FCSC 2021 - Intro - Waterfall

May 03, 2021   
radio-frequencies writeup 
Also available in:  🇫🇷 


Challenge

One of the ways to study a signal is to look at the evolution of the frequencies used over time. This can be done with a signal spectrogram (also called a "waterfall" diagram).

Find the message contained in the challenge.iq file.

challenge.iq (68MB) : https://files.france-cybersecurity-challenge.fr/dl/waterfall/challenge.iq.7z

SHA256(challenge.iq) = 445c04fd72b33915bd694ebdf062c4083108848fa17e4a55c9ec6f3d54baa80d.

Files :


Solving the challenge

To solve this challenge, I opened the IQ capture in the GQRX software with the following parameters:

file=./challenge.iq,freq=10e6,rate=1e5,repeat=true,throttle=true

Then we start reading the capture, and we see the flag appear character by character in the waterfall of the Fourier transform:

And after a long copy by hand, we get the flag:

FCSC{7db25e20bed7c6324609c8c81dc266be31ef216ef8580e95bc11b432f74900ca}