The precise technical details are closely-guarded military secrets, to hinder any development of enemy counter-measures.
Details of Diana's Royal wedding dress were kept a closely-guarded secret, until she stepped out of the carriage at St Paul's.
This was once a closely-guarded secret among my people, but there is a time when secrets need to be revealed.
Her income from investments is a closely-guarded secret.
From that moment on, everything that happened to Pamella for the next month or so is a closely-guarded secret.
Decisions on interest rate movements and other market interventions by the monetary authorities are the most closely-guarded secrets in the financial markets.
The exact nature of the toxin used to attack Mr. Meshal, who now appears fully recovered, is a closely-guarded secret here and in Israel.
For many French voters, support for the National Front is still a closely-guarded secret.
Evan has revealed her closely-guarded secret to you, and has thus put all of us in a kind of jeopardy.
Manchester United striker says return of midfielder was a closely-guarded secret.