Physical-layer authentication based on the channel response over a wireless medium are described. In a first authentication scheme, a count is kept of how many significant multipath taps remain empty or occupied from one channel estimate to the next for authentication purposes. In a second authentication scheme, which does not require an alternative means of initial authentication, the presence of a multipath component based on a reported position of a transmitter and the side knowledge of planar reflectors in the environment may be leveraged for authentication purposes.