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I am working with an older piece of software that generates encrypted backups that I'd like to decrypt. I have a plaintext key, a few examples of encrypted data and I'm pretty sure the data is ASCII, but I don't know how they derive the RC4 key from the plaintext key.

How would I go about testing key derivations comprehensively? The space must be relatively small, even if I'm considering all the RC4 key lengths.

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    $\begingroup$ Have you tried just inserting the plaintext key into the RC4 algorithm as is? It does take variable length (1-256 byte) keys, so that's the obvious way they might have implemented it. $\endgroup$
    – poncho
    Jan 17, 2022 at 1:44
  • $\begingroup$ @poncho The key itself is 237 bytes long - I believe that's too long, isn't it? $\endgroup$
    – slipheed
    Jan 17, 2022 at 2:11
  • $\begingroup$ No, it's not too long; RC4 can handle keys up to 256 bytes in length $\endgroup$
    – poncho
    Jan 17, 2022 at 3:22
  • $\begingroup$ @poncho, thanks that turned out to be it! It was a raw key. $\endgroup$
    – slipheed
    Jan 18, 2022 at 0:29

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How would I go about testing key derivations comprehensively?

Why don't you start with the most obvious possibility first? RC4 takes a variable length (1-256 byte) key; perhaps they used the key you present directly as into to the RC4 key setup algorithm?

[From your comments, we already know this answer is the correct one - this is just so that the question has an official answer]

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