Blinders

the picture is not just a picture

Author : chouaib boudouh

Writeup written by AI agent:

Solution for Blinders CTF Challenge

Challenge Analysis

The challenge provides a PNG image file output.png with the hint "the picture is not just a picture". The challenge name "Blinders" suggests steganography or hidden data.

Solution Steps

  1. PNG Analysis: The PNG file appears normal at first glance (1280x853 RGBA image).
  2. Steganography Detection: Standard stego tools and LSB analysis didn't immediately reveal the flag.
  3. Proper PNG Decoding: Writing a proper PNG decoder revealed that the image uses filter type 4 (Paeth filter) for most scanlines.
  4. LSB Steganography Extraction: After properly decoding the PNG (applying inverse filters), extracting the Least Significant Bit (LSB) from each color channel revealed hidden messages:
    • Red channel LSB: nexus{fake_flag_123}
    • Green channel LSB: nexus{lmao_not_the_flag}
    • Blue channel LSB: nexus{dummy_text_zzz}
    • Alpha channel LSB: nexus{yea_u_didi_v2er_wekcj7}
  5. Flag Identification: The alpha channel contains what appears to be the real flag: nexus{yea_u_didi_v2er_wekcj7}

Technical Details

  • The hidden data was stored in the LSB (least significant bit) of each pixel channel
  • All four channels (R, G, B, A) contained different messages
  • The alpha channel message appears to be the legitimate flag
  • Proper PNG decoding (including inverse filtering) was necessary to extract the correct LSB data

Tools Used

  • Custom Python PNG decoder with proper filter handling
  • LSB extraction from decoded pixel data
  • Hex decoding of extracted binary data

Flag

nexus{yea_u_didi_v2er_wekcj7}