trap door

trap door n.  (alt. `trapdoor') 1. Syn. {back door}
   -- a {Bad Thing}.  2. [techspeak] A `trap-door function' is
   one which is easy to compute but very difficult to compute the
   inverse of.  Such functions are {Good Thing}s with important
   applications in cryptography, specifically in the construction of
   public-key cryptosystems.