in strict confidence

behind closed doors behind someone's back by stealth clandestinely confidentially covertly furtively hush-hush in camera in confidence in holes and corners in secret insidiously intimately o obscurely on the Q.T.

clandestinely covertly furtively personally privately quietly stealthily surreptitiously

behind closed doors behind someone's back by stealth confidentially hush-hush in camera in confidence in holes and corners in secret insidiously intimately obscurely on the Q.T. on the quiet on the sly privily slyly sub-rosa under the table underhandedly unobserved

publicly

openly