Petter Reinholdtsen

Every mobile phone announce its existence over radio to the nearby mobile cell towers. And this radio chatter is available for anyone with a radio receiver capable of receiving them. Details about the mobile phones with very good accuracy is of course collected by the phone companies, but this is not the topic of this blog post. The mobile phone radio chatter make it possible to figure out when a cell phone is nearby, as it include the SIM card ID (IMSI). By paying attention over time, one can see when a phone arrive and when it leave an area. I believe it would be nice to make this information more available to the general public, to make more people aware of how their phones are announcing their whereabouts to anyone that care to listen.

I am very happy to report that we managed to get something visualizing this information up and running for Oslo Skaperfestival 2017 (Oslo Makers Festival) taking place today and tomorrow at Deichmanske library. The solution is based on the simple recipe for listening to GSM chatter I posted a few days ago, and will show up at the stand of Åpen Sone from the Computer Science department of the University of Oslo. The presentation will show the nearby mobile phones (aka IMSIs) as dots in a web browser graph, with lines to the dot representing mobile base station it is talking to. It was working in the lab yesterday, and was moved into place this morning.

We set up a fairly powerful desktop machine using Debian Buster/Testing with several (five, I believe) RTL2838 DVB-T receivers connected and visualize the visible cell phone towers using an English version of Hopglass. A fairly powerfull machine is needed as the grgsm_livemon_headless processes from gr-gsm converting the radio signal to data packages is quite CPU intensive.

The frequencies to listen to, are identified using a slightly patched scan-and-livemon (to set the --args values for each receiver), and the Hopglass data is generated using the patches in my meshviewer-output branch. For some reason we could not get more than four SDRs working. There is also a geographical map trying to show the location of the base stations, but I believe their coordinates are hardcoded to some random location in Germany, I believe. The code should be replaced with code to look up location in a text file, a sqlite database or one of the online databases mentioned in the github issue for the topic.

If this sound interesting, visit the stand at the festival!

A little more than a month ago I wrote how to observe the SIM card ID (aka IMSI number) of mobile phones talking to nearby mobile phone base stations using Debian GNU/Linux and a cheap USB software defined radio, and thus being able to pinpoint the location of people and equipment (like cars and trains) with an accuracy of a few kilometer. Since then we have worked to make the procedure even simpler, and it is now possible to do this without any manual frequency tuning and without building your own packages.

The gr-gsm package is now included in Debian testing and unstable, and the IMSI-catcher code no longer require root access to fetch and decode the GSM data collected using gr-gsm.

Here is an updated recipe, using packages built by Debian and a git clone of two python scripts:

1. Start with a Debian machine running the Buster version (aka testing).
2. Run 'apt install gr-gsm python-numpy python-scipy python-scapy' as root to install required packages.
3. Fetch the code decoding GSM packages using 'git clone github.com/Oros42/IMSI-catcher.git'.
4. Insert USB software defined radio supported by GNU Radio.
5. Enter the IMSI-catcher directory and run 'python scan-and-livemon' to locate the frequency of nearby base stations and start listening for GSM packages on one of them.
6. Enter the IMSI-catcher directory and run 'python simple_IMSI-catcher.py' to display the collected information.

Note, due to a bug somewhere the scan-and-livemon program (actually its underlying program grgsm_scanner) do not work with the HackRF radio. It does work with RTL 8232 and other similar USB radio receivers you can get very cheaply (for example from ebay), so for now the solution is to scan using the RTL radio and only use HackRF for fetching GSM data.

As far as I can tell, a cell phone only show up on one of the frequencies at the time, so if you are going to track and count every cell phone around you, you need to listen to all the frequencies used. To listen to several frequencies, use the --numrecv argument to scan-and-livemon to use several receivers. Further, I am not sure if phones using 3G or 4G will show as talking GSM to base stations, so this approach might not see all phones around you. I typically see 0-400 IMSI numbers an hour when looking around where I live.

I've tried to run the scanner on a Raspberry Pi 2 and 3 running Debian Buster, but the grgsm_livemon_headless process seem to be too CPU intensive to keep up. When GNU Radio print 'O' to stdout, I am told there it is caused by a buffer overflow between the radio and GNU Radio, caused by the program being unable to read the GSM data fast enough. If you see a stream of 'O's from the terminal where you started scan-and-livemon, you need a give the process more CPU power. Perhaps someone are able to optimize the code to a point where it become possible to set up RPi3 based GSM sniffers? I tried using Raspbian instead of Debian, but there seem to be something wrong with GNU Radio on raspbian, causing glibc to abort().

For noen dager siden publiserte Jon Wessel-Aas en bloggpost om «Konklusjonen om datalagring som EU-kommisjonen ikke ville at vi skulle få se». Det er en interessant gjennomgang av EU-domstolens syn på snurpenotovervåkning av befolkningen, som er klar på at det er i strid med EU-lovgivingen.

Valgkampen går for fullt i Norge, og om noen få dager er siste frist for å avgi stemme. En ting er sikkert, Høyre og Arbeiderpartiet får ikke min stemme denne gangen heller. Jeg har ikke glemt at de tvang igjennom loven som skulle pålegge alle data- og teletjenesteleverandører å overvåke alle sine kunder. En lov som er vedtatt, og aldri opphevet igjen.

Det er tydelig fra diskusjonen rundt grenseløs digital overvåkning (eller "Digital Grenseforsvar" som det kalles i Orvellisk nytale) at hverken Høyre og Arbeiderpartiet har noen prinsipielle sperrer mot å overvåke hele befolkningen, og diskusjonen så langt tyder på at flere av de andre partiene heller ikke har det. Mange av de som stemte for Datalagringsdirektivet i Stortinget (64 fra Arbeiderpartiet, 25 fra Høyre) er fortsatt aktive og argumenterer fortsatt for å radere vekk mer av innbyggernes privatsfære.

Når myndighetene demonstrerer sin mistillit til folket, tror jeg folket selv bør legge litt innsats i å verne sitt privatliv, ved å ta i bruk ende-til-ende-kryptert kommunikasjon med sine kjente og kjære, og begrense hvor mye privat informasjon som deles med uvedkommende. Det er jo ingenting som tyder på at myndighetene kommer til å være vår privatsfære. Det er mange muligheter. Selv har jeg litt sans for Ring, som er basert på p2p-teknologi uten sentral kontroll, er fri programvare, og støtter meldinger, tale og video. Systemet er tilgjengelig ut av boksen fra Debian og Ubuntu, og det finnes pakker for Android, MacOSX og Windows. Foreløpig er det få brukere med Ring, slik at jeg også bruker Signal som nettleserutvidelse.