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CrowdNotifier - Decentralized Privacy-Preserving Presence Tracing

This repository puts forward a proposal for a secure, decentralized, privacy-preserving presence tracing system. Our proposal aims to simplify and accelerate the process of notifying individuals that shared a semi-public location with a SARS-CoV-2-positive person for a prolonged time without introducing new risks for users and locations. Existing proximity tracing systems (apps for contact tracing such as SwissCovid, Corona Warn App, and Immuni) notify only a subset of these people: those that were close enough for long enough. Current events have shown the need to notify all people that shared a space with a SARS-CoV-2-positive person. The proposed system aims to provide an alternative to increasing use of apps with similar intentions based on invasive collection or that are prone to abuse by authorities. Our preliminary design aims to minimize privacy and security risks for individuals and communities, while guaranteeing the highest level of data protection and good usability and deployability.

With this proposal, we seek feedback from a broad audience on the high-level design, its security and privacy properties, and the functionality it offers, so that the merits of the design can be discussed, and protection mechanisms can be added to correct weaknesses. We feel it is essential that designs be made public so the community as a whole can discuss the proposal and verify the claimed privacy guarantees before applications are deployed.


The following repositories hold further information about the project:

Work in Progress

The CrowdNotifier protocol underwent changes to improve its security and privacy properties. See CrowdNotifier for updates on the design. This reference implementation reflects the latest changes as of 2020/12/17

CrowdNotifier Reference Implementation

In order to show the different parts of CrowdNotifier from a programming point of view, this demo implements the full chain of the CrowdNotifier project as different classes. If you want to see how it works, go to As the current repo is private, stackblitz cannot reference it directly. This reference implementation has been made possible by the C4DT.

The following figures show the different components of the system:

Overview of system

This figure shows the steps to publish a trace location:

Publish Trace Location

Goal of the demo

  • Reference implementation of the description in the white paper
  • Being able to print intermediate results for test data
  • Insert test data from the main
  • Test with different versions of the QRCode
  • Offer a library for other apps


  • lib - the library of the v2 version, including the old v1 and v1_1 for reference
  • app - example of how to use the library in an app

Starting the tests

To start the tests of the library, run the following commands:

cd lib
npm ci -D
npm run test

For the tests of the app, go into the app-directory, and do the same:

cd app
npm ci -D
npm run test

Library usage

To use the library, first you need to install it:

npm i -S @c4dt/libcrowdnotifier

If you're using libsodium or mcl in your own app, be sure to import them from the libcrowdnotifier package! This is a limitation due to the fact that both libsodium and mcl need to initialize first. If you're using your own libsodium and mcl imports, node will treat them as a separate package!


  • 1.3.0 - dd/mm/12 - Library-version v3 with new QRcodes
  • 1.1.0 - 21/03/12 - Allow the manager to drop the secret of the partial HA key
  • 1.0.9 - 21/01/17 - Correctly set notificationKey and message and remove unused protobufs
  • 1.0.8 - 21/01/13 - Build fixes, better creation of app-classes: Organizer, HealthAuthority
  • 1.0.7 - 21/01/11 - Add Organizer method to use managed mode
  • 1.0.5 - 21/01/08 - Adding test-vectors, better field names in protobufs
  • 1.0.3 - 21/01/05 - Update the types
  • 1.0.0 - 20/12/17 - First version


Everyone interacting on the CrowdNotifier projects codebases, issue trackers, etc. is expected to follow the code of conduct.


This project is licensed under the terms of the MPL 2 license. See the LICENSE file.