1) Common knowledge 2) Ill-kept confidence 3) Known or unknown 4) Mystery in name only 5) What everybody knows 
An open secret is a concept or idea that is `officially` secret or restricted in knowledge but is actually widely known; or it refers to something that is widely known to be true but which none of the people most intimately concerned is willing to categorically acknowledge in public. ==Popular examples== ===Government and military=== One famous .....
[n] - something that is supposed to be secret but is generally known
noun something that is supposed to be secret but is generally known; `their love affair was an open secret`
something supposedly secret but actually known quite generally.