Chapter D Codebooks

Abstract

Simple substitutions generalize the Caesar cipher. One step further are the nomenclators and codebooks, which we present in this chapter. They work like simple substitions, except that they have much larger alphabets: not just letters, but also digrams, syllables, words, and names of people and places. Examples exist already from the 14th century, and a century later we find code factories at work that output series of codebooks by minor variation of a general template. In the First World War, top secret diplomatic messages were encrypted in this way, for example the Zimmermann telegram discussed in Chapter I.