The invention relates to a portable, electronic device such as a mobile phone. The device is provided with a transponder and may be used for authentication purposes. The device includes means for writing user-specific information into a memory unit, included in the transponder. Thus, when an interrogating reading device stimulates the transponder, the transponder emits the user-specific information. This enables authentication of a user rather than of a device. In a preferred embodiment use is made of a user-specific key, by means of which a single-use code is generated, which is used as the emitted user-specific information. The invention also relates to a method and a system, which may work in connection with such a device.