Diff is a stego challenge that was part of the 2022 After Dark CTF - Fall. The challenge description was:

I like listening to my music in mono, but someone broke my converter. Here, listen to this. It’s supposed to be mono, but the left and right channels are slightly different. Could you help me figure out what the difference is, so I can try to fix it?

(All letters lowercase, separate words by _, and wrap with flag{})

doomsday.wav (this)

Solution

Like the description suggests, we have to find the difference between the left and right channel of the given audio file.

We can do this with Audacity by splitting the audio file into left and right channel.

Splitting the channel in Audacity

Then invert one of them and join them together again.

Inverting one of the tracks in Audacity

The result is a new audio file that contains the difference between the left and right channel.

diff_doomsday.wav

If we upload this file to https://academo.org/demos/spectrum-analyzer/ we can see Morse code:

The visualized signal at academo.org 
-... . . .--. -... --- --- .--.

This translates to BEEPBOOP.

Flag

The flag is “flag{beep_boop}”.