HN user
In the case of single key FHE with server-side processing (the demo setup), the server can never gains any insight, even as an active adversary that can arbitrarily deviate from the protocol because it only has access to public evaluation keys and ciphertexts. The worst it can do is return tempered output to the client, but it doesn't help it learning anything about the client request.
A local model works when one party owns everything. But often you'll have more than one party, for example different machine operators, the machine manufacturer, the sensor provider, all involved, each with data they cannot or IP they do not want share. That’s where FHE becomes truly useful and more than an additional layer of protection by enabling collaborations that were simply not possible before.
This is specific to the current state of the demo. Additional techniques can reduce fresh encryptions to only a few times the plaintext size, but they have not been deployed yet.
It uses randomized symmetric encryption with fresh public randomness for every encryption. Ciphertexts are computationally indistinguishable from random and do not reveal whether two encryptions correspond to the same plaintext, different plaintexts, the same key, or different keys.