Bankrupting Sybil Despite Churn

TL;DR

ERGO is a novel Sybil defense mechanism that ensures a persistent minority of malicious IDs and significantly reduces resource consumption by good IDs under attack, outperforming prior methods both theoretically and empirically.

Contribution

ERGO introduces a new Sybil defense guaranteeing a minority of bad IDs and asymptotically optimal resource efficiency for good IDs during attacks, even with high churn rates.

Findings

ERGO maintains a Sybil minority under various attack conditions.

ERGO reduces resource burning for good IDs by up to 3 orders of magnitude.

Empirical results show ERGO outperforms previous defenses in resource efficiency.

Abstract

A Sybil attack occurs when an adversary controls multiple identifiers (IDs) in a system. Limiting the number of Sybil (bad) IDs to a minority is critical to the use of well-established tools for tolerating malicious behavior, such as Byzantine agreement and secure multiparty computation. A popular technique for enforcing a Sybil minority is resource burning: the verifiable consumption of a network resource, such as computational power, bandwidth, or memory. Unfortunately, typical defenses based on resource burning require non-Sybil (good) IDs to consume at least as many resources as the adversary. Additionally, they have a high resource burning cost, even when the system membership is relatively stable. Here, we present a new Sybil defense, ERGO, that guarantees (1) there is always a minority of bad IDs; and (2) when the system is under significant attack, the good IDs consume asymptotically less resources than the bad. In particular, for churn rate that can vary exponentially, the resource burning rate for good IDs under ERGO is O(\sqrt{TJ} + J), where T is the resource burning rate of the adversary, and J is the join rate of good IDs. We show this resource burning rate is asymptotically optimal for a large class of algorithms. We empirically evaluate ERGO alongside prior Sybil defenses. Additionally, we show that ERGO can be combined with machine learning techniques for classifying Sybil IDs, while preserving its theoretical guarantees. Based on our experiments comparing ERGO with two previous Sybil defenses, ERGO improves on the amount of resource burning relative to the adversary by up to 2 orders of magnitude without machine learning, and up to 3 orders of magnitude using machine learning.