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Mixnet missing (specification and implementation)

It seems that mixnets will be used in order to prevent the server from learning the link betwen an element in the contact list (LocalProximityList) and the user (mobile app) that has recorded this contact. The statement of the CNIL (n° 2020-056 of May 25th 2020) relies precisely on the assumption that the server cannot know the contact list of an infected user (see point 41).

Mixnets are only vaguely mentionned in the ROBERT specification as one possible solution. Some mixing seems envisionned in the implementation but is marked as todo: "TODO: implement a multiplexing queue with random delays in order to mix the treatment of LocalProximityList elements." https://gitlab.inria.fr/stopcovid19/accueil/-/blob/master/documentation/robertImplementationModalities.txt

Issue: the specification of mixnets is missing. Their implementation seems missing as well.

Questions:

  • Will mixnets be implemented before the deployment of the application? If not, when it is planned?
  • What kind of mixnets will be considered? What is the articulation with the "authentication" of the contact list through the code given by the doctor when Alice has a positive test? How the mixers will learn the valid codes and the already used code? What are the delays when mixing? Mixnets actually form a real protocol, a specification is needed. When the specification of ROBERT will be updated with the specification of mixnet?
  • What kind of infrastructure is planned? How many servers? Administrated by who? What independence?
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