User-level privacy

[ffuuugor]

There's an ongoing research into user-level privacy (e.g. https://openreview.net/pdf?id=74lwg5oxheC, https://proceedings.neurips.cc/paper/2021/file/ec7f346604f518906d35ef0492709f78-Paper.pdf)

There's also a demand for user-lever privacy guarantees for simulating FL tasks: FL is naturally suited for user-level accounting, but this property is not reflected in server-side simulations.

This could be implemented by changing data-loading side only. There are two options:

• Two step-sampling: first select user, then select event from that user
• User-level clipping. Instead of implementing per-sample clipping, do per-user clipping