A device worn by a person to be authenticated has one or more sensors recording environmental data, which may include parameters related to the device, to the person wearing the device or the external environment of the device. The device is preferably attached to a user via a wrist strap and has a memory, processor and timer means. The data can include temperature or humidity data, stress/strain of the wrist strap, whether the device is worn or not and positional data. Data about the wearer can include vascular/cardiac pulse data including pulse shape, frequency or amplitude or respiratory data. The device processes the data and produces an indication of whether the current wearer appears to be the historic habitual wearer of the device preferably by correlating current data with stored data such as current behaviour patterns. Figure 1 shows how a private key may be generated by the devices processor to allow the wearer to be authenticated in response to an identity challenge. A signal may be sent via a wired or wireless connection.