phasors to stun

ground-segment/phasors-to-stun/README.md

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+# 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](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>