Colonnele Frank's Indecipherable Chiffre

Abstract

Colonnele Frank's 23-folio-long French-language description of what he calls a "chiffre indéchiffrable" survived in the cipher collection of Leopold, grand duke of Tuscany, and Holy Roman Emperor. The author lists the usual weaknesses of the ciphers and offers his technique to remedy them. It is a polyalphabetic method helped by a circular instrument, by no means a fundamental invention in the late 18th century. What makes it relevant is that it is a relatively early practical application of the polyalphabetic method, where not only letters of the alphabet, but even whole words are manipulated. Not much is known about the colonel author, but on the basis of the document, it becomes clear that he was a real user of military cryptography.