Best of two worlds: Efficient, usable and auditable biometric ABC on the blockchain

In García-Rodríguez et al. 2024, two generic constructions for biometric-based non-transferable Attribute Based Credentials (biometric ABC) are presented, which offer different trade-offs between efficiency and trust assumptions. In this paper, we focus on the second scheme denoted as BioABC-ZK that tries to remove the strong (and unrealistic) trust assumption on the Reader R, and we show that BioABC-ZK has a security flaw for a colluding R and Verifier V. Besides, BioABC-ZK lacks GDPR-compliance, which requires secure processing of biometrics, for instance in form of Fuzzy Extractors, as opposed to ($i$) storing the reference biometric template $a_{Bio}$ in the user’s mobile phone and ($ii$) processing of biometrics using an external untrusted R, whose foreign manufacturers are unlikely to adjust their products according to GDPR.

The contributions of this paper are threefold. First, we review efficient biometric ABC schemes to identify the privacy-by-design criteria for them. In view of these principles, we propose a new architecture for biometric ABC of Sarier 2021 by adapting the recently introduced core/helper setting. Briefly, a user in our modified setting is composed of a constrained core device (a SIM card) inside a helper device (a smart phone with dual SIM and face recognition feature), which – as opposed to García-Rodríguez et al. 2024 – does not need to store $a_{Bio}$. This way, the new design provides Identity Privacy without the need for an external R and/or a dedicated hardware per user such as a biometric smart card reader or a tamper proof smart card as in current hardware-bound credential systems. Besides, the new system maintains minimal hardware requirements on the SIM card – only responsible for storing ABC and helper data –, which results in easy adoption and usability without loosing efficiency, if deep face fuzzy vault and our modified ABC scheme are employed together. As a result, a total overhead of 500 ms to a showing of a comparable non-biometric ABC is obtained instead of the 2.1 s in García-Rodríguez et al. 2024 apart from the removal of computationally expensive pairings. Finally, as different from García-Rodríguez et al. 2024, auditing is achieved via Blockchain instead of proving in zero-knowledge the actual biometric matching by the user to reveal malicious behavior of R and V.