L3akctf 2025 | Beneath the Surface

File#

Solution#

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exiftool beneath_the_surface.wav
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ExifTool Version Number         : 13.30
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File Name                       : beneath_the_surface.wav
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Directory                       : .
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File Size                       : 6.3 MB
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File Modification Date/Time     : 2025:07:13 12:40:18+02:00
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File Access Date/Time           : 2025:07:13 12:40:19+02:00
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File Inode Change Date/Time     : 2025:07:13 12:40:18+02:00
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File Permissions                : -rw-r--r--
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File Type                       : WAV
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File Type Extension             : wav
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MIME Type                       : audio/x-wav
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Encoding                        : Microsoft PCM
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Num Channels                    : 1
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Sample Rate                     : 8000
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Avg Bytes Per Sec               : 16000
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Bits Per Sample                 : 16
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Title                           : Generated audio
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Software                        : fldigi-4.2.07 (libsndfile-1.0.28)
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Comment                         : WEFAX576 freq=14011.900
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Date Created                    : 2025:07:11T10:21:36z
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Duration                        : 0:06:35

The Software and Comment fields are useful, these indicate that the audio file was generated by fldigi which contains a WEFAX (Weather Facsimile) image, transmitted at 576 lines at a frequenct of 14011.900 kHz. So, I install fldigi with yay -S fldigi (I use Arch, btw). Open the program and go to Op mode -> WEFAX -> WEFAX576, and then, on the main window, set the frequency to 14011.9 kHz. Now, upload the file on File -> Audio -> Reproduction. Please be patient and wait for the program to decode the entire audio and get the flag.

flag: L3AK{R4diOF4X_1S_G00d_4_ImAG3_Tr4nsM1sSiON}