The invention relates to a method for generating a data authentication key for allowing data communication over a wireless channel between a first mobile device and a second mobile device, comprising: associating said mobile devices to each other by means of said key. The invention comprises: allowing a shared physical or mechanical condition to be applied generally simultaneously upon said devices; detecting, in said first mobile device, said condition; defining, in said first mobile device, said authentication key based on the detected condition; and transmitting said key to said second mobile device. The invention also relates to a mobile device configured for generating a data authentication key for allowing data communication over a wireless channel to a further mobile device.