Wanna

Platform: CUSTOM | Category: Crypto | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: image_reconstruction, known_plaintext_attack, xor_key_recovery

Summary

Walkthrough of the CUSTOM Crypto Wanna. Task: Decrypt a ransomware-encrypted BMP file. Solution: Used known-plaintext attack to recover 16-byte XOR key from black pixels, then decrypted image data to reveal the flag.

Description

Ransomware-encrypted BMP file. The task name references WannaCry and ECB mode.

Analysis

• File size 900064 bytes = 64-byte header + 900000 bytes of image data
• XOR encryption with a 16-byte repeating key
• Encryption starts at offset 64 (header is not encrypted or encrypted separately)
• BMP images often contain areas with zero/black pixels
• Data size 900000 = 1000 × 300 × 3 (RGB) — standard image dimensions

Key Recovery

Using known-plaintext attack: if pixels in the original were black (0x00), then encrypted bytes = key XOR 0x00 = key. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Recovered 16-byte key: cc5c1c2ca8e0b1570fb1be945cc91d98

Solution

 python read-only • copy allowed Copy
#!/usr/bin/env python3
from PIL import Image

# Read encrypted file
with open("encrypted.bmp", "rb") as f:
    data = bytearray(f.read())

# 16-byte XOR key (recovered from known plaintext)
key = bytes.fromhex("cc5c1c2ca8e0b1570fb1be945cc91d98")

# Decrypt data starting from offset 64
for i in range(64, len(data)):
    data[i] ^= key[(i - 64) % 16]

# Extract RGB data (after 64-byte header)
rgb_data = bytes(data[64:])

# Create 1000x300 RGB image
# BMP stores rows bottom-to-top, so we flip
width, height = 1000, 300
img = Image.frombytes('RGB', (width, height), rgb_data)
img = img.transpose(Image.FLIP_TOP_BOTTOM)  # BMP bottom-to-top
img.save("decrypted.png")
img.show()
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# Flag visible in image: REDACTED

Alternative Key Recovery Method

 python read-only • copy allowed Copy
#!/usr/bin/env python3
# Key recovery from known plaintext (black pixels)

with open("encrypted.bmp", "rb") as f:
    data = f.read()

# Look for repeating pattern in encrypted data
# If original contained zeros, encrypted bytes = key
encrypted_body = data[64:]

# Analyze first 16 bytes as potential key
# (assuming first pixels were black)
potential_key = encrypted_body[:16]
print(f"Potential key: {potential_key.hex()}")
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# Check periodicity
for i in range(0, min(160, len(encrypted_body)), 16):
    chunk = encrypted_body[i:i+16]
    if chunk == potential_key:
        print(f"Key repeats at offset {i}")

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Key Takeaways / Lessons

• Mastering image_reconstruction, known_plaintext_attack, xor_key_recovery was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-09 • CUSTOM • Wanna • Difficulty: Medium