Ransomware and the Legacy Crypto API.

Ransomware are malicious software that encrypt their victim’s data and only return the decryption key in exchange of a ransom. After presenting their characteristics and main representatives, we introduce two original countermeasures allowing victims to decrypt their files without paying. The first one takes advantage of the weak mode of operation used by some ransomware. The second one intercept calls made to Microsoft’s Cryptographic API. Both methods must be active before the attack takes place, and none is general enough to handle all ransomware. Nevertheless our experimental results show that their combination can protect users from 50% of the active samples at our disposal.