Enter RedStar OS: North Korea’s own Linux based operating system, designed to monitor its users and remain resilient to any attempts to modify or otherwise exert control over it.

…“They are using something that is supposed to support free-speech,” Grunow said.

…It comes with everything a user might need, including word processing and music creation software, and a modified Firefox browser. These applications, the desktop environment, and the underlying structure of the file system attempts to mimic that of Mac OS X.

But that is where the similarities with other operating systems end, and RedStar’s totalitarian bent begins. RedStar enforces its dominance by rigorously monitoring any changes that a user might make, reacting accordingly, as well as creating “watermarks” on the files on any USB stick inserted into it.

In short, whenever a USB storage device containing documents, photos or videos is inserted into a RedStar computer, the operating system takes the current hard-disk’s serial number, encrypts that number, and then writes that encrypted serial into the file, marking it.

Source.