The RTL-SDR software defined radio can be used to analyze cellular phone GSM signals, using Linux based tools GR-GSM (or Airprobe) and Wireshark. This tutorial shows how to set up these tools for use with the RTL-SDR.

Example - Analysing GSM with RTL-SDR Software Defined Radio

Here is a screenshot and video showing an example of the type of data you can receive. You can see the unencrypted GSM packet information. You will not be able to see any sensitive information like voice or text message data since that part is encrypted. Decryption of messages that are not your own is very difficult, illegal and is not covered in this tutorial.

First, you will need to find out at what frequencies you have GSM signals in your area. For most of the world, the primary GSM band is 900 MHz, in the USA it starts from 850 MHz. If you have an E4000 RTL-SDR, you may also find GSM signals in the 1800 MHz band for most of the world, and 1900 MHz band for the USA. Open up SDRSharp, and scan around the 900 MHz (or 850 MHz) band for a signal that looks like the waterfall image below. This is a non-hopping GSM downlink signal. Using NFM, it will sound something like the example audio provided below. Note down the strongest GSM frequencies you can find.

The rest of the tutorial is performed in Linux and we assume that you have basic Linux skills in using the terminal. For this tutorial we used Ubuntu 14.04 in a VMWare session. You can download the various ready to go Ubuntu VMWare images from here, and the free VMWare player from here. Note that virtual box is reported not to work well with the RTL-SDR, as its USB bandwidth capabilities are poor, so VMWare player should be used. 

Install GR-GSM

This tutorial is heavily based on the instructions from the gr-gsm GitHub readme at https://github.com/ptrkrysik/gr-gsm.

1. The easiest way to install gr-gsm is to use Pybombs. Pybombs will automatically install gr-gsm, and all the required dependencies including GNU Radio.

   $ sudo apt-get update
   $ sudo apt-get install git python-pip
   $ sudo pip install PyBOMBS
   $ sudo pybombs prefix init /usr/local -a default_prx
   $ sudo pybombs config default_prefix default_prx
   $ sudo pybombs recipes add gr-recipes git+https://github.com/gnuradio/gr-recipes.git
   $ sudo pybombs recipes add gr-etcetera git+https://github.com/gnuradio/gr-etcetera.git
   $ sudo pybombs install gr-gsm
   $ sudo ldconfig

2. Plug in your RTL-SDR and connect it to your VM if necessary. Run grgsm_livemon by typing grgsm_livemon at the terminal. A new window should open.
3. In the new window tune to a GSM downlink frequency which you determined while browsing in SDR# and set the gain appropriately.
4. Start Wireshark by using sudo wireshark -k -Y '!icmp && gsmtap' -i lo which will automatically start wireshark in the loopback mode with the gsmtap filter activated. You may get an error when opening Wireshark but this can be ignored.
5. You should now see the GSM data scrolling along in Wireshark.

[expand title = "Old Method using Airprobe (Click to Expand)"]

Install GNU Radio

You will need to install GNU Radio first in order to get RTL-SDR to work. An excellent video tutorial showing how to install GNU Radio in Kali Linux can be found in this video shown below. Note that I had to run apt-get update in terminal first, before running the build script, as I got 404 not found errors otherwise. You can also use March Leech's install script to install the latest version of GNU Radio on any Linux OS. Installation instructions can be found here. I recommend installing from source to get the latest version. http://www.youtube.com/watch?v=B8Acp6_3DA0

Update: The new version 3.7 GNU Radio is not compatible with AirProbe. You will need to install GNU Radio 3.6. However, neeo from the comments section of this post has created a patch which makes AirProbe compatible with GNU Radio 3.7. To run it, place the patch file in your airprobe folder and then run patch -p1 < zmiana3.patch.

Install Airprobe

Airprobe is the tool that will decode the GSM signal. I used multiple tutorials to get airprobe to install. First from this University of Freiberg tutorial, I used their instructions to ensure that the needed dependencies that airprobe requires were installed.

Install Basic Dependencies

sudo apt-get –y install git-core autoconf automake libtool g++ python-dev swig libpcap0.8-dev

Update: Thanks to shyam jos from the comments section who has let us know that some extra dependencies are required when using the new Kali Linux (1.0.5) for airprobe to compile. If you've skipped installing GNURadio because you're using the new Kali 1.0.5 with SDR tools preinstalled, use the following command to install the extra required dependencies.

 sudo apt-get install gnuradio gnuradio-dev cmake git libboost-all-dev libusb-1.0-0 libusb-1.0-0-dev libfftw3-dev swig python-numpy

Install libosmocore

git clone git://git.osmocom.org/libosmocore.git
cd libosmocore
autoreconf –i
./configure
make
sudo make install
sudo ldconfig

Clone Airprobe

Now, I discovered that the airprobe git repository used in the University tutorial  (berlin.ccc.de) was out of date, and would not compile. From this reddit thread I discovered a more up to date airprobe git repository that does compile. Clone airprobe using the following git command.

git clone git://git.gnumonks.org/airprobe.git

Now install gsmdecode and gsm-receiver.

