Rating:
We are given an image and the source code used to create that image.
The image is an A quality meme, shown below.
In the encoding script, the flag was converted to its binary form first.
with open("flag.txt", "rb") as f:
flag = f.read()
binstr = bin(int(binascii.hexlify(flag), 16))
bits = [int(x) for x in binstr[2:]]
Then, those bits were hidden in the second least significant bit of each pixel.
pixel = (pixel[0], pixel[1]&~0b10|(bits[i]<<1), pixel[2], pixel[3])
All we have to do is restore it. I decided to just brute force it, because it's easier and I'm lazy.
flag = b''
## Bruteforce, yeah!
i = 0
gB = '' ## guess bits
while True:
gB += bits[i]
try:
flag = bytes.fromhex(hex(int(gB, 2))[2:])
except:
int()
if b'}' in flag:
print("Flag found! Flag:", flag)
break
i += 1
$ py sol.py
Flag found! Flag: b'tjctf{n0t_th3_le4st_si9n1fic4nt}'
If this was an actual message and not a flag, you could search for a line break instead.
if b'\n' in flag:
print("Flag found! Flag:", flag)
break
$ py sol.py
Flag found! Flag: b'tjctf{n0t_th3_le4st_si9n1fic4nt}\n'
Flag: tjctf{n0t_th3_le4st_si9n1fic4nt}
