Mitigating fraud through privacy-enhancing technologies

The PAYMENT project, part of the National Research Centre on Privacy, Harm Reduction and Adversarial Influence Online (REPHRAIN) focuses on the application of privacy-enahcning technology to address fraud, including transparency around financial institutions compliance with their level of care, without disclosing sensitive details about the institutions or rules; and profiling typical customer behaviour so as to quantify the definition of “gross negligence” without violating customer privacy.

Papers

Earn While You Reveal: Private Set Intersection that Rewards Participants

Aydin Abadi and Steven J. Murdoch (preprint)

Abstract: In Private Set Intersection protocols (PSIs), a non-empty result always reveals something about the private input sets of the parties. Moreover, in various variants of PSI, not all parties necessarily receive or are interested in the result. Nevertheless, to date, the literature has assumed that those parties who do not receive or are not interested in the result still contribute their private input sets to the PSI for free, although doing so would cost them their privacy. In this work, for the first time, we propose a multi-party PSI, called “Anesidora”, that rewards parties who contribute their private input sets to the protocol. Anesidora is efficient; it mainly relies on symmetric key primitives and its computation and communication complexities are linear with the number of parties and set cardinality. It remains secure even if the majority of parties are corrupted by active colluding adversaries.

A Forward-secure Efficient Two-factor Authentication Protocol

Steven J. Murdoch and Aydin Abadi (preprint)

Abstract: Two-factor authentication (2FA) schemes that rely on a combination of knowledge factors (e.g., PIN) and device possession have gained popularity. Some of these schemes remain secure even against strong adversaries that (a) observe the traffic between a client and server, and (b) have physical access to the client’s device, or its PIN, or breach the server. However, these solutions have several shortcomings; namely, they (i) require a client to remember multiple secret values to prove its identity, (ii) involve several modular exponentiations, and (iii) are in the nonstandard random oracle model. In this work, we present a 2FA protocol that resists such a strong adversary while addressing the above shortcomings. Our protocol requires a client to remember only a single secret value/PIN, does not involve any modular exponentiations, and is in a standard model. It is the first one that offers these features without using trusted chipsets. This protocol also imposes up to 40% lower communication overhead than the state-of-the-art solutions do.

Recurring Contingent Service Payment

Aydin Abadi, Steven J. Murdoch, Thomas Zacharias (preprint)

Fair exchange protocols let two mutually distrustful parties exchange digital data in a way that neither party can cheat. They have various applications such as the exchange of digital items, or the exchange of digital coins and digital services between a buyer and seller. At CCS 2017, two blockchain-based protocols were proposed to support the fair exchange of digital coins and a certain service; namely, “proofs of retrievability” (PoR). In this work, we show that these protocols are susceptible to a free-riding attack which lets a buyer/client receive the service without paying the seller/server. We also show that these protocols are not suitable for cases where parties’ privacy matters, e.g., when the server’s proof status or buyer’s file size must remain private from the public. To simultaneously mitigate the attack and preserve the parties’ privacy, we formally define and propose a generic blockchain-based construction called “recurring contingent service payment” (RC-SP). RC-S-P lets a fair exchange of digital coins and verifiable service reoccur securely while ensuring that the server is paid (if and only if it delivers a valid service) and the parties’ privacy is preserved. It supports arbitrary verifiable services, such as PoR or verifiable computation and imposes low on-chain overheads. Also, we present a concrete efficient instantiation of RC-S-P when the verifiable service is PoR. We have implemented the concrete instantiation and analysed its cost. When it deals with a 4-GB outsourced file, a verifier can check a proof in 90 milliseconds, and a dispute between prover and verifier is resolved in 0.1 milliseconds.

Payment with Dispute Resolution: A Protocol For Reimbursing Frauds Victims

Aydin Abadi and Steven J. Murdoch (preprint)

Abstract: An” Authorised Push Payment”(APP) fraud refers to the case where fraudsters deceive a victim to make payments to bank accounts controlled by them. The total amount of money stolen via APP frauds is swiftly growing. Although regulators have provided guidelines to improve victims’ protection, the guidelines are vague and the victims are not receiving sufficient protection. To facilitate victims’ reimbursement, in this work, we propose a protocol called” Payment with Dispute Resolution”(PwDR) and formally define it. The protocol lets an honest victim prove its innocence to a third-party dispute resolver while preserving the protocol participants’ privacy. It makes black-box use of a standard online banking system. We evaluate its asymptotic cost and runtime via a prototype implementation. Our evaluation indicates that the protocol is efficient. It imposes only O (1) overheads to the customer and bank. Also, it takes a dispute resolver 0.09 milliseconds to settle a dispute between the two parties.

Multi-party Updatable Delegated Private Set Intersection

Aydin Abadi, Changyu Dong, Steven J. Murdoch, Sotirios Terzis (published at Financial Cryptography 2022)

Abstract: With the growth of cloud computing, the need arises for Private Set Intersection protocols (PSI) that can let parties outsource the storage of their private sets and securely delegate PSI computation to a cloud server. The existing delegated PSIs have two major limitations; namely, they cannot support (1) efficient updates on outsourced sets and (2) efficient PSI among multiple clients. This paper presents “Feather”, the first lightweight delegated PSI that addresses both limitations simultaneously. It lets clients independently prepare and upload their private sets to the cloud once, then delegate the computation an unlimited number of times. We implemented Feather and compared its costs with the state of the art delegated PSIs. The evaluation shows that Feather is more efficient computationally, in both update and PSI computation phases.

Recurring Contingent Payment for Proofs of Retrievability

Aydin Abadi, Steven J. Murdoch, Thomas Zacharias (preprint)

Abstract: Fair exchange protocols let two mutually distrusted parties exchange digital data in a way that neither can cheat. At CCS 2017, Campanelli et al. proposed two blockchain-based protocols for the fair exchange of digital coins and a certain service, ie,“proofs of retrievability”(PoR), that take place between a buyer and seller. In this work, we identify two serious issues of these schemes; namely,(1) a malicious client can waste the seller’s resources, and (2) real-time leakage of information to non-participants in the exchange. To rectify the issues, we propose “recurring contingent PoR payment”(RC-PoR-P). It lets the fair exchange reoccur while ensuring that the seller’s resources are not wasted, and the parties’ privacy is preserved. We implemented the RC-PoR-P. Our cost analysis indicates that the RCPoR-P is efficient. The RC-PoR-P is the first of its kind that offers all the above features.

Polynomial Representation Is Tricky: Maliciously Secure Private Set Intersection Revisited

Aydin Abadi, Steven J. Murdoch, Thomas Zacharias (published at ESORICS 2021, and also available as a preprint)

Abstract: Private Set Intersection protocols (PSIs) allow parties to compute the intersection of their private sets, such that nothing about the sets’ elements beyond the intersection is revealed. PSIs have a variety of applications, primarily in efficiently supporting data sharing in a privacy-preserving manner. At Eurocrypt 2019, Ghosh and Nilges proposed three efficient PSIs based on the polynomial representation of sets and proved their security against active adversaries. In this work, we show that these three PSIs are susceptible to several serious attacks. The attacks let an adversary (1) learn the correct intersection while making its victim believe that the intersection is empty, (2) learn a certain element of its victim’s set beyond the intersection, and (3) delete multiple elements of its victim’s input set. We explain why the proofs did not identify these attacks and propose a set of mitigations.