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Badertscher, C., Sedaghat, M., & Waldner, H. 2023. Fine-grained accountable privacy via unlinkable policy-compliant signatures. [Cryptology ePrint Archive, Paper 2023/1070]. 
Added by: Rucknium (2024-01-05 22:07)   Last edited by: Rucknium (2024-01-05 22:08)
Resource type: Miscellaneous
BibTeX citation key: Badertscher2023
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Categories: Not Monero-focused
Creators: Badertscher, Sedaghat, Waldner
Views: 87/205
Attachments   2023-1070.pdf [26/50] URLs
Privacy-preserving payment systems face the difficult task of balancing privacy and accountability: on one hand, users should be able to transact privately and anonymously, on the other hand, no illegal activities should be tolerated. The challenging question of finding the right balance lies at the core of the research on accountable privacy that stipulates the use of cryptographic techniques for policy enforcement, but still allows an authority to revoke the anonymity of transactions whenever such an automatic enforcement is technically not supported. Current state-of-the-art systems are only able to enforce rather limited policies, such as spending or transaction limits, or assertions about participants, but are unable to enforce more complex policies that for example jointly evaluate both, the private credentials of sender and recipient-let alone to do this without an auditor in the loop during payment. This limits the cases where privacy revocation can be avoided as the method to fulfill regulations, which is unsatisfactory from a data-protection viewpoint and shows the need for cryptographic solutions that are able to elevate accountable privacy to a more fine-grained level.

In this work, we present such a solution. We show how to enforce complex policies while offering strong privacy and anonymity guarantees by enhancing the notion of policy-compliant signatures (PCS) introduced by Badertscher, Matt and Waldner (TCC'21). In more detail, we first define the notion of unlinkable PCS (ul-PCS) and show how this cryptographic primitive can be generically integrated with a wide range of systems including UTxO-based ledgers, privacy-preserving protocols like Monero or Zcash, and central-bank digital currencies. We give a generic construction for ul-PCS for any policy, and optimized constructions tailored for special policy classes, such as role-based policies and separable policies.

To bridge the gap between theory and practice, we provide prototype implementations for all our schemes. We give the first benchmarks for policy-compliant signatures in general, and demonstrate their feasibility for reasonably sized attribute sets for the special cases.
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