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Comment by vletal | Hacker News Reader

> Now, if this social app is reading accelerometer data on your phone as well as the passenger’s phone, the app can easily figure out that both phones experience the same vibration pattern. ... Don’t be surprised if you receive a recommendation from the app to add this passenger as a friend.

Overall I find the article interesting but this quote is borderline tinfoily. Given the amount of noise in the accelerometer data and signal s much closer to the sensor than the bus itself - such as body movements, it would be hardly precise. Moreover the cost of doing all that research and computation as well as data transfer would hardly pay off.

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Also remember that apps do not run continuously in background, which would make the data only useful if the user was actively running your app.

I don't believe the data is precise enough to do much that could be nefarious.

Also trying to matching two phones (without location info) that have the same highly imprecise vibration is not worth the effort on the server side.

Just an FYI, Apple does give its developer the ability to have the app wake up from background on-demand or at scheduled intervals to perform specific tasks. This is something that can be done as long as the phone is ON and connected to the public gateway either via cell towers or WiFi.

An example would be the Facebook app icon with notification count or the Mail app with unread emails count. This counter is updated based on background processing of Fb or email notifications.

I don't know exactly how it works, but I know notifications are their own separate concept that gets handled by the system and doesn't require the app to be running arbitrary code in the background

There is also the "allow background activity" permission though, which I'm not sure the bounds of, but you can disable it

Notifications and background activities are orthogonal for the use case you mentioned.

Background activity on iOS is severely restricted and mostly relegated to finishing a long or regular upload or download.

Unless you are a phone communication app — and a reason why you may see voice added to apps.

You can do obvious things, like detect when the user put down the device after the ad stats playing.

How does it relate to the paragraph about pinpointing precise location which the original comment refers to?

Sounds plausible to me. The question is whether anyone is really using this technique. Sounds like it would require nontrivial bandwidth and analysis.

> The question is whether anyone is really using this technique.

No. This is not at all the question. A possible privacy breach is a serious issue, regardless of whether there is a working POC. If this data is somehow compromised, a stalker could get your identity just by following you a few minutes on the street.

Sure, but the article makes it seem that this is a thing done in a wild on a mass scale which is implausible.

a stalker wouldn't be able to accelerate the same as you

If they were on the same public transport vehicle, that would likely be a different situation.

I recall that Snapchat explicitly mentioned accelerometer data in their privacy policy - why would you mention it unless you did or have plans to use accelerometer data in the future?

I don't think it would require significant bandwidth; the data is just integers which can be collected, compressed and uploaded asynchronously (as part of another heavy upload such as someone sending a picture). The analysis part could be similar to how Shazam works, but that can be done on the server side so on-device performance isn't a concern.

A single simple would be three doubles and a timestamp (64 bit int). Accelerometer max update rate is 100hz. So all possible samples each second would be 3200 bytes. So bandwidth doesn't appear to be a big issue.

Which also means reduced battery.

Most of the aforementioned apps already have autoplaying media and/or keep the camera running in the background (officially for quicker access to it when the user opens the camera view, as it normally takes a second or so to initialize it). Seems like collecting accelerometer data is a drop in the bucket in comparison.

I find it pretty funny, since to me that seems completely normal and easy and wouldn't even require much from an algorithm to implement.

Vehicle-induced vibrations would result as significant (overpowering any noise) peaks on one or more accelerometer axis, and their timestamp (relative to other peaks) will precisely align with peaks on any other device on that vehicle.

As a layman I think you can just sum all axis for each person, overlay the resulting track with everyone else's and try to find a position where enough peaks correlate (constrained within a 5 minute timeframe from the on-device timestamp to account for clock drift while limiting the search space) and that should work well enough.

I'm sure the sociopaths working for Facebook will have a smarter way of doing this that's even more accurate.

The timing. Just look at two phones that are vibrating for the exact same length of time; they are in the same vehicle.

The app could figure out who you are traveling with (two phones with roughly the same accelerometer data at the same time).

I think there would be far too much noise. The accelerometer data is going to be different based on a number of factors:

Orientation? Is the phone in your hand? Your pocket? Your purse? Front seat / back seat?

By way of comparison I worked on some automatic breaklights that trigger when an accelerometer detects you slowing down (bikes, skateboards, scooters). It turned out to be way more complicated that we expected. Hitting potholes, naturally slowing when you go uphill, taking a turn.

I've mentioned this before, but the Bosch 9-DOF sensors are accurate and have enough fidelity to track me walking through my house while sitting in the back room on a desk. That's the accelerometer. The gyros can detect music playing at the opposite end of the house assuming the speakers are big enough - phone playing YouTube, no, but sound bar, yes.

Make no mistake that MEMs sensors are really very good.

I was trying to wire something together to track what vehicles were travelling down my forest road and left the room with the data streaming, and came back to be impressed.

I think the key is that the data would be in sync. You don't particularly have to care what the vibration is or which direction, the important bit is that these patterns line up with similar patterns collected from the other phone.

It's an accelerometer and gyroscope. The acceleration is/can be reported relative to gravity, which always points down.

You can't tell what component is due to gravity and what is not, without other information.

This is the equivalence principle of general relativity, no less.

It could easily be/have been a success story for contact tracing.

except for, you know, the "we totally violated your privacy" part of it.

People would volunteer their data for it

If they volunteer, why not just use GPS, or Bluetooth LowEnergy? The accelerator seems to shine mostly as a convert vector.

Is the accelerometer sensitive enough to detect vibrations from sound and voices?

Yes - read the article

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