no-sudo-for-you's Security Lab | DawgCTF

ASK Me Again (MISC)

Category Miscellaneous

Points 200

Difficulty Hard

This challenge deals with signals and modulation techniques, specifically Amplitude-Shift Keying (ASK) as hinted by the title. The challenge provides a .wav file that needs to be analyzed to recover the hidden data.

Understanding ASK Modulation

Amplitude-Shift Keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In its simplest form, the presence of a carrier wave at a certain amplitude represents a binary "1", while its absence (or a lower amplitude) represents a binary "0".

ASK Modulation:
In ASK, the carrier signal's amplitude changes to represent different binary values. This is commonly used in short-range wireless communications and can be demodulated with relatively simple techniques.

Analyzing the Signal

To analyze the provided .wav file, I used GNU Radio Companion, a graphical tool for creating signal processing applications. I built a simple flowgraph to demodulate the ASK signal and extract the binary data.

GNU Radio Flowgraph for ASK Demodulation

The key components of the flowgraph included:

A file source to read the .wav file
A low-pass filter to remove noise
An AM demodulator to extract the amplitude variations
A threshold detector to convert the analog signal to digital (binary)
A Binary Slicer to clean up the digital signal
A file sink to save the demodulated bits

After running the flowgraph, I converted the resulting bit stream to ASCII characters to reveal the flag.

$ python3 decode_bits.py Decoded data: DawgCTF{D3M0DUL4710N_1S_FUN}

FLAG: DawgCTF{D3M0DUL4710N_1S_FUN}

Key Learning: This challenge demonstrates basic concepts of digital signal processing and modulation techniques used in telecommunications. ASK is one of the simplest forms of digital modulation, but the principles extend to more complex modulation schemes like FSK (Frequency-Shift Keying) and PSK (Phase-Shift Keying).