A decryptor (Wanakiwi) that has been developed for WannaCry/WannaCrypt/wCrypt. There is a catch though, it only works for some operating systems. This tool essentially searches the systems memory for prime numbers and pieces together the encryption key used. It relies on current running memory so once you reboot it will be gone and if youve done too much on the system since infection, the key wont be found. The tool will automatically identify the WannaCrypt applications running on your system if they are called wnry.exe or wcry.”]
Source: https://blog.malwarebytes.com/cybercrime/2017/05/wannadecrypt-your-files/