The invention relates to an authentication system (14) comprising a data terminal (1) with a data terminal device, a communication network (15), and an authentication service (16), said data terminal device having an image capturing device (3), an image preparing module (4), and a wireless communication interface (5). A communication connection (17) is established between the data terminal device and the authentication service (16) via the communication network (15), said data terminal (1) being provided in the form of a watch.