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The question has morphed over time. I am answering the following. So to be sure, with DES, only when you encrypt something twice with a weak key. You get the back the original plaintext? That is correct as that is the definition of a DES weak key, a key for which encryption and decryption have the same effect. So when using DES in OFB mode with a ...

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An important point for both the one-time pad as well as other (synchronous) stream ciphers is: don't reuse your key stream. For the one-time pad, the key stream is the key itself, so this means: don't use a key twice. The two-times-pad is broken. For OFB and other stream ciphers with an initialization vector (IV), the key stream is decided by both key and ...

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OFB is a method of converting a block cipher, like AES or DES, into a stream cipher. Because One Time Pad is theoretically secure (though impractical) people often try to make stream ciphers that try to approximate one time pad. OFB is a way of taking a block cipher to try to emulate the security you get from one time pad. A key distinction is that OTP is ...

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A quick follow up, there is a problem with using DES in OFB mode when you are not using the full feedback register. The generated keystream will become cyclic with on average a period of the order $2^{32}$ instead of $2^{64}$. See (R.R. Jueneman, “Analysis of certain aspects of Output Feedback Mode,” Advances in Cryptology, Proceedings Crypto’82, D. ...

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