On Measuring Linkability of Multiple Protected Biometric Templates Using Maximal Leakage

With the rapid development of biometric recognition systems, users can be simultaneously enrolled in multiple biometric recognition systems, either with a single or multiple biometric characteristics (e.g., face, voice, etc.). With such a growth of biometric systems, it is important to secure the sensitive information used within these systems. In particular, considering the privacy issues in such systems, several biometric template protection schemes are proposed in the literature. According to the ISO/IEC 24745 standard, each template protection scheme should satisfy the unlinkability property. While previous measures to evaluate unlinkability were based on two protected templates, the adversary may have access to more information. Such information can correspond to multiple templates from different biometric systems, a single multi-modal biometric system, or even a single unimodal biometric system. In this paper, we focus on measuring the linkability of multiple protected biometric templates, and define maximal linkability in the presence of multiple similarity scores. We define different scenarios where the adversary gains access to multiple similarity scores and evaluate the linkability of protected templates in each scenario. We investigate the theoretical properties of the maximal linkability measure, and compare the theoretical prediction with the calculated linkability of the compositive systems in our experiments. To our knowledge, this is the first work on measuring the linkability of multiple protected biometric templates. The source codes of our measure and all experiments are publicly available.