HCSC 2024 - Cool o’ red Met Alls

Description

A flag formátuma: HCSC24{...}

Hint 1 (cost 0): Admin bácsi közli velem
megy ez, mint a veszedelem…
Ami nem megy, nehéz vagyon
súgjunk nekik, kicsit-nagyot…
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Segítségül egy kis hint:
A zip fájlban van több kincs!
Hogy hogyan nyílik, képen rejlik:
Az arányok is ezt segítik!
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Metadata

• Tags: zip, steghide, ratios
• Points: 400
• Number of solvers: 1
• Filename: bogyo.jpg, fs.jpg, vonalzo.jpg, flag.zip

Solution

I was not able to solve this challenge, the solution is from bombera

If we check the size of the image (and use the hint), we might figure out that it is the golden ratio (968/598 = 1.6187290969899666)

$ exiftool bogyo.jpg
Exif Image Width                : 968
Exif Image Height               : 598
$ exiftool fs.jpg
Image Width                     : 967
Image Height                    : 667

The flag.zip is an encrypted ZIP file, with the password: golden_ratio

There are 4 new files:

$ zipinfo flag.zip 
Archive:  flag.zip
Zip file size: 1542852 bytes, number of entries: 4
-rw-r--r--  3.0 unx   226072 BX defN 24-Feb-25 11:41 bookcover.jpg
-rw-r--r--  3.0 unx   911508 BX defN 24-Feb-25 17:08 divines.jpg
-rw-r--r--  3.0 unx   355432 BX defN 24-Feb-25 17:16 Number.png
-rw-r--r--  3.0 unx    57781 BX defN 23-Oct-27 13:50 problem.png

The last step is a steghide extraction with the password Fibonacci on bookcover.jpg. I have no idea how we get this password (maybe the Problem.png with the rabbits).

$ steghide extract -sf bookcover.jpg 
Enter passphrase: 
wrote extracted data to "flag.txt".

$ cat flag.txt 


   HCSC24{
        p H y m S o a 3 b u 4 e 
        q X x O P R v h u g B B 
        G 0 U v A j z B m e 8 R 
        T V 6 Y F B t 9 U c B t 
        J 2 8 r y F n 7 a p w S 
        1 N O e t 5 v g 5 d x 0 
        F G P V X f W S X f K A 
        X b M f c W L c 7 d M o 
        z c n v n L M 3 Y G J L 
        P m i c 8 F K G o d A K 
        1 4 I f o c R 6 v s G F 
        n 2 x D B S 3 d n b A 3 
    }


That's easy... 

I have no idea what is the last step…

Now we have to start from p and step as the Fibonacci sequence: p -> +1 -> H -> +1 y +2 -> S -> +3 -> 3 -> +5 -> q -> …

Flag: HCSC24{pHyS3quenc3}