So for this challenge we got an image file provided `encoded.png`. All we have to know is in the title AND in the description. 
![encoded.png](https://github.com/1Tyron140/writeups/blob/main/Bluehens_CTF/2023/misc/least_significant_color/least_significant_color.png?raw=true)
Before giving the solution, we need to have in mind two things: Least significant bit (LSB) and the representation of an image.
## Reminder
### Images
An image is basically a 2D list of tuples (R=red, G=green, B=blue) of 3 values.The tuple define a number from 0 to 255 defining the intensity of each color 
The width of an image define the number of tuples per lines.The height of an image define the number of lines.
![representation of an image](https://raw.githubusercontent.com/1Tyron140/writeups/main/Bluehens_CTF/2023/misc/least_significant_color/pixel_in_image.png)
### Least Significant Bit 
The least signifcant bit is the last bit of byte. It is used in stego to hide text or else.So we can get multiple bytes and get edit the LSB of each so if we concatenate the LSB of each we get a message. Here is in image before and after editing the LSB (no difference, i think)
![LSB vs original](https://raw.githubusercontent.com/1Tyron140/writeups/main/Bluehens_CTF/2023/misc/least_significant_color/lsb_orginal_vs_edited.png)
## Solution
As the description tell us, i will xor the red value of a tuple with it's green one and iterate this for 30 character = 30 bytes = 30*8 LSB = 240 pixels = 240 tuples.
### Script
I will use a python script with the PIL module for image processing found after some research.
```from PIL import Image
flag = Image.open('encoded.png')
lsb = []
b = ""
pixels = list(flag.getdata())
for c,p in enumerate(pixels):
    b+=bin(p[1]^p[0])[-1]     if len(b) == 8:        lsb.append(int(b, 2))        b=""    if c == 240:        break for char in (lsb):    print(chr(char), end="")```
1. XOR red with green and append the least significant bit to b2. If b is 8 bits, empty it's value in 'lsb' list3. For each bytes in lsb[], print its ascii value
Output after cleaning : `UDCTF{y0u_R_1mag3_wizZarD}`.