Sky Security Seminar: Alin Tomescu (Aptos Labs) – UTT: Sensibly-Anonymous Decentralized Payments without zkSNARKs

Speaker: Alin Tomescu

Title: UTT: Sensibly-Anonymous Decentralized Payments without zkSNARKs

In-person location: Soda 465H

Abstract: We present UTT, a system for decentralized e-cash with accountable privacy.
UTT is the first e-cash system that obtains three critical properties: (1) it provides decentralized trust by implementing the ledger, bank, auditor, and registration authorities via threshold cryptography and Byzantine Fault Tolerant infrastructure; (2) it balances accountability and privacy by implementing anonymity budgets: users can anonymously send payments, but only up to a limited amount of currency per month. Past this point, transactions can either be made public or subjected to customizable auditing rules; (3) by carefully choosing cryptographic building blocks and co-designing the cryptography and decentralization, UTT is tailored for high throughput and low latency. With a combination of optimized cryptographic building blocks and vertical scaling (optimistic concurrency control), UTT can provide almost 1,000 payments with accountable privacy per second, with latencies of around 100 milliseconds and less. Through horizontal scaling (multiple shards), UTT can scale to tens of thousands of such transactions per second. With 60 shards we measure over 10,000 transactions with accountable privacy per second, with latencies around 500 milliseconds.
We formally define and prove the security of UTT using an MPC-style ideal functionality. Along the way, we define a new MPC framework that captures the security of reactive functionalities in a stand-alone setting, thus filling an important gap in the MPC literature. Our new framework is compatible with practical instantiations of cryptographic primitives and provides a trade-off between concrete efficiency and provable security that may be also useful for future work.

Bio: Alin is a Research Scientist at Aptos Labs. He received his PhD from MIT in 2020. Alin’s research focus is on designing, proving and implementing new cryptographic primitives for real-world use, including authenticated data structures, threshold cryptography and anonymous-yet-accountable payment schemes. Alin was previously a Research Scientist at VMware, where he worked on anonymous payments for central banking, and Head of Research & Development at Private Machines where he worked on building tamper-proof clouds. Alin often muses about cryptography and other fantastic beasts on his blog at