Ah, but it's a per-user secret, not per-device. So each user needs a keypair which gets shared across all of their devices, transparently but also in a way that

Ah, but it's a per-user secret, not per-device. So each user needs a keypair which gets shared across all of their devices, transparently but also in a way that protects the private key from MS. Which sounds an awful lot like the original problem of sharing secret WIFI passwords without MS being able to read them... I agree that it must be do-able, it's just tricky to make it work transparently and simply so every user benefits.