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Wow. I've just for the first time ever realised that you might be able to derive the content of the SMS from the specific pattern of interference you would hear on speakers. Like, it seems plausible that is the case. Is that what you mean here by broadcast - or just the fact there was a message was broadcast?

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I thought communication between the tower and the device was encrypted?

That's not to say it's not using some archaic weak encryption scheme left in for backwards compatability though.

yes and yes indeed

https://en.m.wikipedia.org/wiki/A5/1

Not really, speakers just reacted to the bursts of power, because the transmission frequency is way too high for us to hear.

GSM uses time division multiplex, which means it shares the 8-timeslot channel with 6 more devices (7 devices + 1 signalling), meaning your phone transmits for 577us, then waits quietly for all the 7 timeslots that other devices use (so, for 4.039 milliseconds - 7x577us), and again transmits for 577us, and again waits. Bursts of power every ~4.6milliseconds make a noise that we can actually hear at a bit over 200Hz.

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