The advantage of turning mobile devices into secure authenticators is that everybody has one. It is a much more secure factor than a browser, can identify a user through a biometric factor, and has a built-in secure memory. In our solution, the mobile device stores one of the shares that constructs the secret, meaning that only two out of the three shares are sent over the network for each authentication session. This means that the authentication notification we send can only go to the intended user and the intended mobile device.