[fidget]

Unannounced side channels seems like by far the easiest thing to deal with there; send a 2mb file, observe network patterns, raise an eyebrow if 2mb gets sent over a channel that you didn't expect.

As for using the correct key, dismantle the signal message envelope until you get your blob of encrypted message. Then see if the same blob appears on the target device. Multiple keys? I imagine either correlating message size and network traffic (encrypting stuff twice could well show up), or going at it with a debugger.

Which is really the answer to all of these questions instead of any network shenanigans. You root your phone and attach a debugger, then step through what signal is doing.

Not a security researcher, never reverse engineered anything for security reasons in my life.

[snovv_crash]

What if it only does it at a much later point in time, or slowly via adding data to other comms channels? What if it only does it for small payloads by padding packet sizes to 1k? There are so many ways to get around this, unless we have open source and reproducible builds.

[fidget]

Right, which is why I said those blackbox methods were pretty rubbish. You step through it with a debugger.

[jimmies]

>Unannounced side channels seems like by far the easiest thing to deal with there; send a 2mb file, observe network patterns, raise an eyebrow if 2mb gets sent over a channel that you didn't expect.

Facebook distributes your 2MB pic to many people, does it technically require more than 2MB of your upload bandwidth? No. You only need to upload it once to their server.