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)
gnuradio.ogg