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Herd: A Scalable, Traff i c Analysis ResistantAnonymity Network for VoIP SystemsStevens Le BlondMPI-SWSDavid ChoffnesNortheastern UniversityWilliam CaldwellMPI-SWSPeter DruschelMPI-SWSNicholas MerrittMPI-SWSABSTRACTEffectively anonymizing Voice-over-IP (VoIP) calls requiresascalableanonymitynetworkthatisresilienttotraff i canaly-sisandhassuff i cientlylowdelayforhigh-qualityvoicecalls.The popular Tor anonymity network, for instance, is not de-signed for the former and cannot typically achieve the latter.In this paper, we present the design, implementation, and ex-perimental evaluation of Herd, an anonymity network wherea set of dedicated, fully interconnected cloud-based prox-ies yield suitably low-delay circuits, while untrusted super-peers add scalability. Herd provides caller/callee anonymityamong the clients within a trust zone (e.g., jurisdiction) andunder a strong adversarial model. Simulations based on atrace of 370 million mobile phone calls among 10.8 millionusers indicate that Herd achieves anonymity among millionsof clients with low bandwidth requirements, and that super-peers decrease the bandwidth and CPU requirements of thetrusted infrastructure by an order of magnitude. Finally, ex-periments using a prototype deployment on Amazon EC2show that Herd has a delay low enough for high-quality callsin most cases.CCS Concepts•Networks → Network privacy and anonymity;KeywordsAnonymity networks; Voice-over-IP; Intersection attacks;Strong anonymityPermission to make digital or hard copies of part or all of this work for personalor classroom use is granted without fee provided that copies are not made ordistributed for prof i t or commercial advantage and that copies bear this noticeand the full citation on the f i rst page. Copyrights for third-party components ofthis work must be honored.SIGCOMM ’15 August 17-21, 2015, London, United Kingdomc ? 2015 Copyright held by the owner/author(s).ACM ISBN 978-1-4503-3542-3/15/08.DOI: http://dx.doi.org/10.1145/2785956.27874911. INTRODUCTIONVoice-over-IP (VoIP) systems are popular and their sup-port for pseudonymous identities and strong encryptionmakes them appealing to users concerned about communi-cation privacy. However, many nation states use Internetsurveillance to monitor and repress critics [18, 14], and evenwestern democracies conduct mass surveillance [9, 10, 8, 4].Inthisenvironment, VoIPsystemsarenotsuff i cienttoensuretheanonymityofInternetusers, northesafetyofpoliticalac-tivists and whistleblowers, as illustrated, for example, by theNSA’s ability to intercept Skype conversations [13].Further, existing anonymity networks are not designed forlatency suff i ciently low to support VoIP, or scalability to mil-lionsofusersunderastrongadversarialmodel. Forexample,Tor [28] scales to large numbers of users but is vulnerable toan adversary who can eavesdrop on the ingress and egresspaths of Tor circuits. Recent revelations indicate that theUK’s GCHQ is pursuing this to deanonymize Tor users [1].We demonstrate in Section 4 that a simple attack usingonly the start and end times would trace 98.3% of the calls ofa large, real voice workload, if they had been made over Tor.(A more sophisticated attack that also considers the time se-riesofencryptedpacketswouldlikelytraceevenmorecalls.)In addition, Tor typically incurs round trip delays between2–4 seconds on established, sender-anonymous circuits be-cause of random proxy selection and high-latency connec-tions [43, 15], which is prohibitive for VoIP.Other anonymity systems resist traff i c analysis at the costof performance or scalability [37, 47, 26, 30, 45]. For exam-ple, Dissent, the state-of-the-art Dining Cryptographers net-work (DC-Net), is resilient under a strong threat model butrequires one broadcast channel per anonymity set, impact-ing anonymity and scalability, as well as several round-triptimes, yielding delays signif i cantly higher than Tor [47].In this paper, we present the design, implementation, andevaluation of Herd, a scalable anonymity network that com-bines delays suitable for VoIP with anonymity under a strongadversarial model. Herd achieves these properties using anovel design tailored to the specif i c requirements and traff i ccharacteristics of VoIP. In particular, (1) Herd leverages the639Publication rights licensed to ACM.Request Permissions from Permissions@acm.org.