Serial Killer 5 - CTF Writeup

Challenge Name: Serial Killer 5
Category: Osint
CTF: MOJO-JOJO
Description: What…? This can’t be right.
All trails lead back to where everything started — the first location, the first clue.
By retracing the suspect’s movements and searching every possible lead, investigators finally uncovered one last piece of evidence:
An encrypted ZIP archive, with nothing but a weird chessboard position, seemingly arranged without logic or rules.
But this board is no random game — it leads the final message.
Decrypt the archive. Analyze the board. Only then will you be able to uncover the real identity of the killer.
Flag Format: MOJO-JOJO{Killer_Identity}

Solution

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Step 1: Identify the Password Length

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Returning to Serial Killer 4, The YouTube video titled “the password length is 9” provided the critical first clue — the ZIP archive was password-protected with a 9-character password.

Step 2: Decode the Image Handout

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Using CyberChef, we decoded the image handout by interpreting it as binary data where:

• 0 = White pixel
• 1 = Black pixel

This binary-to-image conversion revealed the digit: 6

Step 3: Combine Previous Flags

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From earlier stages of the investigation, we had collected 4 flags containing 8 digits in total. Combining these with the newly decoded digit (6), we constructed the complete 9-character password:

716722036

Step 4: Extract the ZIP Archive

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Using the password 716722036, we extracted the contents of the encrypted ZIP archive.

unzip -P 716722036 handout.zip

Step 5: Analyze the Extracted Picture

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The ZIP archive contained a picture file. Upon examination of the image, it revealed the hidden message containing the killer’s identity.

Final Flag

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Reading the decrypted image yielded the final flag:

MOJO-JOJO{Saffe7_Lboudreya}