Cipher discs are paper-and-string encrypting machines made up of two or more rotating dials called volvelles (from the Latin, volvere, “to turn”). Embellished with alphabets, numbers, or other symbols, these ciphering wheels encode or decode messages: when the cells of the volvelle’s inner dial align with those in its outer dial, the facing pairs of symbols or letters determine the cipher’s plaintext-to-ciphertext substitutions. In the cipher disc shown in figure 1, for instance, g replaces A, k replaces B, and l replaces C, etc. Though made of simple materials — paper, ink, paste, vellum strips, and string — and constructed on basic principles, cipher discs are a foundational tool in the history of cryptology.
The cipher disc reached a high point of artistry in the Renaissance, a period as enamored of learning and beauty as it was of “concealment, hidden revelation, and secrecy,” as Jonathan Sawday reminds us (Sawday 2007, 103). Its invention is often credited to that consummate Renaissance figure, Leon Battista Alberti (1404–1472) — mathematician, poet, athlete, and architect. But similar instruments probably date to antiquity: the Caesar Cipher — a “shift” cipher named for Julius Caesar, its earliest documented user — can employ a cipher disc to track its cryptic substitutions, for instance.
Alberti’s writing on ciphers and cipher discs appears in a short treatise written in 1466 or 1467 titled De Cifra (“On Ciphers”). This was the first western work on cryptography and, as Arielle Saiber has argued, “one of the earliest treatises on coding” (2017, 22). First circulated in manuscript among Alberti’s friends and patrons, De Cifra was eventually printed in Venice in 1568 as part of a posthumously published collection of essays titled Opuscoli Morali. The Opuscoli also prints Alberti’s brief and experimental works on mathematical perspective, family life, architecture, and morality — hence the collection’s name: “moral essays,” opuscoli meaning “little works” (figure 2). In a display of Alberti’s intellectual range and drift, the Opuscoli also includes a short treatise on the house fly.
Near the beginning of the treatise, Alberti lists the advantages of ciphers, noting that their primary function is to shield strategic information from prying eyes. This posed a problem for intelligence-hungry diplomats and spies, who relied on spycraft and code breaking to reveal “the schemes and machinations of enemies.” But, Alberti adds, “in my opinion it is even more advantageous to be able to communicate our intentions to another, no matter how far away, in a way that no other mortal except the intended recipient… is able to read them” (Williams et al. 2010, 170).
In this passage Alberti sets up a useful distinction: the first branch of cryptology, cryptanalysis (from analýein [ἀναλύειν], the Greek verb meaning to untie, unknot, or unbind), involves uncovering secrets that one’s enemies would prefer to keep hidden. The second, and according to Alberti, more important, branch is cryptography: the art of designing ciphers of sufficient complexity to frustrate code breaking. While Alberti treats both cipher-making and cipher-breaking in De Cifra, his first interest is in the design of new ciphers that evade the cryptanalyst’s probing art. For what use is a cipher, Alberti implies, if it fails to conceal?
Alberti’s solution was a technology of his own invention: polyalphabetic ciphers. These are cryptographic systems that employ multiple alphabets to disguise messages. Unlike simple substitution ciphers that replace each occurrence of a plaintext letter with the same ciphertext letter, polyalphabetic ciphers periodically change, allowing the same letter to be enciphered differently within the same message. Five hundred years later, this remains a concept fundamental to algorithmic methods of encryption.
But as Alberti well knew, complex ciphers are difficult to use — the human brain is poorly adapted to tracking the thousands of permutations involved in even basic polyalphabetic systems. To address this, Alberti devised a cipher instrument: a metal volvelle made up of two concentric brass plates, each marked with an alphabet around its perimeter. Reflecting his work on mathematical games, Alberti named his metal cipher instrument the “formula,” and advised making at least two: one for the sender and another for the recipient. Agreeing on an initial position, or “index,” for their discs, two (or more) correspondents could disguise their communications in a code made legible only by means of their carefully calibrated devices.
An illustration of Alberti’s cipher wheel appears alongside its description in De Cifra, but only its outer, stationary disc is shown (figure 3). This was intentional. Rather than provide a prefabricated disc, Alberti invites the reader to craft one of their own and mount it onto the page. Having done so, Alberti’s printed illustration would function as a paper replica of his brass prototype. Although made of flimsier stuff, this paper surrogate was no less powerful; whether made of paper or metal, Alberti assures us, the device will “serve as a sort of key that opens a door onto even the most intimate of secrets” (serve come quasi per una chiave da aprirci la entrata per potere penetrare negli intimi segreti).
A century or so later, Alberti’s cipher disc inspired more intricate forms. In 1563, the Italian scientist Giambattista Della Porta published a book-length study of cryptography titled De Furtivis Literarum. Unlike Alberti’s De Cifra — which only provided instructions on making his “formula” — Della Porta’s work is a cipher machine in the shape of a book: its pages carry several printed cipher discs with concentric, rotating rings filled with arcane symbols that seem to conceal divinatory secrets and obscure arts (figure 4).
