P. and V. κρυπτός, ἀφανής.
clandestine: P. and V. λαθραῖος, κρυφαῖος (Plato), V. κρύφιος.
abstruse: P. and V. ἀσαφής, ἄδηλος, αἰνιγματώδης, V. δυσμαθής, ἄσημος; see dark.
