> Now, however, the server can no longer understand the changes you send. If you want to see your friend’s latest changes, you’ll need to both be online at the same time.
What? No, the server sends you the changes you've not seen yet, you decrypt and merge them, and so you get the latest version of the document. Right?
The homomorphic encryption is a fascinating topic, but it's almost never an answer if you need anything resembling reasonable performance and/or reasonable bandwidth.
I've seen a paper that ingeniuously uses homomorphic encryption to implement arbitrary algorithmic computations, totally secret, by encoding a (custom-crafted) CPU together with RAM and then running "tick a clock" algorithm on them. And it works, so you can borrow some AWS huge instance and run you super-important calculations there — at 1 Hz. I am not kidding, it's literally 1 virtual CPU instruction per second. Well, if you are okay with such speed and costs, you either have very small data — at which point just run your computation locally, or you're really, really rich — at which point just buy your own goddamn hardware and, again, run it locally.
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