机密

confidential letters

confidential document

secret mission

confidential information; classified intelligence/information

secret archives

confidential report

be considered highly/strictly confidential; be considered top secret

Every citizen must strictly guard state secrets.

inside secret

military secret

trade secret

lock a secret in one's heart/breast; closely guard a secret

give away/leak/betray a secret

reveal/disclose/break a secret (to)

make a secret public

pry into a secret