2020-05-26 06:29:17 +00:00
|
|
|
# Phasors to Stun
|
|
|
|
|
|
2020-05-27 02:00:32 +00:00
|
|
|
**Category:** Communication Systems
|
2020-05-26 06:29:17 +00:00
|
|
|
**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:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
This looks suspiciously like Phase Shift Keying (PSK) and it's a very clean signal (this is also
|
2020-05-26 06:30:24 +00:00
|
|
|
hinted at by the challenge name). We can use [Universal Radio Hacker](https://github.com/jopohl/urh)
|
2020-05-26 06:29:17 +00:00
|
|
|
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>
|
