Ransomware is a Cryptovirology attack carried out using covertly installed malware that encrypts the victim’s files and then requests a ransom payment in return for the decryption key that is needed to recover the encrypted files. Thus, ransomware is an access-denial type of attack that prevents legitimate users from accessing files since it is intractable to decrypt the files without the decryption key. Other attacks superficially lock the system and display messages intended to coax the user into paying. Ransomware typically propagates as a Trojan, whose payload is disguised as a seemingly legitimate file.
While initially popular in Russia, the use of ransomware scams has grown internationally; in June 2013, security software vendor McAfee released data showing that it had collected over 250,000 unique samples of ransomware in the first quarter of 2013, more than double the number it had obtained in the first quarter of 2012. Wide-ranging attacks involving encryption-based ransomware began to increase through Trojans such as CryptoLocker, which had procured an estimated US$3 million before it was taken down by authorities, and CryptoWall, which was estimated by the US Federal Bureau of Investigation (FBI) to have accrued over $18m by June 2015.