Efficient Densest Flow Queries in Transaction Flow Networks

Transaction flow networks are crucial in detecting illicit activities such as wash trading, credit card fraud, cashback arbitrage fraud, and money laundering. Our collaborator, Grab, a leader in digital payments in Southeast Asia, faces increasingly sophisticated fraud patterns in its transaction flow networks. In industry settings such as Grab's fraud detection pipeline, identifying fraudulent activities heavily relies on detecting dense flows within transaction networks. Motivated by this practical foundation, we propose theS-T densest flow(STDF) query. Given a transaction flow networkG, a source setS, a sink setT, and a size thresholdk, the query outputs subsetsS^⊆ SandT^⊆ Tsuch that the maximum flow fromS^toT^is densest, withS^∪ T^≥ k. Recognizing the NP-hardness of the STDF query, we develop an efficient divide-and-conquer algorithm,Conan. Driven by industry needs for scalable and efficient solutions, we introduce an approximate flow-peeling algorithm to optimize the performance ofConan, enhancing its efficiency in processing large transaction networks. Our approach has been integrated into Grab's fraud detection scenario, resulting in significant improvements in identifying fraudulent activities. Experiments show thatConan, outperforms baseline methods by up to three orders of magnitude in runtime and more effectively identifies the densest flows. We showcaseConan's applications in fraud detection on transaction flow networks from our industry partner, Grab, and on non-fungible tokens (NFTs).