If you have some viscous fluid between two concentric, rotating cylinders, blobs of dye in it will be smeared out -but then re-formed when the movement is reversed.
Today’s invention uses this laminar flow reversibility as an aid to security.
An instruction about how to open a lock or access a safe can be printed in the form of a coarse, dye-blob message within the apparatus above. The cylinders are then rotated backwards and forwards in a sequence of moves known only to those authorised to access the system.
Such a person can walk up (seeing only an indecipherable smear), reverse the moves and read the secondary message (which might be a vault or pin code).
The fluid would then be flushed and the access details rewritten with a new sequence of moves for the next person.