The DP-3T algorithm for contact tracing (with Nicky Case)

as far as covid-19 cares there are only three kinds of people not infected yet infected and contagious but with no symptoms and infected contagious and showing symptoms if you have widespread testing you can get people to self-isolate as soon as they show symptoms the problem is that the virus still spreads because of all the contacts that happened while people are contagious but asymptomatic however if when someone shows symptoms and tests positive you isolate not only them but everyone they've been in contact with you're staying one step ahead of the virus the old school way to do this is with interviews but that's slow it's inefficient and frankly it's quite the intrusion on people's privacy another approach in the modern world would be to ask people who've tested positive to forfeit all the geolocation information from their phones and then to track down the people who have been in those same spots but now we are well into Big Brother territory so do we have to sacrifice privacy for health well I'll just let Nikki's illustration speak for itself here there are several clever algorithms that let you alert everybody who's recently been in contact with someone who tests positive for covid-19 but without compromising the privacy of anybody involved side note here I found this very surprising I know it shouldn't have been since I've gone through this dance many times of thinking something's impossible only to see that cryptography makes it actually possible but I would not blame anybody at all for assuming that downloading an app that can alert everybody you've been in contact with must NE necessarily be tracking and revealing your location and a lot of other information the code for these apps is entirely open so you don't have to trust me or whoever wrote the app or Nikki or anyone to believe that it's doing what it really claims to be doing anyway back to the post let's see how this works with the help of Alice and Bob Alice gets a tracing app every 5 minutes her phone sends out some uniquely pseudo random gibberish to all the nearby devices using Bluetooth because these messages are pseudo random they don't use GPS and they contain no information about Alice's identity not her location not anything it really is gibberish but the key point is that this gibberish is unique now while her phone sends out messages it also listens for messages from nearby phones for example Bobs Bob also has a privacy first tracing app that's compatible with or the same as Alice's if Alice and Bob stay close to each other for more than 5 minutes their phones will exchange respective strings of unique gibberish both of these phones remember all of the messages that they said and heard over the last 14 days again because the random messages contain no information Alice's privacy is protected from Bob and vice versa the next day Alice develops a dry cough and a fever Alice gets tested Alice has covid-19 this is not a good day for Alice but she won't suffer in vain Alice tells her app to upload all of the random gibberish messages that it's been sending out to a hospital database and to do this she uses a one-time passcode given to her by her doctor this code is to prevent spam the database then stores Alice's gibberish and again the random messages give no information about Alice where she was who she was with what she was doing or even how many people Alice met it really is meaningless to the hospital but it's not meaningless to Bob's phone often checks this Hospital list of random messages that have come in from covid-19 positive cases essentially the hospital's database is saying to all the phones out there hey we just got this new random gibberish if you've seen that same random gibberish sometime in the last 14 days it means you've been in contact with someone who just tested positive for covid-19 once Bob's phone recognizes some of these numbers that are the gibberish nippet now known to be associated with positive test cases it can warn Bob to self quarantine and so bob cuts off the chain of Transmissions we're staying one step ahead of the virus and that's it that's how digital contact tracing apps can proactively prevent the spread of covid-19 while also protecting our rights thanks Alice and Bob stay safe