Summary#

This challenge combines two concepts: image steganography (using steghide) and the Atbash cipher. The goal is to extract hidden data from a JPEG image and decode it using Atbash.

Analysis#

We are provided with a file named atbash.jpg:

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$ file atbash.jpg
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atbash.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 465x455, components 3

So the file is a normal JPEG image:

Step 1: Extracting Hidden Data#

The most common steganography tool for JPEGs is steghide, which can hide text or files inside images.

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$ steghide atbash.jpg
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wrote extracted data to "encrypted.txt".

This gives us a new file:

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$ cat encrypted.txt
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krxlXGU{zgyzhs_xizxp_8z0uvwwx}

So the hidden message inside the JPEG is:

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krxlXGU{zgyzhs_xizxp_8z0uvwwx}

It clearly looks like a flag, but encoded.

Step 2: Understanding Atbash#

Step 3: Manually Decoding the String#

Let’s decode it character by character.

• k → p
• r → i
• x → c
• l → o
• X → C
• G → T
• U → F

So:

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krxlXGU  → picoCTF

Continuing the Atbash mapping over the rest of the string gives : picoCTF{atbash_crack_8a0feddc}

Manual decoding works, but to speed things up we can also use an online tool such as the Atbash Decoder

⚡ Raikiri

🎉 Flag pwned!

Final flag : picoCTF{atbash_crack_8a0feddc}

💡 TL;DR / Lesson Learned

This challenge shows how to extract hidden data from a JPEG using steghide and decode it using the Atbash cipher, revealing the final flag.