Hackvent 2024: Day 24

[HV24.24] Stranger Sounds

Santa received a file with some very strange audio. He's kind of scared, it sounds like some monster who's about to hunt him down. Help him get to the bottom of it.

Analyze the audio and get the flag.
Flag format: HV24{}
sha256sum of stranger-sounds.mp3: f382c74ec2d1030eb3a1df6df9f0debf62b7ca652f56b44215e58031f2c68955

This challenge was written by coderion. Last, but definitely not least.

Sound file:

Solution

The sound file provided is an MP3 file with a length of 35:20 minutes. The audio itself sounds like strange noises, somewhat resembling ultrasound tones. We spend some time exploring the spectrogram view and inspecting the LSB for the flag, as these are two common methods of hiding messages in audio files. However, this yields no result.

At this stage, the critical piece of information to consider is that Coderion enjoys Rickrolling participants in their challenges. The MP3 duration is also quite long for a challenge. We look up the official Rick Astley video on YouTube and realise it is 3:32 minutes long. This is exactly 10 times shorter than our current audio. Our audio also shows subtle signs of voice distortion or stretching.

A hint reveals to us that we should reverse the audio as a first step. We do this and then speed up the audio by a factor of 10. This gives us an audio file that is the Rickroll audio:

Unfortunately, we must listen to the entire song for further clues. At 2:17 in the audio, we hear a TTS voice begin speaking and enunciating each character in the flag. It is a little hard to hear, but we eventually make out what it says and retrieve our daily flag.

Flag:

HV24{g3t_r1ckr0ll3d}