1. Introduction
The advantages offered by biometric systems in contrast to traditional PIN or token based authentication systems are well known [15]: a stronger link between the subject and the physical authentication factor (e.g. a passport), or no need to carry items, which can be lost, or to struggle to remember complex passwords. These facts have allowed a wide deployment of biometric systems in high security environments (e.g., border crossing) and everyday applications (e.g., smartphone unlocking).