A few days after Thanksgiving last year, Kurtis Minder got a message from a man whose small construction-engineering firm in upstate New York had been hacked. Minder and his security company, GroupSense, got calls and e-mails like this all the time now, many of them tinged with panic. An employee at a brewery, or a printshop, or a Web-design company would show up for work one morning and find all the computer files locked and a ransom note demanding a cryptocurrency payment to release them.