1) Cautious 2) Clandestine 3) Close 4) Closelipped 5) Closemouthed 6) Incommunicative 7) Not open 8) Reticent 9) Secret 10) Sneaky 11) Stealthy 12) Surreptitious 13) Tightlipped 14) Uncommunicative 1) Cloak and dagger 2) Closelipped 3) Close mouthed 4) Confidential 5) Hole-and-corner 6) Incommunicative 7) Secret 8) Subversive 9) Tightlipped 10) Uncommunicative 11) Wary 
• (a.) Tending to secrete, or to keep secret or private; as, a secretive disposition.
Se·cret'ive adjective Tending to secrete, or to keep secret or private; as, a secretive disposition. 
secretive 1. Unwilling to reveal information. 2. Having or marked by an inclination to secrecy; not open, forthright, or frank.
to do things in secret
not open; living in secrecy