I’ve been reading about the history of encryption (without understanding everything, written as it is by people who assume that everyone shares the same background knowledge). I enjoyed being reminded of the idea of writing a message on a spiral strip of paper wound around a baton of a particular diameter, to form a continuous sheet. Unwinding the strip allows you to pass it, as a coded message, to someone who knows the correct baton diameter to be able to read the writing.
Today’s invention is related to that approach.
It consists of a bundle of fibres, with say half black and half white (‘on’ or ‘off’). At different locations along the length of this cable, the fibres are arranged to form a crude ‘image’ of eg a letter or some other piece of information (<200 fibres would be enough).
Only the intended recipient would know where to section the rope to be able to see the intended letters in the right order. At other locations, there would be merely noise or decoy symbols.
This could be made so that the bundle was of optical fibres, bonded together to preserve the images and interrogatable from outside using eg a lightmeter device spun in a tight helix around the cable circumference.
Message symbols within the rope could occur very spatially frequently (limited by the wavelength of light) making it possible to compress a lot of information into a small space. This would potentially allow it to be understood only by someone with a machine capable of extreme precision in determining where to take the readings along the cable length.