Author: Sudesh Gowda J
Flag: CTF{PLEASEBUYATOUCHPHONE}
Agent "Le chiffrage indéchiffrable" recieved an audio from an invincible device named "Kinoa". Help the agent decode the message.
Flag format: CTF{FLAGINCAPSWITHNOSPACES}
Proposed difficulty level: Medium
When chall.mp3 is opened in an audio editor like Audacity, it can be seen that it is a stereo track(this is also evident while listening). There are DTMF tones in both tracks. Splitting the tracks and then using some online tool to detect the tones will give 22 7 9 7777 7777 55 333 44 6 8 55 7777 9 55 555 55 555 777 9999 444 and 6 33 7777 7777 2 4 33 66 666 8 777 33 222 444 33 888 33 3.
Applying Multi-tap cipher on the obtained sequence of numbers will give BPWSSKFHMTKSWKLKLRZI and MESSAGENOTRECIEVED, respectively. The name "Le chiffrage indéchiffrable" leads us to Vigenere cipher. We have two strings, using MESSAGENOTRECIEVED as the key for decrypting the ciphertext BPWSSKFHMTKSWKLKLRZI will provide the flag PLEASEBUYATOUCHPHONE.