Table of Contents

HackTheBox - Challenges - Crypto - Call

Provided Files:

You are provided one file:

  • sound.mp3

Walk Through:

Playing the sound.mp3, you’ll hear a familiar sound. If you’re not familiar the sounds you’re hearing are hearing DTMF (dual tone multi frequency) tones. The same tones you used to hear while dialing on a pay phone or while getting through automated teller menus.

Each tone has a specific frequency. You could get the numbers manually, but who has the time for that? DialABC has a great tool for this, but doesn’t support mp3 files. First, you’ll have to convert it to .wav with this tool

Take the converted file to DialABC and you’ll get the following output:

2331434783711923431767372331117714113

Take notice that if you listen to the audio file carefully or open it in Audacity or Sonic Visualizer that, with one exception, the numbers are paired in groups of two. If you separate out the number you get the following:

23 31 43 47 83 71 19 23 43 17 67 37 23 31 11 7 71 41 13

Organized like this, you might be confused and think that it might be HEX. It isn’t.
Pay close attention to the numbers. What mathematical trait do each grouping of numbers share?…. They are all prime numbers. Which should bring you to try the lesser known prime number cipher.

We’ll use Decode.fr to complete this challenge.
Submit the cipher text from before we separated it out and you’ll get the flag.

2331434783711923431767372331117714113

Flag Example:

HTB{xxxxxxxxxxxxxxxxxxx}