After an analysis of the encryption routine of the malware used in the Petya/ExPetr attacks, we have thought that the threat actor cannot decrypt victims' disk, even if a payment was made.
This supports the theory that this malware campaign was not designed as a ransomware attack for financial gain. Instead, it appears it was designed as a wiper pretending to be ransomware.
Below the technical details are presented. First, in order to decrypt victim's disk the attackers need the installation ID:
In previous versions of "similar" ransomware like Petya/Mischa/GoldenEye, this installation ID contains crucial information for the key recovery. After sending this information to the attacker they can extract the decryption key using their private key.
Here's how this installation ID is generated in the ExPetr ransomware:
This installation ID in our test case is built using the CryptGenRandom function, which is basically generating random data.
The following buffer contains the randomly generated data in an encoded "BASE58" format:
If we compare this randomly generated data and the final installation ID shown in the first screen, they are the same. In a normal setup, this string should contain encrypted information that will be used to restore the decryption key. For ExPetr, the ID shown in the ransom screen is just plain random data.
That means that the attacker cannot extract any decryption information from such a randomly generated string displayed on the victim, and as a result, the victims will not be able to decrypt any of the encrypted disks using the installation ID.
What does it mean? Well, first of all, this is the worst-case news for the victims - even if they pay the ransom they will not get their data back. Secondly, this reinforces the theory that the main goal of the ExPetr attack was not financially motivated, but destructive.
Our friend Matt Suiche from Comae Technologies independently came to the same conclusion.