Recently, a new authentication method based on 3D signatures created in air is proposed for mobile devices [4]. The 3D signature is created in air using a properly shaped magnet (a rod or ring) taken in hand. It is based on influencing compass sensor embedded in the new generation of mobile devices. In this paper, we present implementation of this technology on a mobile device (iPhone 3GS). It can demonstrate authentication process using a gesture in the form of a 3D signature freely created in the space around the device by a magnet held in hand. Movement of the magnet in the from of a signature produces a temporal change in the magnetic field sensed by the embedded compass sensor, and can be used as a basis for authentication. As magnetic signatures are performed in 3D space, they can provide a wider choice for authentication, and they can not be easily hardcopied.