The challenge starts with this image:


By modifying the saturation and the chroma hue of the image, we can see a string in base64:


I then decode the base64 string:

echo -n "YW50aW1pc3NpbGVkZWZlbnNl" | base64 --decode

With this command, I get in output the value antimissiledefense

This value does not correspond to the flag, so I suppose that it will be used to retrieve the real information.

I decide to use the steghide tool which allows to hide or extract data in image or audio files.

So I run the following command using the passphrase during the interactive mode: antimissiledefense

steghide extract -sf classified.jpeg

The tool extracts the data and generates the file flag.txt.

All I have to do is display the contents of the file to retrieve the flag:

cat flag.txt

Flag : MCTF{cL@$Sif1eD_D0cUm3N7}