That’s part of the rationale for homomorphic encryption, even if the device performing the operations is completely compromised it never has access to the unencrypted data, so given a theoretical perfect HE system your data would still be safe.

I’m off to read this paper S I don’t understand how they are using a side channel to extract information that the operating device theoretically shouldn’t have. If there’s a measurable power usage difference over the operations that implies that the encrypted bits do leak information about what is encrypted.

In a homomorphic scheme, you have a machine A (trusted), which encrypts some data and sends it to machine B (untrusted) and B then does operations on the data and returns the results to A, which decrypts.

This attack is on machine A, not machine B. The "leak" happens as the data is being encrypted. They're sidechanneling a machine that has the plaintext on it.

Ah, I was super confused (as I said I hadn’t yet read the paper, I was hoping for TLDR at the time)