[sjaak]

In the case of Signal, I imagine people assume all of the following:

1. the protocol between client and server is setup in such a way, even if Signal wanted to store interesting information, they could not access anything interesting even if they wanted to (for example, messages), thus they don't store anything since it's useless

2. the app implements the protocol faithfully and this has been checked by people perusing the source code

3. the binary downloaded from the app/play store phone is compiled from the sources listed on github

[rozab]

This might seem like a naive question, but how is it possible to verify that 3 is true?

I get how it might be done in theory but real life is complicated. Has anyone attempted to do this?

[floreen]

Signal has reproducible builds for at least Android: https://github.com/signalapp/Signal-Android/tree/master/repr...

You compare the result you get compiling the app yourself with what you downloaded from the Play Store. For iOS this might be harder

[CarelessExpert]

> I get how it might be done in theory but real life is complicated. Has anyone attempted to do this?

This is mentioned elsewhere, but the answer is: reproducible builds.

You can take the Signal client source (which is available on Github), build an APK or whatnot yourself, then get the SHA256 hash or whatever and compare that to the artifact downloaded from the app store and validate that it's the same.

Has anyone done it? No idea!

[bigiain]

The signatures on app store apks mean that doesn't work.

You can though, decompile both your version and the app store version and compare them that way.

[CarelessExpert]

Any reason you can't just zero out the signatures and then hash?

[tdons]

I imagine it's non-trivial. I think it would involve (in case of iOS):

1. downloading the binary

2. jailbreaking the phone to extract the binary (pretty sure this is necessary on iOS)

3. check the version of the binary, then compile the original sources of the version

4. ??? compare the two binaries, this is likely the most difficult part, they won't be identical because of things like codesigning (and build flags, timestamps, ...)

I know noone that does this.

[evgen]

You don't compare builds because you probably don't actually have sources. What you do is use a special iPhone (a Security Research Device) that Apple grants some researchers or you use an emulator like the one from Corellium (to whom Apple recently lost a lawsuit over this emulator) to probe and step through the code. Find the key sections that do the real crypto work and make sure that they do what they are supposed to do and that they are getting the correct inputs.

There is a large group of people who do this sort of research, and some fraction of them do this research and actually talk about it or publish papers. If you could find a deliberate weakness in the security of an app like what we are talking about (or WhatsApp or iMessages) then you have just printed your own golden ticket to whatever mobile cybersecurity job you want for the next decade or two, so there is a bit of an incentive to publish if something like this was discovered...

[aidenn0]

> You don't compare builds because you probably don't actually have sources

https://github.com/signalapp/Signal-iOS