Bleicher, managing director at cybersecurity consulting firm Arete Incident Response, is a specialist who helps companies deal with ransomware — the type of cyberattack in which hackers lock up a company's computers and then demand payment to undo the encryption. He has given CNBC a rare and exclusive look inside a shadowy world where American companies find themselves paying millions of dollars to known criminals. It's a corner of the criminal underworld that has seen explosive growth. According to a report by Chainalysis, the total amount paid by ransomware victims increased by 336% in 2020 to reach nearly $370 million worth of cryptocurrency. And some big players are scoring huge gains: The report found the digital hostage-takers are dominated by large players who are raking in millions of dollars a year. Just 199 cryptocurrency deposit addresses receive 80 percent of all funds sent by ransomware addresses in 2020, Chainalysis found.