state secret
official secret

That can't be a state secret, yet no one will tell me.

There were some state secrets involved, but I'm not very sure.

Let's be clear here: This isn't some kind of a state secret.

Because it is said that they once knew state secrets.

He then added, "As of now, everything about this project is a state secret."

She was later asked not to discuss this "state secret".

Or had knowledge of the contacts been made a state secret, meant only for those with a need to know?

Another problem is that Russia still has no law on state secrets.

So long ago and he still treated them as state secrets.

The final design has been guarded like a state secret.