## Cryptology ePrint Archive: Report 2020/670

Inverse-Sybil Attacks in Automated Contact Tracing

Crypto Group at IST Austria

Abstract: Automated contract tracing aims at helping with the current COVID-19 pandemic by alerting users of encounters with infected people. There are currently many proposals for protocols (like the decentralized" DP-3T and PACT or the centralized" ROBERT and DESIRE) to be run on mobile phones, where the basic idea is to regularly broadcast (using low energy Bluetooth) some values, and at the same time store (a function of) incoming messages broadcasted by users in their proximity. Should a user get diagnosed, he can upload some data to a backend server, which then is used to alert users that were in proximity with the diagnosed user.

There are many important aspects one wants those protocols to achieve, in particular simplicity/efficiency, privacy and robustness, the latter including some security against false positives, that is, users getting alerts despite not having been in proximity with a diagnosed user.

In the existing proposals one can trigger false positives on a massive scale by an inverse-Sybil" attack, where a large number of devices (malicious users or hacked phones) pretend to be the same user, such that later, just a single person needs to be diagnosed (and allowed to upload) to trigger an alert for all users that were in proximity to any of this large group of devices.

We propose the first protocols that do not succumb to such attacks assuming the devices involved in the attack do not constantly communicate, which we observe is a necessary assumption. Our first protocol requires devices to non-interactively exchange values (like e.g. in DESIRE), while the second requires that the devices have access to some location dependent coordinate (like coarse GPS coordinates or cell tower IDs).

The high level idea of the protocols is to derive the values to be broadcasted by a hash chain, so that two (or more) devices who want to launch an inverse Sybil attack will not be able to connect their respective chains and thus only one of them will be able to upload. Apart from achieving strong privacy and good efficiency, a main challenge is to force the chains on different devices to divert, which we do by infusing unpredictable data (randomness from encounters in the first, location data in the second protocol). Our protocols also achieve security against replay, and the second even against relay attacks.

Category / Keywords: cryptographic protocols / COVID-19, tracking, security, privacy, inverse-sybil