has-writeup/ground-segment/phasors-to-stun

Phasors to Stun

Category: Ground Segment Points (final): 62 Solves: 71

Demodulate the data from an SDR capture and you will find a flag. It is a wav file, but that doesn't mean its audio data.

Write-up

by haskal

The provided WAV file contains a signal that looks like this:

This looks suspiciously like Phase Shift Keying (PSK) and it's a very clean signal (this is also hinted at by the challenge name). We can use Universal Radio Hacker to demod this with very little effort.

Select PSK modulation, then click "Autodetect parameters". Then move to Analysis:

We discovered that the signal is NRZI (non-return-to-zero inverted) coded, and after selecting this in URH the flag is decoded in the data view.

Resources and other writeups

• https://github.com/jopohl/urh
• https://en.wikipedia.org/wiki/Phase-shift_keying
• https://en.wikipedia.org/wiki/Non-return-to-zero#NRZI