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Phone Home

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Challenge Overview

The challenge provides an audio file containing DTMF tones. The goal is to extract the hidden message from the audio and then decode it to obtain the final flag.

Note: DTMF stands for Dual-Tone Multi-Frequency, a signaling system used in telecommunications to transmit keypad input (0-9, *, #) over voice channels.

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DTMF Decoding

First, the audio file was analyzed using multimon-ng to decode the DTMF signals:

multimon-ng -t wav -a DTMF phonehome.wav

This command processes the WAV file and outputs a sequence of digits generated by the DTMF tones. The resulting output represents an octal-encoded string.

15558675309#115145164141103124106173147060164137171060165162137156165155142063162065175

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Octal Decoding

The extracted octal string after "#" (the preceding digits likely representing the dialed phone number) was then decoded using CyberChef:

  • Operation: From Octal

  • Input: DTMF-decoded numeric output

After decoding, the operation reveals the plaintext message, which contains the final flag.

made by k0d

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