Secure aggregation: let the server see only "the sum of everyone's updates," never any single update
In one sentence use secure multiparty computation so the server can compute only "the sum of all clients' updates," never any single update — the single point is hidden, and inversion loses what it stands on. It's communication-efficient and robust to client dropout, and is already used in Google's production FL (Bonawitz et al., MLSys 2019 secure aggregation downgrades "the server is trusted" to "the server only sees the aggregate sum"; but it's not a cure-all — it defends against "seeing a single update," not "the aggregate sum itself leaking" or "parties colluding," so still pair it with DP.