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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 ...


2

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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Yes, it is correct. Just follow the bits in the decryption pictures on the Wikipedia page about modes of operation. Modes of operation don't have to have a meaning compared to other modes of operation. I don't see CFB or OFB used too much anymore. OFB with partial feedback has been shown to be less secure, so that shouldn't be used anymore. Currently the ...


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Actually, I think I found the answer to my question while writing it, but I'll post it anyway, since it might be interesting to others: Yes, OFB mode is secure even with 8-bit feedback, at least as long as IVs are chosen randomly. Specifically, in the paper "New proof for old modes" (IACR Cryptology ePrint Archive, 2008), which I've cited earlier here, ...


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No OFB_DRBG isn't directly vulnerable to attacks. However from a theoretical standpoint there is one point speaking against it and in favor of CTR_DRBG. Speed. CTR can be parallelized (can encrypt counters in parallel), OFB can't (encrypting state over and over again) So the main reason is speed I'd guess and of course adding OFB_DRBG wouldn't have added ...



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