Anantya ByteMe CTF Writeup Series: The Raven of the North

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Welcome Back to the Official Write-Up Series of ByteMe CTF!

The OWASP PCCOE Student Chapter is excited to present the 4th installment in our series. This challenge, The Raven of the North, moves us into the realm of Steganography and Classical Cryptography, requiring a mix of encoding analysis and ciphertext decryption.

Category: Steganography / Cryptography

Difficulty: Easy-Medium

Author: Zeeshan

Theme: Game of Thrones / Hidden Messages


1. The Encrypted Archive

We begin with a password-protected ZIP archive named wbua.zip. To proceed, we need a password, but the filename itself feels like a clue.

The Hint in the Name: In many CTF challenges, simple substitution ciphers are used for hints. Applying ROT13 to the filename:

  • wbua -> john

This is a direct pointer to John the Ripper, the famous password-cracking utility.


2. Cracking the ZIP Password

Using the hint, we apply John the Ripper to wbua.zip. The tool successfully recovers the password in seconds.

Result:

  • Password: a1b2c3

  • Extracted File: raven.png





3. Inspecting the Raven

The extracted file, raven.png, appears to be a standard image. However, in steganography, data is often appended to the file structure where the human eye can't see it.

By running the strings command, we look for printable sequences within the binary file:

strings raven.png



Hidden at the end of the output, we find a strange string:

JHGJQX JH QJKA


4. Identifying the Cipher

The challenge provided a secondary hint file with two critical clues:

  1. "Vigenere is old, but gold."

  2. "The King in the North holds the key."

The Breakdown:

  • Cipher: Vigenère (a polyalphabetic substitution cipher).

  • Key: The "King in the North" in the context of our theme is JON (Jon Snow).


5. Decrypting the Message

We now have our ciphertext and our key. Using a Vigenère decoder (like dCode or a custom script):

  • Ciphertext: JHGJQX JH QJKA

  • Key: JON

  • Decrypted Plaintext: ATTACK AT DAWN


6. Capturing the Flag

By wrapping the decrypted message in the standard event format, we successfully claim the points.

Flag: ByteMe{ATTACK_AT_DAWN}


Final Words

The Raven of the North reminds us that hints are everywhere—even in the filename. Whether it's ROT13, metadata, or hidden strings, a keen eye for "out of place" data is a security researcher's best tool.

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