Abstract: A method performed by a client access device includes (1) receiving, at the client access device, a signal from a client authorizing device, the signal including an environmental detection instruction, the environmental detection instruction instructing the client access device to detect an aspect of a local environment, (2) detecting, at the client access device, the aspect of the environment indicated by the environmental detection instruction to yield a first environmental detection result, (3) sending the first environmental detection result from the client access device to a remote server, and (4) in response to sending the environmental detection result to the remote server, receiving a proximity signal from the remote server indicating whether or not proximity between the client access device and the client authorizing device has been established by comparing the first environmental detection result to a second environmental detection result sent from the client authorizing device to the server.