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)[https://github.com/jopohl/urh] 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