I'm trying to find a story I read in a printed book in the UK around 1990.
It was aimed at kids, probably about computers, and had some line drawings.
The only bit I can really remember is a couple of kids going to see a bed-bound kid who is lying down wearing prism glasses so he could see his computer screen and having a discussion of the "Knapsack" encryption system. I seem to recall the line picture of this on the left hand side.
I think it was a "children's novel" rather than a short story.
Any ideas of this vague recollection?