state secret

noun2

1. •
   Information which pertains to the affairs of a country, access to which is restricted by the government.

   • I'll tell you one great State secret: the Queen, sensible how much she was governed by the late Ministry, runs a little into t'other extreme, and is jealous in that point. — 1711
2. •
   (humorous) Personal information which someone would like to keep private, even if ultimately not important.