Petya is a relatively new ransomware variant that first appeared on the cyber-crime scene at the beginning of 2016. Its developers were not content with encrypting all the important files found on the victims hard-drive but also encrypting its Master-File-Table (MFT) This renders the entire file system useless until the ransom is paid. The original encryption method called SALSA20 uses a 32-byte encryption key and an 8-byte key-stream to produce the final 512-stream encryption key. The first encryption flaw is located in the type type type of the type of stream-stream.”]
Source: https://blog.checkpoint.com/2016/04/11/decrypting-the-petya-ransomware/