Score:1

Can you use Output Feedback Mode (OFB) to construct a Message Authentication Code (MAC)

mx flag

Suppose $f_k$ is an arbitrary block cipher. Can OFB via $f_k$ be used to make a MAC. I argue that you can't even with fixed l, $m = l \times k$ because in OFB, $\mu = f_k(f_k(...((IV))) \oplus m$, and with an oracle, you would be able to get $\mu$ for a given message and find $f_k(f_k(...(IV)))$ and so can find $\mu$ for any message. However, I'm not sure I've used the oracle correctly, that is I'm not sure I can use the MAC provided by the oracle in such a way. Any help would be appreciated.

mangohost

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.