> What if coffee shops/bus stations/shopping centers broadcast beacons and essentially became fixed known users -- doesn't that defeat this? No, because the l

> What if coffee shops/bus stations/shopping centers broadcast beacons and essentially became fixed known users -- doesn't that defeat this? No, because the list of beacons you received is only stored in your phone, so that would only allow you to determine that you went to a coffee shop often, and no one would be able to get a list of anyone who was close to the coffee shop. This of course assumes that the app/OS vendor does not include hidden functionality that uploads your received beacons, but in that case they could just activate the GPS and upload the location regardless of the contact tracing system chosen. The coffee shop would be able to determine how many smartphones are present near its location at any time (although this could be mitigated in principle by having smartphones send many beacons if they are receiving few, so the number is approximately constant), but would not know anything about them, including whether they are the same smartphones that it detected at a different time.