# Radio Intercept - Midnightsun CTF 2020

Alpha, Brabo, Delta! We need to recover radio signal from noise

## Problem

The problem provides two files. ```receiver.png``` and ```recv.bin```. Image file shows flow graph of GNu radio.


RTL-SDR Source is FM radio reciving doggle for USB port. Receive disignate radio signal from certain frequency. Pass it to Band Pass filter and multiply sine wave. Since we know all parameter of sine wave, we can recover origianl wave. ```Keep 1 in N``` align output. Now, convert complex type data to byte and save to output file (```recv.bin```).

## Walkthrough


First we look over what's in the given data file. It seems like there are audio data over two different frequency. So, I decide to cut off below with High-pass filter block.

Second step is to reverse multiplied cosince wave. Using divide block in GNu radio, we can recover original radio data.


WBFM Receive block is work as Demodulator of FM radio signal. FM radio is travel with other carrier wave. Compare to AM, FM transmission requires frequency demoluation (AM requires Amplitude Demoulation). Choosing parameter is somewhat difficult, but FM radio tutorial in Youtube help us (plus, some parameter from origial flowgraph). Within retrived audio file, a speaker said bunch of English words similar to Alpha, Bravo, etc. Some words are hard to identify, but googling with ```nato phonetic alphabet "guitarist"``` lead to ```PAG phonetic alphabet```, which is phonetic alphabet for ASCII symbol.

Now, it was time to dictation and guessing. Finally, we retrieved following flag.

goggles : m
guitarlist : i
flytrap : d
headwaters : n
gazelle : i
graduate : g
Frighten : h
hydraulic : t
kickoff : {
enterprise : S
Gremlin : o
hazardous : m
fracture : e
hydraulic : t
gazelle : i
hazadous : m
fracture : e
Hurricane : s
eyetooth : _
decadence : A
Glucose : l
hesitate : p
frighten : h
frequency : a
eyetooth : _
december : B
highchair : r
frequency : a
inverse : v
Hemisphere : o
eyetooth : _
guitarlist : i
hockey : s
Forever : _
Goldfish : n
Hemisphere : o
indoor : t
forever : _
fracture : e
headwaters : n
gremlin : o
impartial : u
freedom g
gravity : h
klaxon: }