Paul Clark

New GRC Block for Reversing Simple Protocols

Hello SDR fans. A long-standing wish of mine is to have a grc block that detects a preamble on an incoming byte stream and tags it with a fixed length. There has been a block in grc for a long time called the Correlate Access Code – Tag Stream (kind of a mouthful, I know) which almost does this but not quite. This original block scans the 32 samples after the preamble and tries to extract a frame length field, which it will then use for the tag value. That function is kind of neat if you’re building your own transmitter/receiver pair, but it doesn’t help when you’ve got a mystery signal on your hands that won’t have that header in place.

So for a while a worked around the issue. On short projects, I’d use the now-deprecated Correlate Access Code block, dump the data to a file and whip up a quick Python script to handle things (you can see some examples of this in Volume 3 of our book series). For more complex situations, I employ WaveConverter, which I built to extract and analyze large amount of payload data. Despite these options, I always wanted a simple grc-only method of quickly looking at payload data.

All of which brings me to the block I built, Correlate Access Code – Tagged Stream – Fixed Length (yeah, I didn’t really reduce the mouthful any). I simply created a new module and block with the gr_modtool utility and copied over the *.cc, *.h and *.xml files from the original block. I then cut out the header parsing and added a property for you to set the Packet Length. After a bit of debug and cleanup, I had a useable block, which you can grab from:

https://github.com/paulgclark/gr-reveng

The example flowgraph below shows how easy it is to use:

The new block allows you to detect the preamble and tag it such that the subsequent Tagged Stream to PDU block can convert the payload (and only the payload) to a Message PDU, which you can then print out with a Message Debug sink. The PDUs simply appear in the console window as you run your flowgraph.(I’m using the print input of the Message Debug block because I have ASCII data, but if you have binary data, you should use the print_pdu input instead.)

Note that you can also dump the message PDUs to a File Sink, which gives you a nice file containing only payload data, none of the preamble or inter-frame dead air. A couple lines of Python can read this file and extract each payload into a list, from which you can do all sorts of fun stuff.

Getting Started with SDR on Black Hills InfoSec Webcast

I’ll be on the Black Hills InfoSec webcast tomorrow talking about all the things you need to know to get started with SDR. You can register for the webcast at the link below:

https://register.gotowebinar.com/register/3587817882602530306

Please bring your questions on SDR hardware, software, installation issues or anything else you can think of. See you there!

Installing gnuradio with Lime, HackRF and UHD support

The LimeSDR Mini is a great combination of things: affordable, full-duplex and incredibly configurable. Until recently, though, it wasn’t the easiest thing to get running. Scanning the forums at MyriadRF (the folks who make the Lime products) shows that a number of people have been able to get the Mini working but also that a number have not had so much success.

I was a pre-production supporter of the CrowdSupply campaign and had my LimeSDR Mini delivered in March. At first it was rough going, but after numerous attempts, I’ve been able to get a repeatable installation flow that works on Ubuntu 16.04 and supports UHD and HackRF hardware as well (it may support BladeRF, but I don’t have the hardware to test it).

You can grab the install script from my GitHub at:
https://github.com/paulgclark/grc-install

The git bundle also contains a number of simple grc files to validate your installation versus each of the three hardware platforms. After installation, my advice is that you test things with:
./grc/audio_tone.grc
./grc/lime-test/fm_receiver_hardware_lime.grc
(make sure to tune to one of your local stations)

Next, if you’ve got a second SDR (and computer) you can use the GFSK transmit and receive flowgraphs to send simple digital data back and forth between them. You should be able to send and receive from any combination of the three SDR platforms, though you will likely need to adjust the squelch levels depending on the physical distance between your SDRs. If a HackRF is involved, you’ll also need to fine tune the receiver flowgraph to compensate for their frequency error.

One thing to note about this install flow: you get version 3.7.10 of gnuradio-companion, not the latest as you’d get with a PyBOMBS installation.

Hope this helps!

Addendum: This flow will work for Ubuntu 18.04, but you’ll need to go into the install script and tweak one line per the commented instructions. You’ll also need to adjust the device args for the hackrf transmit flowgraphs to “driver=hackrf,soapy=0” (but do NOT type a space between the comma and “soapy”). A bonus to using 18.04 is that you’ll get a slightly newer version of gnuradio companion (3.7.11).

