Writeup for Basic LSB

Category: Steganography
Author: Tom

Basic LSB was part of the SharkyCTF 2020 competition. With the challenge, we were given the following image, called pretty_cat.png:

pretty_cat

The name was a big and immediate giveaway that this might be LSB steganography, or Least Significant Bit steganography.

LSB stego is based around slightly modifying colours in an image, the idea being that if you change the least significant bit of pixel's colour in order to encode data. If you attempt to hide too much data using LSB stego, you run the risk of visually altering the image, which is what may have happened here. Zooming close onto the image shows various miscoloured pixels that look out of place.

zoomed

There are numerous tools you can use in order to extract data from images that have LSB encoded data - I used this one just because it was on the first page of Google and it looked like it might work. It did work.

flag

The flag is at the end - shkCTF{Y0u_foUnD_m3_thr0ugH_LSB_6a5e99dfacf793e27a}

Here is the image of the cat with uninverted colours, if you care.

inverted_cat

Pretty cat <3

You can get the original image from here.

chall