Six Group counts its profit in millions, but the financial pipework it controls moves billions. Its operations, which include the Spanish and Swiss stock exchanges, count as critical national infrastructure and this gives it a close relationship with governments and regulators in Madrid and Zurich. Those relationships are critical in an age where digital warfare makes financial infrastructure a prime target for hackers linked to hostile states. Jos Dijsselhof, the Dutch chief executive of the Swiss-based stock exchange group, is open about the scale of the challenge. “I employ a lot of hackers,” he says, tapping the table sharply. “Sometimes...