As codes go, Potter’s wasn’t inordinately complicated. As Wiltshire explains, it was a “mono-alphabetic substitution cipher code,” in which each letter of the alphabet was replaced by a symbol — the kind of thing they teach you in Cub Scouts. The real trouble was Potter’s own fluency with it. She quickly learned to write the code so fast that each sheet looked, even to Linder’s trained eye, like a maze of scribbles.
from The Secret Code of Beatrix Potter