Last week the cyber world has faced a lot of damages from CIA's leaked exploits which have been used by a group of hackers who created WanaCry ransomware, but not so long till it security researchers to find the kill switch to halt the spread and especially create a decryption tool WanaKiwi.
WanaCry ransomware which has affected over 100,000 computer over the world demanding $300 in bitcoins to be sent in the first 3 days which would double to $600 when the ransom is not paid, and eventually victims would be unable to decrypt their files after 6 days when they don't pay any ransom. WanaCry hacker team have already gathered $92,096 from their victims (47.79272564 BTC) and their files not getting decrypted.
How does WanaCry Decryption tool work?
The WannaCry's encryption scheme works by generating a pair of keys on the victim's computer that rely on prime numbers, a "public" key and a "private" key for encrypting and decrypting the system’s files respectively.
To prevent the victim from accessing the private key and decrypting locked files himself, WannaCry erases the key from the system, leaving no choice for the victims to retrieve the decryption key except paying the ransom to the attacker.
But here's the kicker: WannaCry "does not erase the prime numbers from memory before freeing the associated memory," says Guinet.
Based on this finding, Guinet released a WannaCry ransomware decryption tool, named WannaKey, that basically tries to retrieve the two prime numbers, used in the formula to generate encryption keys from memory.
"It does so by searching for them in the wcry.exe process. This is the process that generates the RSA private key. The main issue is that the CryptDestroyKey and CryptReleaseContext does not erase the prime numbers from memory before freeing the associated memory." says Guinet
So, that means, this method will work only if:
The affected computer has not been rebooted after being infected.
The associated memory has not been allocated and erased by some other process.
"In order to work, your computer must not have been rebooted after being infected. Please also note that you need some luck for this to work (see below), and so it might not work in every case!," Guinet says.
"This is not really a mistake from the ransomware authors, as they properly use the Windows Crypto API."
While WannaKey only pulls prime numbers from the memory of the affected computer, the tool can only be used by those who can use those prime numbers to generate the decryption key manually to decrypt their WannaCry-infected PC’s files.
WanaKiwi: WannaCry Ransomware Decryption Tool
Good news is that another security researcher, Benjamin Delpy, developed an easy-to-use tool called "WanaKiwi," based on Guinet's finding, which simplifies the whole process of the WannaCry-infected file decryption.
All victims have to do is download WanaKiwi tool from Github and run it on their affected Windows computer using the command line (cmd).
WanaKiwi works on Windows XP, Windows 7, Windows Vista, Windows Server 2003 and 2008, confirmed Matt Suiche from security firm Comae Technologies, who has also provided some demonstrations showing how to use WanaKiwi to decrypt your files.
Although the tool won't work for every user due to its dependencies, still it gives some hope to WannaCry's victims of getting their locked files back for free even from Windows XP, the aging, largely unsupported version of Microsoft's operating system.