LWN: Comments on "Android wallpaper fingerprints"

Android Apps As Adversaries

mcortese — Thu, 02 Dec 2021 23:17:15 +0000

An ad-blocking proxy does exist. Look for Blokada on f-droid.

Android Apps As Adversaries

sammythesnake — Sat, 13 Nov 2021 10:58:55 +0000

I've long fantasised about having some kind of virtualized environment for the apps on my Android phone.

I want to be able to run an app that insists on all sorts of permissions that I'd rather not give it. There are many that could be given more restricted access to sensitive data or fake data rather simply.

Back in the day, CyanogenMod did some of this stuff, but it went under before getting to the point I thought they were heading for :-(

Local file storage should be (by default) restricted to one directory used only by that one app and the file manager

Requests for location data could easily always return some fixed value, ideally with the indicated accuracy set to a low value

Apps that insist on being a device administrator could be told they are without their changes to system settings changing anything other than what they're told the system settings are

Information about wi-fi/mobile networks etc. could be configured to always give the correct lie configured for each app

Demands to be always active or start on boot-up can be ignored

All sorts of fingerprinting relevant stuff like wallpapers, installed apps, localisation, etc. could be faked to an extremely bland default (just the Google default Google config)

While we're at it, an advert blocking proxy would be nice, though probably somewhat more subtle to implement, many apps will simply fail to work if they don't get their adverts to serve and faking them might be both fiddly and app specific...

Android wallpaper fingerprints

rschroev — Mon, 01 Nov 2021 10:42:42 +0000

Maps, weather apps, timetable apps for public transportation, ... . And then there are all the apps that don't really need to access a server, but do it anyway; to show ads, or to collect telemetry data.

In short, probably most of the apps on most people's phones.

Android wallpaper fingerprints

ttuttle — Sun, 31 Oct 2021 16:05:06 +0000

Games, for one.

Android wallpaper fingerprints

giraffedata — Sat, 30 Oct 2021 18:53:32 +0000

I don't get it. What kind of mobile apps are we talking about? The ones I'm thinking of (that access a server) have my email address. What more identity could they want?

Android wallpaper fingerprints

derobert — Sat, 30 Oct 2021 09:15:24 +0000

That's not an analogy; Firefox is an Android app (as well as desktop of course). It also happened to be the app I was using at the moment (to read LWN).

Android wallpaper fingerprints

mkbosmans — Sat, 30 Oct 2021 08:59:37 +0000

Sure, but in that analogy the browser is the toolkit and whatever is running inside the tab is the app. So the app itself does not need to know the pixel output.

Android wallpaper fingerprints

derobert — Thu, 28 Oct 2021 23:38:54 +0000

One example, Firefox's tab chooser shows what each tab looks like. Easiest way to implement that is of course to just scale down the pixels that were displayed.

Android wallpaper fingerprints

immibis — Thu, 28 Oct 2021 15:47:09 +0000

I dunno, some kind of picture-in-picture or other postprocessing effect. I know there are some games that screenshot themselves for the pause screen background, but that's not on Android. The point is that you'd generally expect apps to be able to read their own pixels. Changing that will probably break something.

IIRC Android apps work by sending screenfuls of pixels to the compositor process. How will you stop them seeing what they're sending?

Android wallpaper fingerprints

dskoll — Thu, 28 Oct 2021 15:31:58 +0000

Why would most apps need to take screenshots of themselves?

Android wallpaper fingerprints

immibis — Thu, 28 Oct 2021 14:11:08 +0000

And how are those widgets implemented? Either in-process - in which case you can just get access to them - even if they are protected by Java mechanisms you can scan memory with native code. Or out-of-process - in which case your opportunities for UI customization are severely limited. And you have to restrict the app from taking screenshots of itself!

Android wallpaper fingerprints

syrjala — Thu, 28 Oct 2021 13:55:24 +0000

If only we had this thing (let's call it a "toolkit") that can isolate the business logic from the UI rendering. If the application just describes its UI using abstract elements (say "widgets") then there is no need for it to know anything about specific colors/etc. used to actually render the UI.

Android wallpaper fingerprints

cpitrat — Thu, 28 Oct 2021 06:55:02 +0000

On my side, I have picked a "basic color" from the parameters in wallpaper and style so I'd expect the system to return it to the app, but no. It returns the colors from the wallpapers. Looks like a bug to me...

Android wallpaper fingerprints

felixfix — Wed, 27 Oct 2021 15:12:34 +0000

Awesome place, LWN is, has the most interesting users, all 90 of 'em :-)

Android wallpaper fingerprints

bojan — Wed, 27 Oct 2021 09:22:43 +0000

You mean 72, right? :-)

Android wallpaper fingerprints

lkundrak — Wed, 27 Oct 2021 09:14:05 +0000

who's the other 73 then

Android wallpaper fingerprints

developer122 — Wed, 27 Oct 2021 09:12:09 +0000

I would expect frequently changing wallpapers to increase identifiability, no decrease it.

Suppose I have Alice_app and Bob_app, and they want to know if they've been installed on the same phone together. They could look at the three theme colors and both realize that they're on default wallpaper A, of system provided wallpapers A, B, C, and D. Now, maybe lots of people use wallpaper A, so that's not a great hint.

However, suppose that they both take a series of measurements, timestamped with the system time? If they both see the same sequence of transitions from A to C to B to D to A to D to C, then there's a much higher probability that they're both on the same phone, as it's unlikely two different phones would pick the same randomly generated sequence. Here we're able to record changes tot he environment over time and match them. Unless the OS was programmed to pick backgrounds at scheduled time in a predetermined order, I think is is a potentially powerful (if not immediate) fingerprinting technique.

Who says you need to match users together immediately? You could fold the histories of matching pones together after you determine a match, then continue to merge them going forward.

Android wallpaper fingerprints

wsy — Wed, 27 Oct 2021 07:50:58 +0000

Same as me :)

Android wallpaper fingerprints

felixfix — Wed, 27 Oct 2021 05:45:15 +0000

I have a solid black background and lock screen, and it says 75 users out of 13xx.