Can I consistently derive a private key that isn’t the PassKey from a PassKey?
Eg. so that I could use that derived key in-application across multiple different devices without having to use a both-online-at-the-same-time exchange method (eg. Diffie-Hellman) to securely share a generated key between devices?
There's a PRF extension to Webauthn, which can be used to derive a secret from a FIDO2 credential such as a passkey. Now, some browsers and password managers may support it and some may not, but it is available.