Three years ago we took our depot Wi-Fi off shared passwords. Every handheld scanner and laptop now proves who it is with a certificate, checked against our own certificate authority before the device is let on. The certificates renew themselves every year without anybody noticing, which is how it should work.

The certificate authority was built in 2016 by a contractor, as a single server running Microsoft's certificate services, and its root certificate was given a life of ten years. Everything it issues is signed by that root, so nothing it issues can outlive it. When a device renewed during the root's final year, the authority quietly cut the new certificate short to end at the same moment as the root. By this summer every certificate in use, about eleven hundred of them, was due to expire in the same minute.

That minute arrived at twenty past two on a Thursday afternoon in September.…