WordNet (r) 3.0 (2006):

under wraps
    adj 1: kept secret; "the plan was kept carefully under wraps"

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "under wraps":
   abstruse, arcane, beclouded, blind, buried, cabalistic, censored,
   classified, close, closed, clouded, concealed, covered, covert,
   cryptic, dark, eclipsed, enigmatic, esoteric, hermetic, hid,
   hidden, hush-hush, in a cloud, in a fog, in eclipse, in purdah,
   in the wings, incommunicado, latent, mysterious, obfuscated,
   obscure, obscured, occult, recondite, restricted, secluded,
   secluse, secret, sequestered, smothered, stifled, suppressed,
   top secret, ulterior, unbreatheable, under an eclipse, under cover,
   under house arrest, under security, underground, undisclosable,
   undisclosed, undivulgable, undivulged, unknown, unrevealable,
   unrevealed, unspoken, untellable, untold, unutterable, unuttered,
   unwhisperable, wrapped in clouds