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MSB

  • Description: This image passes LSB statistical analysis, but we can't help but think there must be something to the visual artifacts present in this image...
  • Difficulty: Medium

🔎 Solution​

The challenge provides an image file along with a hint referencing MSB.

Unlike LSB - a commonly used technique in steganography challenges - MSB refers to the Most Significant Bit, which is the leftmost bit in a byte (bit 7, if counting from 0). It holds the highest value because it contributes the most to the overall value of the byte. In contrast, the Least Significant Bit (LSB) is the rightmost bit (bit 0), carrying the smallest value (1).

For challenges involving either LSB or MSB, a simple script can often be used to extract hidden data:

  • Read each pixel of the image
  • Extract the MSB from each color channel (bit 7)
  • Concatenate the bits into a binary stream and convert it to ASCII to reveal any embedded message.
from PIL import Image

def extract_msb_bits(image_path):
    img = Image.open(image_path)
    pixels = list(img.getdata())
    
    bitstream = ""

    for pixel in pixels:
        if isinstance(pixel, tuple):
            for channel in pixel[:3]: 
                msb = (channel & 0b10000000) >> 7
                bitstream += str(msb)
        else:
            msb = (pixel & 0b10000000) >> 7
            bitstream += str(msb)

    return bitstream

def bits_to_ascii(bitstream):
    chars = []
    for i in range(0, len(bitstream), 8):
        byte = bitstream[i:i+8]
        if len(byte) < 8:
            continue
        ascii_char = chr(int(byte, 2))
        chars.append(ascii_char)
    return ''.join(chars)

if __name__ == "__main__":
    image_path = "Ninja-and-Prince-Genji-Ukiyoe-Utagawa-Kunisada.flag.png" 
    bitstream = extract_msb_bits(image_path)
    hidden_text = bits_to_ascii(bitstream)
    print(hidden_text)

Running this script on the provided image yielded a very long text. By searching for the string pico within that output, I was able to locate the flag.

> python script.py | grep "pico"
picoCTF{15_y0ur_que57_qu1x071c_0r_h3r01c_3a219174}

🚩Flag​

picoCTF{15_y0ur_que57_qu1x071c_0r_h3r01c_3a219174}