> The reason passkeys have their own name and definition is because they are meant to be a phishing-resistant primary factor that competes with the UX of passwords. And a great usability trait of passwords is that they’re convenient to use across all your devices. With a technology involving public/private keypairs, the only possible way to compete with that UX is to sync the private key across the user’s devices.

Another way would be auto-enrolling passkeys from other devices you own through a standard API. Enroll your trusted Apple device in your Google Account's settings, or your Bitwarden/KeePass, and vice-versa. When your iPhone creates a passkey at a site, iCloud notifies Google, which issues a new passkey and sends the public key to iCloud, which auto-enrolls it at the site alongside the iCloud passkey. BitWarden gets the same treatment. When you open your KeePass vault it checks Google and iCloud and picks up any pending offers for passkey enrollment and completes them.

Simple, secure, opt-in, and users control their devices and passkey vaults with minimal hassle. If a device is lost, the other services can help you automatically delete the compromised passkeys and set up your new replacement device.

Matter does something very similar with cross-compatibility between Apple and Google (and the rest of the ecosystem) when new devices get enrolled with the user's choice of PAA; the only thing missing is roughly cross-PAA enrollment but that would be just one additional trivial trust relationship in both ecosystems.