The Free On-line Dictionary of Computing (30 December 2018):

trap-door function

    A special class of one-way
   function, which is reversible if you know a certain secret
   associated with the function.

   (http://rsasecurity.com/rsalabs/faq/2-3-2.html).
   (http://ieor.berkeley.edu/~jshu/knapsack/Review/OverviewPK.html).

   (2003-05-22)