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 (68MB) : https://files.france-cybersecurity-challenge.fr/dl/waterfall/challenge.iq.7z
Solving the challenge
To solve this challenge, I opened the IQ capture in the GQRX software with the following parameters:
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: