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I've been thinking a little bit about it, and now I think it is possible, but you have to consider the generalization of CFB in ISO 10116 (I don't have access to the ISO 10116 standard, so I will assume that the description by Rogaway is correct). The generalization of CFB from the ISO standard seems to have two main changes: The feedback block (FB), of ...

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I think it is now valid to answer this question as this course is likely to be over. First observer how CTR-mode works: $C_1=E_K(IV)\oplus P_1$ As you can see, there's a linear relation between the plaintext and the ciphertext. You now use $C_1$ (observed) and $P_1$ (known). You want to make $C'_1$ decrypt to $P_1'$. To obtain this you first construct ...

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A predictable nonce that cannot be controlled by the adversary is safe as a CFB IV (with some assumptions), as shown in the other answers. However, a nonce that can be chosen by an adversary is not safe against chosen plaintext attacks, as shown in Evaluation of Some Blockcipher Modes of Operation (page 36): Assume s = n. The adversary asks its oracle to ...

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I have a problem with OFB mode, because I have heard that it is stronger than CFB. On the contrary I would say that CFB is stronger. OFB means encrypting the IV again and again to produce the keystream. If you end up in a cycle, the keystream will start repeating itself. (This should not be a practical weakness, but why chance it?) CFB is more like ...

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Forget OFB mode. You should use CTR (counter) mode. It has the best bounds, and is parallelizable. This means that when you are using the AES-NI instruction set, encrypt with CTR is about 7 times faster than CBC, OFB etc. If you encrypt in OpenSSL you will get this performance. For a good thorough analysis and comparison of modes of operation, see ...

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XTS has been designed for disk encryption, where an attacker typically has access to the disk only a single time (when they steal/confiscate the device). When an attacker sees several ciphertexts encrypted using the same key, they can tell which blocks differ between the versions, but not the content of the blocks. Compare this with CTR mode, which leaks ...

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