Apple and Google partner on Covid-19 contact tracing technology • Hacker News Board

Re: Apple and Google partner on Covid-19 contact tracing technology

#481

Post by jschwartzi » Mon, Apr 13, 2020, 4:34 PM UTC

nradov wrote: post #334

jschwartzi wrote: post #207

Earlier quoted context omitted.

Bluetooth 5.1 devices can do both distance and direction, so if you have a bunch of beacons you can determine your location to sub-meter accuracy.

This app appears to use peer to peer Bluetooth between phones, not beacons. The intention is to determine relative proximity between users, not absolute position.

It's ridiculously easy to turn any Bluetooth device into a beacon. All you have to do is configure the BLE advertising frame for the device and then set the device to advertise. I can turn my laptop into a beacon in about 5 minutes(some web searching to remember exact commands and formats).

Re: Apple and Google partner on Covid-19 contact tracing technology

#482

Post by foota » Tue, Apr 14, 2020, 5:05 AM UTC

tinus_hn wrote: post #461

foota wrote: post #155

Earlier quoted context omitted.

This is very different than what you think when you hear "find my iPhone|Android". Iirc the find my iPhone tech does phone to phone association to identify the location. See: https://www.wired.com/story/apple-find-my-cryptography-bluet...

This technology which has been announced but not released would, as your link states, ‘let you track down your stolen laptop, but not let anyone track you. Not even Apple.’ What issue do you have with that?

I have nothing against it :-) I was stating that what they were describing is nothing like what was implemented.

Re: Apple and Google partner on Covid-19 contact tracing technology

#483

Post by ericlavigne » Thu, Apr 16, 2020, 1:44 AM UTC

prophesi wrote: post #472

ericlavigne wrote: post #433

Earlier quoted context omitted.

Think of the daily key as the seed to a random number generator. If two people pass the same seed into the same random number generator, they can generate the same list of 500 random numbers. This provides a compact way for someone to say: "I just learned that I was infected. These are the 500 identifiers I broadcast on that day. If you recognize one of them, then you might also be infected." https://tcn-coalition.or…

I understand that aspect of it; I'm just confused as to how only having the daily key is enough to generate the identifiers. Wouldn't they also need the TimeIntervalNumber, according to the function?

If each phone generated 500 numbers per day, then TimeIntervalNumber is a number in the range of 1...500. So generate 500 codes using all of the numbers in that range. If any of those 500 codes match one of the codes that you actually saw in the wild, then you were near that person.

Re: Apple and Google partner on Covid-19 contact tracing technology

#484

Post by prophesi » Thu, Apr 16, 2020, 5:25 PM UTC

ericlavigne wrote: post #483

prophesi wrote: post #472

Earlier quoted context omitted.

I understand that aspect of it; I'm just confused as to how only having the daily key is enough to generate the identifiers. Wouldn't they also need the TimeIntervalNumber, according to the function?

If each phone generated 500 numbers per day, then TimeIntervalNumber is a number in the range of 1...500. So generate 500 codes using all of the numbers in that range. If any of those 500 codes match one of the codes that you actually saw in the wild, then you were near that person.

Thanks! I actually brainstormed with a friend later that day on how it'd work and we finally came to a similar conclusion.

According to the spec, the phone only generates a new identifier when the MAC address changes or on a new day. But since it's generated in accordance to a 10-minute time window, that means you'd try to derive their key with all 144 possible time windows for that day. And if you find one of those ID's in your list of contacts, then you know you were in contact with someone infected.