1
$\begingroup$

Assume the receiver have a wrong IV (initialization vector), in the CFB mode only the first block of plaintext is wrong but in OFB mode the second and all blocks will be affected.

Is that correct? If it is, what is the meaning of the OFB mode compared with CFB mode?

$\endgroup$
4
$\begingroup$

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 cryptographic community doesn't seem to care too much about error propagation anymore. Instead, everybody tries to make sure that an authentication tag over the IV and ciphertext is present. If that is the case then errors can be detected and the message can be discarded. One of the reasons is that it is often possible to alter the ciphertext in transport protocols (e.g. when open WiFi hotspots are used), and few protocols handle partially correct data that well.

So you are much better off looking at authenticated modes such as GCM and EAX modes of operation.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.