ctf-writeup/2023/NewportBlakeCTF 2023/do you hear that

do you hear that?

I'm not sure why, but when I look at this image I can hear some sort of faint sound. Do you hear it too?

About the Challenge

We were given an image called help.png (You can download the file here) and if we check using zsteg there's a wav file inside the image

root@ubuntu:~# zsteg help.png
[?] 317564 bytes of extra data after image end (IEND), offset = 0x8a24
extradata:0         .. file: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 44100 Hz
    00000000: 52 49 46 46 74 d8 04 00  57 41 56 45 66 6d 74 20  |RIFFt...WAVEfmt |
    00000010: 10 00 00 00 01 00 01 00  44 ac 00 00 88 58 01 00  |........D....X..|
    00000020: 02 00 10 00 64 61 74 61  50 d8 04 00 00 00 00 00  |....dataP.......|
    00000030: 01 00 03 00 05 00 08 00  0a 00 0b 00 0b 00 09 00  |................|
    00000040: 06 00 02 00 fe ff f9 ff  f6 ff f4 ff f5 ff f7 ff  |................|
    00000050: fb ff 00 00 04 00 08 00  09 00 06 00 00 00 f7 ff  |................|
    00000060: ec ff e0 ff d5 ff cd ff  c8 ff c8 ff ce ff d7 ff  |................|
    00000070: e3 ff f0 ff fb ff 03 00  05 00 01 00 f7 ff e9 ff  |................|
    00000080: d8 ff c9 ff be ff b9 ff  bd ff cb ff e0 ff fd ff  |................|
    00000090: 1b 00 39 00 51 00 60 00  64 00 5c 00 4b 00 32 00  |..9.Q.`.d.\.K.2.|
    000000a0: 18 00 02 00 f6 ff f7 ff  08 00 29 00 57 00 8d 00  |..........).W...|
    000000b0: c3 00 f1 00 0e 01 16 01  05 01 db 00 9e 00 55 00  |..............U.|
    000000c0: 0c 00 ce ff a6 ff 9d ff  b7 ff f2 ff 46 00 a4 00  |............F...|
    000000d0: fb 00 36 01 40 01 09 01  88 00 ba ff aa fe 6a fd  |..6.@.........j.|
    000000e0: 17 fc d3 fa c5 f9 10 f9  d2 f8 21 f9 01 fa 6d fb  |..........!...m.|
    000000f0: 4c fd 7b ff cb 01 09 04  04 06 8f 07 89 08 de 08  |L.{.............|
[!] possible image block size is 639x5, downscaling may be necessary

How to Solve?

First, we need to extract the WAV file inside the png using this command

zsteg -e extradata:0 help.png > sound.wav

And then use spectogram analyzer, there's an online tool like https://www.dcode.fr/spectral-analysis or you can use Sonic Visualizer

bctf{y0u_h4v3_s0m3_g00d_34rs}