I may be misunderstanding a few things here and if so I’ll be pleased to be corrected.
My main use case is to use 2FA for unix logins. So far this works well with the pam_duo module in a server’s pam stack. The operation is simply that pam_duo pushes an approval to the phone. But this is not quite U2F as there is no cryptographic relationship proven as part of the authentication.
The strength of the Duo push process is in the proof provided by some biometric. But this is still a level below U2F as a push is subject to an interception attack, before it reaches the device, isn’t it?
Duo has long supported U2F via some token that contains cryptographic material to use. I’m wondering if Duo has or is considering leveraging a smartphone’s TrustZone feature to store keys in order to effect U2F, rather than relying on another physical device?
My own view is that I find USB tokens, attached to my car/house/etc keys a greater level of user friction than a smartphone biometric. I find losing a phone less likely than losing keys and phones often have the advantage of a remote control feature (eg Apple’s Find My Phone) that helps (disabling, locating, alerting) in such cases.