state secret
official secret

I have never known whether this project was an official secret.

The agency's size is an official secret, but some officials said the cut could amount to at least 4,000 people, from a force of perhaps 10,000.

Quite apart from official secrets, there were things Jack did not want any other man to hear.

His works remained an official secret for many years.

However this was to be considered as an official secret.

From the beginning the identify of the station's owner and operator was an official secret.

He saw it as his duty to keep official secrets.

From their view I'm a walking encyclopedia of official secrets.

The scientists have used a model to test the technology, which was an official secret until recently.

Even though I got the reasons for it made an official secret, he knew, he knows.