Paper 2024/502

Best of Two Worlds: Efficient, Usable and Auditable Biometric ABC on the Blockchain

Abstract

In [1], 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 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 aBio 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 [2] by adapting the recently introduced core/helper setting of [3]. 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 [1]- does not need to store aBio. 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 recently introduced key derivation scheme of [4] and the modified ABC scheme of [2] are employed together. As a result, a total overhead of 500 milliseconds to a showing of a comparable non-biometric ABC is obtained instead of the 2.1 seconds in [1] apart from the removal of computationally expensive pairings. Finally, as different from [1], 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.

Note: The appendix is only intended as a supplementary material.