Malware
BlackMatter
According to PCrisk, BlackMatter is a piece of malicious software categorized as ransomware.
BlackMatter is a Windows malware family.
Background
PCrisk classifies BlackMatter as ransomware. It encrypts a victim's data and then demands payment in exchange for the decryption tools; in practice, the affected files become inaccessible and the victim is pressured to pay to regain access.
When encrypting, the malware appends an extension made up of a random character string. A file originally called "1.jpg", for instance, would end up looking like "1.jpg.k5RO9fVOl". Once finished, BlackMatter replaces the desktop wallpaper and drops a ransom note named "[random_string].README.txt" (for example, k5RO9fVOl.README.txt).
Source: Malpedia (Fraunhofer FKIE).