Nomenclator — Bench tool
Published 26 Sep 2026 ·
A nomenclator is half cipher, half dictionary. It substitutes letters as usual but also assigns its own codes to the names that matter — kings, cities, generals, months — so that the words an attacker would most like to find are the words protected best. For three centuries it was the standard instrument of European diplomacy, and its failure mode was never mathematical.
- Era
- Medieval
- Type
- Substitution
- Key space
- codebook size
- Level
- Field
IWhat this tool does
A Venetian nomenclator of the sixteenth century has three parts: an alphabet of two- or three-digit codes for letters, a list of syllables for the sounds its language needs, and a nomenclatura of hundreds of proper nouns each with its own number.
The bench beside this entry reproduces the shape of it. Letters get two- and three-digit codes; THE, AND, KING, POPE, WAR and FRANCE get codes of their own; words are separated with a slash.
| Layer | Example | Purpose |
|---|---|---|
| letter codes | L = 83 | handles anything unlisted |
| word codes | THE = 01 | removes the most predictable text |
| nulls | unused numbers | fillers and traps |
| homophones | several codes per letter | flattens the frequency profile |
The last row is the real craft. If E has four different codes and the encipherer rotates between them, the frequency profile flattens out and the classical attack weakens. But the attacker no longer needs the letters anyway, because the names are the message: a dispatch consisting entirely of 03 06 05 needs no cryptanalysis at all if you have captured a copy of the book.
IIThe key
The key space is the number of entries in the book, and it is entirely administered rather than computed. A diplomatic nomenclator might carry 1,200 codes; a papal one, more.
The advantage is real: the book does something no algorithm can, which is to make the important words the expensive words. Frequency analysis assumes every plaintext symbol costs the attacker the same amount of work. A nomenclator deliberately breaks that assumption.
The cost is equally real, and it is operational: every correspondent needs their own book, every book must be delivered securely, every book must be replaced when it might have been captured, and every clerk must be able to use it accurately under time pressure.
The nomenclator's security parameter is not entropy. It is the number of people who must keep a secret.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →