The technology described in this document can be embodied in a computer-implemented method that includes receiving, at one or more servers from a first computing device, (i) first identification information identifying the first computing device or an application executing on the first computing device, and (ii) second identification information identifying a second computing device. The second identification information is obtained by the first computing device by detecting changes to one or more parameters of a magnetic field generated by the second computing device. The method also includes determining, by the server based on the first information, identity information of a user associated with the first computing device, and transmitting, from the one or more servers to the second computing device, the identity information, such that the identity information is usable by the second computing device to verify an access attempt by the user.