Radio Frequency IDentification, or RFID, is a ubiquitous technology found across many industries, but which is susceptible to breaches of information security. This research introduces analogue fingerprints as a means to authenticate passive RFID tags. An authentication model implemented at the physical layer of a passive RFID tag, using analogue fingerprints is proposed. The use of analogue computing principles increases the amount of potential authentication data whilst reducing the potential for counterfeiting.