Level: Medium
Author: coderion
Santa baked you a pie with hidden ingredients!
The category of this challenge was forensics and we were given a png image of a green cake. It's clear that this challenge would have to do with some form of steganography. I immediately loaded the file in a Python script and looked at the RGB values. The R values were simply the digits of the constant Pi. The B values were also interesting as they all were in the range 32 - 127 (printable ASCII). Printing the B values didn't lead to the flag yet. But if we XOR the R and B values, we obtain a readable text:
from PIL import Image
img = Image.open('cake.png')
pixels = img.load()
flag = ''
for x in range(img.width):
for y in range(img.height):
flag += chr((pixels[x, y][2] ^ pixels[x, y][0]))
print(flag)The script prints a long text of "Never gonna give you up" as well as the flag HV23{pi_1s_n0t_r4nd0m}. I have to admit
that I almost missed the flag there.