Re: So if I understand this correctly?
> So does this mean that if someone mimics the behaviour of the local API and protocol used to communicate with the server, they could fraudulently send bogus authentication messages to the server?
AIUI The server holds a public key for your auth device. The auth device then signs a server-provided nonce with the private key to prove it has control.
That private key might in some way be derived from your gesture or fingerprint, but is more likely to simply be unlocked with it.
So to do what you suggest, the attacker would need to have gained a copy of the private key on your authentication device. If someone lays hands on your private key then it's game over anyway.
I suspect where this will probably fall down in practice though (aside from uptake) is there will inevitably be some crap authenticators hit the market (for example who's fingerprint reader can easily be fooled)