I'm reading Understanding Cryptography by Christof Paar and Jan Pelzl. In chapter 2 (Stream Ciphers) there's a question that goes like this:

Assume we have a stream cipher whose period is quite short. We happen to know that the period is 150–200 bit in length. We assume that we do not know anything else about the internals of the stream cipher. In particular, we should not assume that it is a simple LFSR. For simplicity, assume that English text in ASCII format is being encrypted.

Describe in detail how such a cipher can be attacked. Specify exactly what Oscar has to know in terms of plaintext/ciphertext, and how he can decrypt all ciphertext.

I'm assuming that a Known-Plaintext attack would work for this particular scenario. So, the attacker would have to know some plaintext. My question is could the plain-text be derived by performing LFA (Letter Frequency Analysis)? Since the question assume that English text in ASCII format is being encrypted ? If not how can the attacker guess some of the plaintext ?

Or if Known-Plaintext is an infeasible attack what other attack could be feasible ?

  • 2
    $\begingroup$ If the period is small then you have some form of the Vigenere cipher. $\endgroup$
    – kelalaka
    May 15, 2022 at 21:11
  • $\begingroup$ I'm guessing that the question assumes that XOR is used for creating the ciphertext, and that it is as such not plaintext aware. $\endgroup$
    – Maarten Bodewes
    May 18, 2022 at 14:48
  • $\begingroup$ @MaartenBodewes that's what I assumed. $\endgroup$
    – KetDog
    May 18, 2022 at 20:32


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