If you have been infected by petya ransomware then we have good news for you, recently a researcher published tool to find decrypt key for this ransomware.
Petya is different from the other popular ransomware these days. Instead of encrypting files one by one, it denies access to the full system by attacking low-level structures on the disk. This ransomware’s authors have not only created their own boot loader but also a tiny kernel, which is 32 sectors long.
Petya’s dropper writes the malicious code at the beginning of the disk. The affected system’s master boot record (MBR) is overwritten by the custom boot loader that loads a tiny malicious kernel. Then, this kernel proceeds with further encryption. Petya’s ransom note states that it encrypts the full disk, but this is not true. Instead, it encrypts the master file table (MFT) so that the file system is not readable.