Utilizing Public Blockchains for the Sybil-Resistant Bootstrapping of Distributed Anonymity Services

Distributed anonymity services, such as onion routing networks or cryptocurrency tumblers, promise privacy protection without trusted third parties. While the security of these services is often well-researched, security implications of their required bootstrapping processes are usually neglected: Users either jointly conduct the anonymization themselves, or they need to rely on a set of non-colluding privacy peers. However, the typically small number of privacy peers enable single adversaries to mimic distributed services. We thus present AnonBoot, a Sybil-resistant medium to securely bootstrap distributed anonymity services via public blockchains. AnonBoot enforces that peers periodically create a small proof of work to refresh their eligibility for providing secure anonymity services. A pseudo-random, locally replicable bootstrapping process using on-chain entropy then prevents biasing the election of eligible peers. Our evaluation using Bitcoin as AnonBoot's underlying blockchain shows its feasibility to maintain a trustworthy repository of 1000 peers with only a small storage footprint while supporting arbitrarily large user bases on top of most blockchains.Comment: To be published in the proceedings of the 15th ACM ASIA Conference on Computer and Communications Security (ACM ASIACCS'20

Short Paper: Towards Characterizing Sybil Attacks in Cryptocurrency Mixers

Sybil attacks are a well-studied problem in peer-to-peer networking systems. However, their relevance to cryptocurrency mixers has received little attention in the literature, with only a few papers in recent times aiming to design mixers that are resistant to Sybil attacks. A lot of the research has been primarily driven by independent cryptocurrency enthusiasts. We attempt to provide a few characterizations of Sybil attacks as they pertain to mixers and provide mitigations based on economics in order to disincentive Sybil attacks against mixers. In doing so, we highlight that the security of mixers need not only be analyzed through the use of cryptographic techniques but also with the use of economic techniques. Moreover, we provide future research directions in determining heuristics for detecting Sybil identities in mixers

TumbleBit: an untrusted Bitcoin-compatible anonymous payment hub

This paper presents TumbleBit, a new unidirectional unlinkable payment hub that is fully compatible with today s Bitcoin protocol. TumbleBit allows parties to make fast, anonymous, off-blockchain payments through an untrusted intermediary called the Tumbler. TumbleBits anonymity properties are similar to classic Chaumian eCash: no one, not even the Tumbler, can link a payment from its payer to its payee. Every payment made via TumbleBit is backed by bitcoins, and comes with a guarantee that Tumbler can neither violate anonymity, nor steal bitcoins, nor print money by issuing payments to itself. We prove the security of TumbleBit using the real/ideal world paradigm and the random oracle model. Security follows from the standard RSA assumption and ECDSA unforgeability. We implement TumbleBit, mix payments from 800 users and show that TumbleBits offblockchain payments can complete in seconds.https://eprint.iacr.org/2016/575.pdfPublished versio