As witnesses that the things were not done in a corner. - Gen. Thomas Harrison,

Life within the Kremlin was shrouded in impenetrable secrecy.

Est rosa flos Veneris cujus quo furta laterent. [Roughly meaning, The discourses of the table among true loving friends are held in strict silence.]

When we desire to confine our words, we commonly say they are spoken under the rose. - Sir Thomas Browne,

He who gives up the smallest part of a secret has the rest no longer in his power. [Ger., Wer den kleinsten Theil eines Geheimnisses hingibt, hat den andern nicht mehr in der Gewalt.]

Three may keep a secret if two of them are dead.