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Heard of Hex?

  • Published: 02/06/2024 (#3/11 in round)
  • Category: Cryptography/Steganography
  • Points: 70
  • Author: Vipin

Find the secret message hidden in the image.

Attachments

Write-up

Reveal write-up

A common procedure on downloaded files, particularly in steganography challenges, is to see if the flag is hidden directly inside the file using strings. Therefore, we can run this command:

strings xehedoced.jpg

This command will print out all sequences of at least 4 printable characters in the file, which results in: (only the first 10 lines are shown)

JFIF
tju{TpApEjoFY8WKiCu8s0Vgr0Y7zdURs9326BDM5Ftc}
I:c:pd
CI"c$
gNRRN-8
#&M:i
'`gH(J\
RM2N
&i gvc
)E!'g

The line tju{TpApEjoFY8WKiCu8s0Vgr0Y7zdURs9326BDM5Ftc} looks like the format for our flags, which is csd{flag}. Since the characters are all letters, this can be a ROT/Caesar cipher. We can use a tool like CyberChef to decode it.

In CyberChef, the "ROT13 Brute Force" operation is useful if we know it's a ROT cipher but don't know the number it is being rotated by. We can input input the string and use that operation, which outputs a series of text, one of which is our flag. Since we know that our flag format starts with csd, we can see that it was rotated 9 times.

Amount =  9: csd{CyJyNsxOH8FTrLd8b0Epa0H7imDAb9326KMV5Ocl}

Flag: csd{CyJyNsxOH8FTrLd8b0Epa0H7imDAb9326KMV5Ocl}

Write-up by GodderE2D