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**Hugh** · 05-05-2020, 11:18

The Register (in amongst its standard snark and Government bashing) makes a good point about the UK's tracking app - it probably won't work.

https://www.theregister.co.uk/2020/0...ronavirus_app/

Quote:

But there is a problem with the NHS's approach: it probably won't that well work on your phone, and probably won't be terribly accurate at measuring the spread of the virus.

That's because the proposed system will only work in the way the UK government claims it will if everyone does what it says: a classic failing of the Whitehall mindset that stretches back to the World War One trenches and further back still to the days of Great Houses and Men Who Knew Better.

Despite what the NCSC has continued to imply, the app will not, as it stands, work all the time on iOS nor Android since version 8. The operating systems won't allow the tracing application to broadcast its ID via Bluetooth to surrounding devices when it's running in the background and not in active use. Apple's iOS forbids it, and newer Google Android versions limit it to a few minutes after the app falls into the background.

That means that unless people have the NHS app running in the foreground and their phones awake most of the time, the fundamental principle underpinning the entire system – that phones detect each other – won’t work.

It will work if people open the app and leave it open and the phone unlocked. But if you close it and forget to reopen it, or the phone falls asleep, the app will not broadcast its ID and no other phones around you will register that you've been close by. There is even a handy video of someone in Australia showing this (Australia has gone for a similar system with its COVIDSafe app.)

We cannot state it plainer: on iPhones, apps cannot send out their IDs via Bluetooth when the software is in the background, and on newer Android builds, IDs cannot be transmitted after a few minutes in the background. And Apple and Google have refused to allow the tracing app to send out IDs in the background.

The NHS has insisted its engineers have worked around this problem "sufficiently well" by waking the app after it detects itself running on a nearby phone emitting an ID: the software is blocked from sending out its ID when in the background but it can passively listen for IDs of apps still allowed to broadcast. However, this assumes there are a sufficient number of phones running the tracing app nearby still broadcasting to keep enough people's apps awake: there needs to be a critical mass of users while we're all supposed to be socially distancing. If two or more people pass each other and their apps have stopped broadcasting, the software will never know they came in contact.

And it could be a battery hog, which may make people leave the app off, preventing the app on other phones from waking up.