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Unlinkable Policy-Compliant Signatures for Compliant and Decentralized Anonymous Payments

July/2024, To appear at: PETS '24

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. Current state-of-the-art systems are only able to enforce rather limited policies, such as spending or transaction limits, or assertions about single participants, but are unable to enforce more complex policies that for example jointly evaluate both, the private credentials of sender and recipient, such as admissible cross-border payments, let alone to do this without auditors in the loop during payment. This severely limits the cases where decentralized virtual assets can be used in accordance with regulatory compliance such as the Financial Action Task Force (FATF) travel rule, while further retaining strong privacy features.

We present unlinkable Policy-Compliant Signatures (ul-PCS), an enhanced cryptographic primitive extending the work of Badertscher et al. (TCC 21). We give rigorous definitions, formally proven constructions, and benchmarks using our prototype developed using CharmCrypto. Unlinkable PCS has the following unique combination of features:

1. It is an enhanced signature scheme where the public key encodes in a privacy-preserving way the user's verifiable credentials (obtained from a credential authority).

2. Signatures can be created (and later publicly verified) by additionally specifying a recipient's public key aside of the to-be-signed message. A valid signature can only ever be created if the attributes xS of the signer and the attributes xR of the receiver fulfill some global policy F(xS, xR).

3. The signature can be created by the signer just knowing the recipient's public key; there is no further interaction needed no attributes are leaked (beyond the validity of the policy).

4. Once credentials are obtained, a user can generate fresh public keys without interacting with the credential authority.

By merging the act of signing a transaction with the act of providing an assurance about the involved participants being compliant with complex policies, yet retain that participants are able to change addresses without the involvement of an authority, we show how ul-PCS constitutes a crucial step towards achieving a technology that improves regulatory compliance of privacy coins such as Monero or Zcash.