Methods, apparatus, and systems for authenticating a user taking into account measurement values of characteristics of the purported environment of the user are described. Specifically, in a preferred embodiment, a device is used that comprises a sensor for making said measurement of the location dependent physical property; a memory component for storing a secret value; and a data processing component for generating an electronic signature over said measurement by cryptographically combining said measurement with a secret key comprised in or derived from said secret value.