status: Published

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, situations that occur in 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 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 which gives the first insights into feasibility of PCS. 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 $x_S$ of the signer and the attributes $x_R$ of the receiver fulfill some global policy $F(x_S,x_R)$. 3) The signature can be created by the signer just knowing the recipient's public key; there is no further interaction needed and no information is 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 public keys without the involvement of an authority, we formally show how ul-PCS is a step towards improving regulatory compliance of privacy coins such as Monero or Zcash.

Realizing access control to sensitive data offloaded to a Cloud is challenging in the Internet of Things, where various devices with low computational power and different security levels are interconnected. Despite various solutions, the National Institute of Standards and Technology (NIST)’s Attribute-Based Access Control (ABAC) model is one of the preferred techniques in the literature. In this model, users who satisfy access policies using both static and dynamic attributes are allowed to access the data. However, NIST’s ABAC model does not support encryption and therefore does not satisfy data confidentiality. Attribute-Based Encryption (ABE) is a known cryptographic primitive that enables fine-grained access control over encrypted data. However, currently the existing ABE schemes do not meet NIST’s ABAC requirements or are not computationally efficient enough for IoT applications. In this paper, we propose a Multi-Level Security ABAC (MLS-ABAC) scheme that satisfies the requirements of NIST’s ABAC model. Our construction is efficient and relies on a decryption outsourceable Ciphertext-Policy ABE scheme. Additionally, based on realistic application scenarios, only the authorized data users can decrypt the ciphertext, and check the integrity of the retrieved message. Furthermore, we present both conceptual and formal models for our proposed MLS-ABAC architecture along with performance metrics. The experimental results show that the proposed MLS-ABAC achieves a constant ciphertext size of ∼230 bytes and with encryption and decryption running times of ∼18 and ∼10 ms, respectively, independent of the number of attributes.