Crowdsourcing applications are vulnerable to Sybil attacks where attackers create many accounts to submit bogus or malicious data at scale. The traditional approach to manage Sybil attacks is privacy invasive since it requires contributors to identify themselves when contributing data. In this paper we present a new reporting protocol which supports the anonymous submission of data to crowdsourcing systems by honest contributors, while identifying malicious individuals who attempt to submit multiple reports. Our approach builds on Chaum’s digital cash, and we demonstrate its practicality and deployability on mobile devices based on its low storage, network, runtime, and power requirements.