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README.md

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

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

Audacity showing the signal

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.

URH with the signal open

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

URH Analysis pane

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