Giambattista Della Porta and the De furtivis literarum notis

Naples, 1563. A scholar not yet thirty publishes a treatise in four books on ways of writing in secret, entitled De furtivis literarum notis, that is “on the hidden signs of letters”. The author is Giambattista Della Porta, already known for the first edition of the Magia naturalis, which appeared in 1558, and the book would become one of the reference texts of European cryptography for the whole of the following century.

A naturalist of secrets

Della Porta belonged to a wealthy Neapolitan family and devoted his life to curiosity about natural phenomena, lenses, magnets, physiognomy and the theatre: he also wrote numerous comedies. In Naples he founded a circle of scholars called the Academy of Secrets, which met to discuss discoveries and curiosities of nature. The Inquisition looked on these activities with suspicion, and the circle had to disband.

In 1610 Della Porta was admitted to the Accademia dei Lincei founded by Federico Cesi, the same academy that would welcome Galileo the following year. His figure embodies well a moment of transition, when experimental research still coexisted with a taste for the marvellous, the prodigious and the secret kept by a few initiates.

The structure of the treatise

The De furtivis clearly distinguishes the different ways of hiding a message. The first book deals with ancient scripts and devices of concealment; the others with systems of transposition, substitution with symbols and ciphers with several alphabets. Della Porta also discusses how to break other people's ciphers, suggesting that one start from probable words, those reasonably expected in a letter, and from the study of the most frequent letters.

Among his best-known proposals is a cipher with eleven reciprocal alphabets, guided by a keyword, now known as the Porta cipher. Even more important, for historians, is the first known digraphic substitution: a system in which one does not encipher one letter at a time but a pair of letters, represented by a single symbol chosen from a large table.

Invisible inks and curiosities

Della Porta did not separate cryptography from natural magic. In the Magia naturalis and other works he collects many devices for concealing writing: inks that appear only with heat, messages traced inside an egg, letters hidden in the most ordinary objects. They are described with the taste for wonder typical of the age, and not all of them work as promised, but they show how wide the field of the art of secrecy was.

A European success

The treatise was reprinted several times, also under the title De occultis literarum notis, enlarged by the author. Diplomats and secretaries throughout Europe consulted it, and its ideas helped spread the awareness that a simple cipher was no longer enough. His attention to the analysis of ciphertext makes him, in the eyes of many historians, one of the first true cryptanalysts of the modern age.

With Della Porta secret writing becomes an organised body of knowledge, divided by method and accompanied by examples. The fact that the same author wrote about lenses, magnets and comedies reminds us that in the Italian Renaissance cryptography was part of a universal curiosity, still far removed from modern specialisation and open to every kind of wonder.