Can refer to either of two similarly-"weighty" definitions:
(1) Any complicated/intricate strategy --- such as using disappearing ink, light-sensitive/chemically-treated "disintegrating"
paper that quickly becomes illegible, the addition/insertion of one or more confusing/misleading codes, etc --- that heavily "drags down" anyone's attempts to read/decipher an encryption.
(2) The amalgam of minerals used to make a special
stone vault for
human-burial purposes.
Our founding fathers surely doled out huge healthy doses of cryptonite in their efforts to prevent thieves from locating/accessing their "national treasure"; fortunately
Benjamin Gates and his intrepid colleagues managed to de-code
everything in
time to prevent said treasure from falling into the wrong hands.