Skip to content
Tire Pressure Monitoring System decoding tools.
Branch: master
Clone or download
Latest commit 527066b Mar 8, 2016
Permalink
Type Name Latest commit message Commit time
Failed to load latest commit information.
src
README.md Update README.md Mar 8, 2016

README.md

TPMS

Software for capturing, demodulating, decoding, and assessing data from automotive tire pressure monitors.

Tire pressure monitoring systems (TPMS) are becoming common on automobiles, and in certain countries, are required as a condition of sale. Most TPMS devices use simple wireless communication techniques such as:

  • FSK modulation
  • Manchester bit coding
  • Small CRCs or checksums
  • Unique device identifiers

Background

For more background on this project, please watch Jared Boone's talk from ToorCon 15:

"Dude, Where's My Car?: Reversing Tire Pressure Monitors with a Software Defined Radio" [video] [slides]

...or this interview with Darren Kitchen of Hak5:

"Hak5 1511 – Tracking Cars Wirelessly And Intercepting Femtocell Traffic" [video]

Software

This software was developed for and tested with:

Hardware

I used a variety of hardware for receiving tire pressure monitors. If you don't already have a software-defined radio receiver, a $50 US investment is all you need to get started.

Quick Shopping List for The Impatient

Aside from a computer capable of running GNU Radio, here's what you'll need:

Receiver

If you're just getting started with SDR, I highly recommend getting a DVB-T USB dongle, supported by the rtl-sdr project. They cost $25 US, typically.

Recommended DVB-T dongle vendors include:

If you're looking to do active attacks on TPMS (a topic I haven't explored), I recommend the HackRF. However, my code has not yet been adapted to support the HackRF's much wider bandwidth, so you're on your own for the time being.

Antenna

The antenna that comes with your DVB-T dongle will work well, but you'll get more signal and less noise with a band-specific antenna.

For 315MHz:

  • Linx Technologies ANT-315-CW-RH-SMA 315MHz 51mm (2") helical whip antenna, SMA.
  • Linx Technologies ANT-315-CW-RH 315MHz 51mm (2") helical whip antenna, RP-SMA.
  • Linx Technologies ANT-315-CW-HWR-SMA 315MHz 142mm (5.6") tilt/swivel whip antenna, SMA.
  • Linx Technologies ANT-315-CW-HWR-RPS 315MHz 142mm (5.6") tilt/swivel whip antenna, RP-SMA.

For 433MHz:

  • Linx Technologies ANT-433-CW-RH-SMA 433MHz 51mm (2") helical whip antenna, SMA.
  • Linx Technologies ANT-433-CW-RH 433MHz 51mm (2") helical whip antenna, RP-SMA.
  • Linx Technologies ANT-433-CW-HWR-SMA 433MHz 142mm (5.6") tilt/swivel whip antenna, SMA.
  • Linx Technologies ANT-433-CW-HWR-RPS 433MHz 142mm (5.6") tilt/swivel whip antenna, RP-SMA.

I'm using the Linx Technologies ANT-315-CW-RH-SMA and ANT-433-CW-RH-SMA with good results, but you may prefer bigger antennas, or RP-SMA connectors.

Ideally, I'd build a Yagi-Uda antenna. :-)

Cabling

You'll need a cable to connect the antenna to the DVB-T dongle. The DVB-T dongles from Hacker Warehouse and NooElec have a female MCX connector. The SMA antennas I use have a male SMA connector. So you'll want a 50 Ohm cable with a male MCX connector on one side, and a female SMA connector on the other.

Filtering

I like to use a SAW filter between the antenna and receiver to further cut noise and interference. It's certainly not necessary (and likely overkill). The SAW filter I use is built from a PCB I designed.

Using

Capture data from a vehicle with a software radio receiver like an DVB-T USB dongle, or a HackRF, or other device capable of capturing approximately 1MHz of complex spectrum from 315MHz or 433MHz. The best way to constrain packets received to only one vehicle is to ride in the vehicle as it is driven.

Extract bursts of data from the raw capture:

extract_bursts <filename>.cfile

Visually inspect bursts and assess modulation characteristics (ASK/FSK, carrier frequency, deviation, bit rate, access code or preamble):

burst_inspect.py tpms_314.950m_0.400m_20131013_180516z_rtlsdr/

Demodulate packets with certain characteristics, into raw bit streams:

tpms_fsk.py --rate 400000 --modulation fsk --carrier 53000 --deviation 33000 --symbol-rate 20150 --preamble 1101101011100 */*.cfile | tee demodulated.txt

Examine statistics of packet lengths, assuming Manchester decoding (the most common type of TPMS bit coding):

cat demodulated.txt | packet_stats.py --encoding man --lengthstats

Examine 0/1 distribution of each decoded bit, across all packets:

cat demodulated.txt | packet_stats.py --encoding man --length 70 --bitstats

Make and test some assumptions regarding ranges of bits. First, test 32-bit ranges to find bits that possess only four unique values (one ID for each tire):

cat demodulated.txt | packet_stats.py --encoding man --length 70 --rangestats 0,32
cat demodulated.txt | packet_stats.py --encoding man --length 70 --rangestats 1,33
cat demodulated.txt | packet_stats.py --encoding man --length 70 --rangestats 21,53

Test assumptions about other ranges of bits, using an oft-valid assumption that fields break on byte boundaries. Graph distribution of data and look for data that might represent tire pressure and temperature, or perhaps a CRC or checksum:

cat demodulated.txt | packet_stats.py --encoding man --length 70 --decode | grep 1000110011000 | packet_graph.py --range 5,13
cat demodulated.txt | packet_stats.py --encoding man --length 70 --decode | grep 1000110011000 | packet_graph.py --range 13,21
cat demodulated.txt | packet_stats.py --encoding man --length 70 --decode | grep 1000110011000 | packet_graph.py --range 61,69

If a CRC or checksum field is identified, test for possible CRC polynomials and other characteristics by using bruteforce-crc:

cat demodulated.txt | packet_stats.py --encoding man --length 70 --brutecrc 2 | tee brute.txt
bruteforce-crc --file brute.txt --width 8 --start 5 --end 61 --offs-crc 61

Export decoded packet data and graph using knowledge acquired above:

cat demodulated.txt | packet_stats.py --encoding man --length 70 --decoded | tee decoded.txt
cat decoded.txt | ride_2_decode.py | ride_2_graph.py

Notes and Things to Investigate

Another CRC reversing package: http://reveng.sourceforge.net

Liquid-DSP library for building efficient software defined radio implementations, perhaps on the HackRF ARM Cortex-M4F: https://github.com/jgaeddert/liquid-dsp

License

The associated software is provided under a GPLv2 license:

Copyright (C) 2013 Jared Boone, ShareBrained Technology, Inc.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.

Contact

Jared Boone jared@sharebrained.com

ShareBrained Technology, Inc.

http://www.sharebrained.com/

The latest version of this repository can be found at https://github.com/jboone/tpms

You can’t perform that action at this time.