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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 Feb 1 '14 at 20:48
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If the key is always the same and there is no IV (or fixed IV), then

$$plaintext1 \oplus ciphertext1 \oplus ciphertext2 = plaintext2$$

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