You know it is a bad day when your computer screen blips up with this ransomware message:

“You Hacked, ALL Data Encrypted. Contact For Key (cryptom27@yandex.com)ID:681, Enter.”

But if the day is Black Friday, and your computer is part of a major US city’s municipal transit system, I think we can all agree that this constitutes a very, very, bad day indeed.

This was the ransomware message presented on hundreds of SFMTA (San Francisco’s transit agency) computer screens on Black Friday – 25 November.

The ...