Updated gnuradio Install Instructions

We’ve gotten some feedback recently from a number of our readers that the gnuradio installation process documented in our books (and on our books’ web site) has a few glitches. We knew that we’d need to update the instructions at some point, and the time had definitely come. If you’re planning on installing gnuradio any time soon, please see our updated instructions pdf.

HackRF One’s Minimum Sample Rate Spec – There for a Reason

The HackRF One is an awesome entry platform into the wonderful world of SDR. I do, however, want to point out one thing to be careful about: operating at too low a sample rate.

In a recent class I was teaching, I introduced a new project: one in which students transmitted a simple triangle wave at a specified frequency. As good RF citizens, we of course transmitted these signals at very low power, and for short duration in an appropriate band.

I had tested this exercise several times prior to adding it to the class syllabus, but during the class itself, something went wrong. About half of the students were able to generate a transmission that could be received, demodulated and identified as a clear triangle wave… and about half couldn’t. Since each student had their own HackRF One, we swapped hardware and tried executing the flowgraphs again. Sure enough, all of the flowgraphs worked with some of the HackRF devices, and all of the flowgraphs failed with the other HackRFs. Since we were nearing the end of the class I didn’t have time to do much live debug, but it sure seemed like some of the HackRF boards were behaving differently than others.

After I got back to my lab, I did some research as well as contacting Nooelec – where I had bought the boards. A very quick response from the Nooelec folks (thanks!) led me to assume that I might have burnt out the RF amplifier chip by exposing the HackRF to excessive RF energy. Nooelec has seen this a number of times and recommends putting a 10 dB attenuator in series with your antenna. It’s easy to miss this note, but documentation does state this:

https://github.com/mossmann/hackrf/wiki/HackRF-One#receive-power

I then prepared for PCB surface mount surgery on one of my HackRFs using the following info:

http://www.t4f.org/articles/repairing-the-hackrf/

Before I made the incision, however, I decided to check a few more things. I set up an Ettus B200 in receive mode and tried to fiddle with several HackRF boards in transmit mode. The Ettus board has better ADCs, a better RF front end… pretty much better everything than the HackRF. My hope was that I could figure out a little more about the failure mode of the HackRFs. After poking around for a bit, I came to a startling conclusion: the boards were fine, I just made a boneheaded mistake!

You see, I was a little concerned about the performance of the laptops that my students might be bringing to the class, or that they might be trying to use a VM for the class exercises. As such, I had the not-so-brilliant idea of reducing the sample rates of the class exercise to 1 Msps. The documentation recommends a minimum sample rate of 8 Msps, but I’ve always played a bit fast and loose with that figure without too much trouble. Even as low as 1 Msps, I’d often gotten usable results.

When I upped the sample rate to 8 Msps, however, it was clear that the boards I had thought broken were just fine:

Top Block_012

Even as low as 2 Msps, the triangle wave was clearly visible. At 1 Msps, though, things started to break down for some boards:

Top Block_019

But not others:

Top Block_018

After all of this, I took a look back at the specs, and sure enough, I noticed the 2 Msps is the listed minimum sample rate after all.

A happy, if slightly embarrasing, ending.

Great Time with SDR at Wild West Hackinfest!

A couple of weeks ago, I had the privilege to teach an SDR class at the Wild West Hackinfest in Deadwood, SD. We spent two days working through radio and SDR concepts from the ground up. As always, I kept the lecturing to a minimum and concentrated on a number of hands-on projects.

During the conference itself, I hosted a reverse engineering lab in the main convention floor. We used gnuradio to break down a simple fan controller signal and identify the functions of all its payload bits. We then assembled and transmitted our own control signal to take over the system. It was a simple device to reverse, but each of the steps we took was applicable to more complicated digital radios.

Version 2

I was had a few copies of the Field Expedient SDR series for sale, but unfortunately not quite enough. If you stopped by my table too late to get a copy, you can still order copies in either paperback, Kindle or iBooks formats.

The conference itself was an amazing experience, with a host of awesome speakers and a ton of labs. The vibe was laid back and fun, and I’m grateful to Black Hills InfoSec and John Strand for letting me participate. If you have a chance to make it next year, I’d strongly recommend it.

Welcome Back to Factoria Labs!

After about a year of leaving our old web site in place, we’ve finally gotten around to revamping factorialabs.com with some new stuff – the most significant of which is this blog. Check back with us every week for tips, tricks and general commentary on the wonderful world of SDR!

We’ll also be rolling out public SDR trainings in 2018, and we’ll have the schedule (and sign-ups) on the new-and-improved site as well.