I was looking for historical information on the first definition of “nomenclator” (the assistant to Roman Senators who whispered the names of approaching dignitaries) when I came across this:
An interesting variant is the nomenclator. Named after the public official who announced the titles of visiting dignitaries, this cipher combined a small codebook with large homophonic substitution tables. Originally the code was restricted to the names of important people, hence the name of the cipher; in later years it covered many common words and place names as well. The symbols for whole words (codewords in modern parlance) and letters (cipher in modern parlance) were not distinguished in the ciphertext. The Rossignols’ Great Cypher used by Louis XIV of France was one; after it went out of use, messages in French archives were unbreakable for several hundred years.
Nomenclators were the standard fare of diplomatic correspondence, espionage, and advanced political conspiracy from the early fifteenth century to the late eighteenth century; most conspirators were and have remained less cryptographically sophisticated. Although government intelligence cryptanalysts were systematically breaking nomenclators by the mid-sixteenth century, and superior systems had been available since 1467, the usual response to cryptanalysis was simply to make the tables larger. By the late eighteenth century, when the system was beginning to die out, some nomenclators had 50,000 symbols.