challenge is that the file once decrypted has the INTEL HEX FORMAT which has a quiet regular format (like the : at each line).

At this point, I get some information about the keystream (i.e a partial keystream). My question is there some relation between the element of the keystream, and is it possible to reconstruct the total keystream from my partial keystream?

All what I found on the RC4 was for attacking WEP, based on the fact that it use the same (IV+key) to encrypt multiple packets, and I don't think it's possible to use this in my case.

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What challenge is this from? –  Ben Mar 30 '12 at 16:25
I don't think it's good to give here the link of the challenge because it was released on 16 of this month, So I will send it to you via PM mine is Kiyohime.Shoji[a]mail.com if you want. –  Kiyohime Shoji Mar 30 '12 at 18:54