. Est-a,

, retrieves the permanent identifier id A and epoch i A by decrypting ebid A , as i

K. , U. Sre, and A. Lee-a,

, If mac A is incorrect, the message is silently rejected

, User Notified) in order to check that the user has not already received a "At Risk" notification, 6. verifies that U N A is "f alse

, verifies that (i ? SRE A ) >= T , where T is minimum number of epochs between two consecutive ESR REQU EST . Otherwise, the message is silently rejected

, If the ESR REQU EST A,i is valid, the server: 1. updates SRE A with the current epoch number, i

, Two situations are then possible: ? If the computed score indicates that the user is at risk of exposure, the server sets U N A at "true". It means that App App A cannot perform any new request until user U A is tested and her status updated. An ESR REP LY A,i message set to

, ? If the computed score does not indicate any significant risk, an ESR REP LY A,i message set to

, Decentralized privacy-preserving proximity tracing

R. Canetti, A. Trachtenberg, and M. Varia, Anonymous collocation discovery: Harnessing privacy to tame the coronavirus, 2020.

, Joel Hellewell et a. Feasibility of controlling covid-19 outbreaks by isolation of cases and contacts. The Lancet, 2020.

L. Ferretti, C. Wymant, M. Kendall, L. Zhao, A. Nurtay et al., Quantifying sars-cov-2 transmission suggests epidemic control with digital contact tracing, Science, 2020.

S. Bluetooth, Bluetooth Core Specification

S. Vaudenay, Analysis of DP3T. Cryptology ePrint Archive, 2020.