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What is the the point of an "online" mode for an authenticated cipher?

I understand what "online" means in this context. However, I have trouble coming up with applications that would benefit from such a feature.

Usually, the "messages" that are encrypted and authenticated are not too long and I think that this is due to people not wanting to decrypt "big messages" completely only to throw them away if the tag is finally invalid. So, at least for network protocols, it seems like a very good idea to insert authentication tags more frequently.

But this strategy makes the "online" feature of some AE implementation seem irrelevant especially since support for online modes makes the implementation more complicated.

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Personally I think the "online" feature isn't very important for authenticated encryption. It might be useful for large files downloaded from an untrusted but rarely actually malicious source. But usually I'd prefer a (tree)hash or signed (tree)hash over a MAC in those scenarios. –  CodesInChaos Mar 27 '14 at 17:23
related. Many schemes for constrained devices will be online because it tends to reduce the amount of temporary storage required. Whilst techincally being online doesn't guarantee this (because in theory an online cipher could collect all the previous messages in it's internal state), generally they have relatively small internal state. –  figlesquidge Mar 27 '14 at 18:16
I should add that many online modes are very simple indeed, in some ways more simple than offline modes. Possibly more of interest is whether you implement the scheme in an online fashion or not. –  figlesquidge Mar 28 '14 at 16:52
The reason I ask this is because most candidates of the CAESAR competition advertise their design to be "online". After having implemented NORX with "online support", I'm wondering whether this was really worth the effort. –  sellibitze Apr 4 '14 at 12:44

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