Why is that? Guinet explains that " when WannaCry invades a computer, it will generate a decrypted password string based on prime numbers. It is important that ransomware does not remove them from memory before releasing the associated memory ." " If you're lucky (the memory involved hasn't been deleted or redistributed), these good integers are still in there ."
If you can recover those numbers, you can decrypt the file, Guinet said. He released the software used to decrypt the infected WannaCry computer and called it Wannakey.
Wannakey has not been extensively tested, so it is still uncertain how effective it is. However, it also raised the hope that data encrypted by ransomware could be decrypted in the future. Anyway, that is good news.