backstairs, behind-the-scenes, clandestine, furtive, hole-and-corner, hugger-mugger, hush-hush, private, privy, secret, sneak, sneaking, sneaky, stealth, stealthy, surreptitious, undercover, underground, underhand, underhanded

back-channel, closed-door, off-the-books, off-the-record

classified, confidential, restricted, top secret, undisclosed

concealed, hidden, secreted, subterranean, unadvertised, unexposed

acknowledged, avowed

aboveboard, straightforward, unconcealed, undisguised

unclassified, unrestricted

clear, evident, manifest, obvious, patent, plain

open, overt, public