Comment by arianvanp

These aren't synced over iCloud

What you're thinking of are Passkeys. Which are synced. Somebody would have to write an SecurityKeyProvider that talks to the Passkey API instead.

Actually I don't think it's completely impossible. The only thing is that passkeys are origin-bound. They belong to a specific AppBundle ID or domain name. If say Secretive would add passkey support then that specific public/private keypair can't be used by another app. Though it does sync across instances of the app across devices.