# Phasors to Stun

**Category:** Communication Systems
**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](https://awoo.systems)

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

![Audacity showing the signal](signal.png)

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.

![URH with the signal open](urh1.png)

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

![URH Analysis pane](urh2.png)

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>
