Killer Queen CTF 2021
Underscore in Corrupted [409 pts]
My music is ruined.
kqctf{y0u_r3c0v3r3d_my_m4573rp13c3!_1_c4n_m4k3_34r5_bl33d_4n07h3r_d4y.}
tl;dr
Introduction
We are given a sound file with a phone conversation where a lady types in her credit card number on her mobile phone.
The phone is old, so each character emits a different sound – their Dual-Tone Multi-Frequency (DTMF). Based on these frequencies, we can decode the numbers pressed and get her card number.
This can be done manually by inspecting frequency pairs and using a lookup table, but we can also just download a tool to do it automatically: https://github.com/ribt/dtmf-decoder
With this, we just run
./dtmf.py tippytappiesbutmobile.wav
This doesn’t give an output that looks correct, but we can change the interval. Setting it to half a second looks good:
./dtmf.py tippytappiesbutmobile.wav -i 0.5
This results in
14716097646384761
When we listen to the file, we hear she first types in the 1 as an option and then the card number. So we remove the first 1, leaving the 16-digit number, which is the flag when wrapped in the flag format.