Re: [Discuss-gnuradio] Analyze Waveform with GnuRadio

[Marcus Müller]

Hi Nathan,

I will choose the option of helping to enhance the functionality of the Repeat block. 

We like that attitude here :)

I have an .OGG file that sounds like a data transmission that needs to be demodulated.

So that tells us your signal has audio frequency content only; whoever posted it was confident that vorbis coding doesn't harm decodability too much.
For now, this won't tell you much, but for later on, when you've understood a bit about your signal: Read a bit about the ogg vorbis codec.

  A clue left behind in another challenge indicated that gnuradio with default blocks could be used to solve this one.

Phew.
Err well GNU Radio, at this time, contains decoders for different digital TV standards, satellite images, a lot of constellations (PSK, QAM), FSK, OFDM...
But let's assume this means you won't need "higher order" modulations such as OFDM.

Not knowing anything about radio, or frequencies, or GNUradio,

Not knowing about GNU Radio: not really a problem, something that you can learn by playing around.
Not knowing about "frequencies" means you might be weak on the theory involved, and that can make things quite complicated.
So, this is a hacking challenge, right? So what does a hacker do? He tries to understand the system at hand. GNU Radio's wiki has a page on SuggestedReading, I'd strongly encourage you to at least read through Micheal Ossman's DSP/SDR tutorials; otherwise, it will look to you as if GNU Radio was just a system to plumb together blocks, and that's not giving you the insight you need to understand your signal.

  I've been able to get file output from gnuradio, but I'm not doing it right as it's just jumbled data.

After that, you might want to think about what you hear. Take a few notes. What's special about the sound you hear?

Make yourself acquinted with the GNU Radio blocks that help analyzing stuff: they're under "Instrumentation/Qt".
To make yourself more at ease with working with these, start by reading through chapters 1 and 2 of
https://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorials
Chapter 3 will get really interesting, but you need to understand 1 and 2 first.

"Demodul me! 2400 bauds challange - Basic RZ with no preamble"

Ha! It's awesome that you discovered that.
Now, that means two things:
1) you need to understand what "2400 bauds" means (ok, I guess I didn't need to tell you that), and
2) you see that in a spectrogram. Which, assuming this challenge is not too hard, means that this text is not part of the actual data. Which implies that you can do something with parts of your spectrogram, right? What does that mean (this is basically asking you to understand what a spectrogram shows you).

Best regards,
Marcus

On 26.07.2015 23:05, Nathan Coppersmith wrote:

Hey all, (long post)

I'm trying to solve a cyber security challenge using GnuRadio, so I'm very new, and very lost.  I've spent the last few days reading, playing, experimenting, and trying to decipher the data contained within, but I'm at a loss, so I'm turning to ask for specific help, as the interwebz has not provided a clear path to the solution.  I'm not looking for an outright answer, but rather an analytical process that could be applied via lateral thinking to other challenges later on. (teach me to fish, don't give me a fish outright)

Here's what I know:  I have an .OGG file that sounds like a data transmission that needs to be demodulated.  A clue left behind in another challenge indicated that gnuradio with default blocks could be used to solve this one.  If you use a spectrum analyser (like sonic visualiser) and apply a spectrogram filter, you can see the following words embedded in the file: "Demodul me! 2400 bauds challange - Basic RZ with no preamble". You can also see this in Gnuradio by converting the OGG to WAV and viewing a GUI Waterfall sink. (fyi challange is spelled that way in the file, not sure if it's a typo or a hint)

Not knowing anything about radio, or frequencies, or GNUradio, how can I go about finding out what's inside this file?  I've been able to get file output from gnuradio, but I'm not doing it right as it's just jumbled data.  I've done significant reading on various modulation/demodulation schemes, but nothing seems to work the way I'm applying it.  It looks like the frequency is in the 400-650Hz range, but I don't think it's an RF signal, the embedded clue makes me think it's a packet stream from a dial-up modem.  Examining the waveform leads me to believe it's a frequency shift key type modulation.  I've tried to setup a flow in GRC of Wav File Source -> Throttle -> Float to Complex -> (various demodulators) -> Packet Decoder -> File Sink.

I don't think I need to modulate the input first, as I believe it's the raw modulated sound to begin with.  Not being versed in this area and feeling I've exhausted what's available via Google, I'm hoping someone can point me in the right direction.  I'm attaching the OGG file I'm using as input after converting to wav because it's smaller and I can't seem to get GNUradio to take OGG as a file input for some reason. (hopefully that doesn't break a TOS I didn't see)

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