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I need to crack a stream cipher. I have a set of plain text and encrypted text pairs.

The hint to the question asks me to use entropy analysis and look at the hex dump of the encrypted text. However, I couldn't find anything useful in either of them.

What should be the approach to solve this cipher?

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  • $\begingroup$ Try xoring two ciphertexts and look at the entropy. Also, to recover the encryption stream, xor the pt and ct. Then you could compare multiple encryption streams. $\endgroup$
    – mikeazo
    Commented Feb 1, 2014 at 20:48

2 Answers 2

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If the key is always the same and there is no IV (or fixed IV), then

$$\text{plaintext1} \oplus \text{ciphertext1} \oplus \text{ciphertext2} = \text{plaintext2}$$

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If the key is constantly changing, the stream cipher is probably as secure as a OTP. By brute force, several plain texts can be achieved after significant time but which plain text is correct requires human guessing.

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