Install gsmdecode

cd airprobe/gsmdecode
./bootstrap
./configure
make

Install gsm-receiver

cd airprobe/gsm-receiver
./bootstrap
./configure
make

Testing Airprobe

Now, cd into to the airprobe/gsm-receiver/src/python directory. First we will test Airprobe on a sample GSM cfile. Get the sample cfile which I found from this tutorial by typing into terminal.

cd airprobe/gsm-receiver/src/python
wget ​https://svn.berlin.ccc.de/projects/airprobe/raw-attachment/wiki/DeModulation/capture_941.8M_112.cfile

Note: The tutorial and cfile link is sometimes dead. I have mirrored the cfile on megaupload at this link. Place the cfile in the airprobe/gsm-receiver/src/python folder. Now open wireshark, by typing wireshark into a second terminal window. Wireshark is already installed in Kali Linux, but may not be in other Linux distributions. Since Airprobe dumps data to a UDP port, we must set Wireshark to listen to this. Under Start in Wireshark, first set the capture interface to lo (loopback), and then press Start. Then in the filter box, type in gsmtap. This will ensure only airprobe GSM data is displayed. Back in the first terminal that is in the python directory, type in

./go.sh capture_941.8M_112.cfile

If everything installed correctly, you should now be able to see the sample GSM data in wireshark.

Receive a Live Channel

To decode a live channel using RTL-SDR type in terminal

./gsm_receive_rtl.py -s 1e6

A new window will pop up. Tune to a known non-hopping GSM channel that you found earlier using SDRSharp by entering the Center Frequency. Then, click in the middle of the GSM channel in the Wideband Spectrum window. Within a few seconds some GSM data should begin to show constantly in wireshark. Type ./gsm_receive_rtl.py -h for information on more options. The -s flag is used here to set the sample rate to 1.0 MSPS, which seems to work much better than the default of 1.8 MSPS as it seems that there should be only one GSM peak in the wideband spectrum window.

Capturing a cfile with the RTL-SDR (Added: 13/06/13)

I wasn't able to find a way to use airprobe to capture my own cfile. I did find a way to capture one using ./rtl_sdr and GNU Radio however. First save a rtl_sdr .bin data file using where -s is the sample rate, -f is the GSM signal frequency and -g is the gain setting. (rtl_sdr is stored in 'gnuradio-src/rtl-sdr/src')

./rtl_sdr /tmp/rtl_sdr_capture.bin -s 1.0e6 -f 936.6e6 -g 44.5

Next, download this GNU Radio Companion (GRC) flow graph (scroll all the way down for the link), which will convert the rtl_sdr .bin file into a .cfile. Set the file source to the capture.bin file, and set the file output for a file called capture.cfile which should be located in the 'airprobe/gsm-receiver/src/python' folder. Also, make sure that 'Repeat' in the File Source block is set to 'No'. Now execute the GRC flow graph by clicking on the icon that looks like grey cogs. This will create the capture.cfile. The flow chart will not stop by itself when it's done, so once the file has been written press the red X icon in GRC to stop the flow chart running. The capture.cfile can now be used in airprobe. However, to use this cfile, I found that I had to use ./gsm_receive.py, rather than ./go.sh as a custom decimation rate is required. I'm not sure why, but a decimation rate of 64 worked for me, which is set with the -d flag.

./gsm_receive.py -I rtl_sdr_capture.cfile -d 64

[/expand]

Going Further with Decryption

We don't cover how to decode the actual encrypted GSM data here, but this is possible to do with messages going to your own phone once you extract the encryption code for your sim card. But note that if you want to do this you'll need to put in some good study and research into understanding how GSM actually works before you can even think about trying it. Disclaimer: Only decrypt signals that you are legally allowed to (such as from/to your own cell phone) to avoid breaching privacy.

The most complete video guide is probably the YouTube tutorial by Crazy Danish Hacker, and the most complete web guide is the one by Domonkos P. Tomcsanyi available on his blog here.

A reader wrote in to let us know some information on obtaining the TMSI and Kc numbers, which are useful if you wish to go further and actually decode messages coming from your own phone. He writes:

For some reason, most of posts on the Internet concerning GSM sniffing provide very few examples of how to get our own TMSI and Kc numbers. These rely either on the BlackBerry engineering screen or the use of a SIM-card reader (see for example http://domonkos.tomcsanyi.net/?p=369). I know there are other methods like the one you describe in www.rtl-sdr.com/rtl-sdr-cell-phone-imsi-tmsi-key-sniffer/.

However, I have rarely seen anything related to the Android IMSI-Catcher Detector app. This can be easily installed via the standard repositories and it allows us to send AT commands to the modem provided we root the MS. This procedure works on many devices (I checked it on a Motorola Moto E).

Just a quick reminder of the basic AT+commands:

1. Extraction of IMSI -> AT+CRSM=176,28423,0,0,3.

2. Extraction of Ciphering Key Kc -> AT+CRSM=176,28448,0,0,9 (for SIM),
AT+CRSM=176,20256,0,0,9 (for USIM). First 16 entries.

3. Extraction of TMSI -> AT+CRSM=176,28542,0,0,11. First 8 entries.

The Android IMSI-Catcher Detector provides some additional interesting data, like the cell ID the device is connected to, the LAI, etc.

We note that software such as SimSpyII together with a Sim Card reader can also be used to easily acquire the Kc value.

If you enjoyed this tutorial you may like our book available on Amazon. Available in eBook and paperback formats. The Hobbyist's Guide to the RTL-SDR: Really Cheap Software Defined radio.