I’ve not been paying attention to Petya, because I didn’t notice it had a bootloader impact. Whoops.
[…]How do we explain Petya’s attacks against the master boot record and master file table? These render the entire system unusable. In this case why does encrypting files matter? The attack on the boot record and file table are similar to the behavior of the previous version of Petya, but there is one important difference. In research reported by Hasherezade, the new Petya destroys the Salsa20 cipher key by erasing it from the disk. In previous versions of Petya, the key is backed up in the victim’s ID before being erased—allowing for the recovery of the disk. Hasherezade also shows that the victim’s ID is generated before the random Salsa20 key is made, proving there is no relationship between the Salsa20 key and the victim’s ID. A reboot is required for this overwrite to take effect and supports the priorities we have mentioned. This difference in priorities implies the attackers are looking for pure destruction—closer in behavior to campaigns like Shamoon rather than ransomware such as Cerber, Locky, and WannaCry.[…]