Rensenware is unusual as an example of ransomware in that it does not request the user pay the creator of the virus to decrypt their files, instead requiring the user to achieve a required number of points in the shoot 'em up video game Undefined Fantastic Object before any decryption can take place.

Rensenware was developed by Korean undergraduate student and programmer Kangjun Heo for Windows operating systems out of boredom as a joke within the Touhou Project fandom.

[1][2] When executed, the program scans and encrypts all files on the computer that end in certain extensions using AES-256 and appends ".RENSENWARE" to the filename.

[4] Once the files have been encrypted, a warning window depicting the character Minamitsu Murasa from the Touhou Project is displayed, which cannot be closed.

The program requires the user to play the bullet hell video game Touhou Seirensen ~ Undefined Fantastic Object, which is not included with the software meaning they must download it on their own,[4] and score at least 200 million points in the "Lunatic" level of difficulty before any decryption may take place (the program automatically detects the game's process "th12" and its accumulated points).