Threaded onto a piece of robust string, the central dial of Della Porta’s cipher disc rotates clockwise within two static dials: the first contains a scrambled and modified twenty-letter alphabet (it’s missing letters J, K, U, W, X, Y, and Z and has two Rs), while the second contains the Roman numerals from one to twenty. The hand inside the inner volvelle points to the disc’s dial position. Each position (I–XX) determines which of Della Porta’s strange symbols replace a letter in the plaintext, and vice versa. With the central dial pointing to the first position (“I”), for instance, the letter R could either be encoded as a black square or a circle enclosing a cross (figure 4). As the pointing hand makes its way around the outer disc—advancing one step for each symbol or letter in the message—the process of enciphering or deciphering is simple: when enciphering, each letter in the plaintext is replaced with its aligned, corresponding symbol; and when deciphering, each symbol in the ciphertext is replaced with its aligned, corresponding letter. This amounts to an Alberti-inspired polyalphabetic substitution cipher, though in Della Porta’s case the substitution involves pseudo-hieroglyphic symbols instead of the more familiar characters of the Latin alphabet.
The publication history of Della Porta’s book itself forms a kind of cipher. Originally printed in Naples in 1563, a second, unauthorized edition of the book was printed in London in 1591. The two editions are nearly indistinguishable, suggesting that the publisher of the London edition, John Wolf, sought to disguise his edition as the first, likely in an attempt to conceal his plagiarism: in many surviving copies of Wolf’s edition, the imprint on the title page is false, reading “Naples, 1563” instead of “London, 1591.” Carnegie Mellon’s Posner Center for Special Collections, where I work, holds a copy of the original Naples printing as well as a copy of Wolf’s 1591 edition (figures 5 and 6). Despite their resemblances, these copies differ in one significant way: the cipher discs in Special Collections’ copy of Wolf’s edition are blank, left unassembled by its earliest readers, while those in the Naples edition are present, even retaining their sixteenth-century axial threads. This demonstrates the random nature of historical survival: when Della Porta’s book was printed, a set of readymade central dials were provided on a separate leaf of paper bound at the end of the volume (figure 7). Handwritten numerals (1–3) directed the dials’ placement on Della Porta’s first, second, or third cipher disc. It was left to the original buyer (or a later reader) to carefully cut out these dials and mount them with needle, thread, and paste to the cipher discs’ vacant centers. While an early owner of the copy of the Naples edition held in the Posner Center followed these instructions, no early reader of the copy of the London edition bothered to do so.
Besides its set of carefully assembled and functional cipher discs, Special Collections’ copy of the Naples edition of De Furtivis Literarum also contains extensive evidence of use: across its pages, one early reader has amended Della Porta’s text, experimented with the book’s cipher systems, and made observations on cryptography in the margins (figure 8). This interaction with the text amounts to a uniquely cryptographic example of Renaissance readers’ propensity to “use” their books directly on the page (see Sherman 2009). In this case, the annotations that survive in the Posner Center’s Neapolitan edition of Della Porta’s book show an amateur cryptographer honing their cryptographic skills. In contrast, the Center’s copy of the London edition offers up a comparative void: its margins are blank, and its pages show no sign of readerly labor.
The dissimilarities that set apart these copies of Della Porta’s and Alberti’s books extend to the tone and intellectual framework of the works themselves. Whereas the Opuscoli enacts a detached, rational, and technical account of cryptology, De Furtivis Literarum leans into the field’s reputation for the esoteric and the occult. Cryptology was for Alberti an instrument of state power, but Della Porta claims a wider remit, enlisting cryptography as part of an elaborate system of secrets and coded learning. Even Della Porta’s title imputes a certain mystique to cryptography: De Furtivis Literarum translates as “on secret writing” or “on secret letters.” Della Porta’s cipher wheels, meanwhile, are flanked by sphinxes — guardians and riddlers of Greek myth who make apt emblems for the cryptologic arts (figure 9). In a similarly emblematic mode, Della Porta’s invented alphabet of obscure characters recalls the “lines, circles, signs, letters, and characters” Faustus uses to summon infernal spirits in Marlowe’s play (Marlowe 2014, 34). Not by chance, perhaps, Della Porta was placed under surveillance by the papal curia and came to be known as a magus and “professor of secrets.” He would eventually help found the Accademia dei Segreti (the Academy of Secrets), a learned society whose elite members were charged with investigating the unexplained phenomena of nature.
This preoccupation with mysteries and the occult explains, in part, the strangeness of Della Porta’s ciphers. But this strangeness had a methodological edge: while for Alberti, ciphers entailed substituting letters for letters, Della Porta’s pseudo-hieroglyphs showed that encryption was not limited to alphabetic pairs. Alighting on this same idea in the 1570s, the English philosopher Francis Bacon (1561–1626) devised a cipher based on only two characters: A and B. With this “biliteral,”, or two-lettered symbology, Bacon realized that all twenty-six letters of the Latin alphabet could be encoded in strings of As and Bs, as follows:
A = aaaaa
B = aaaab
C = aaaba
D = aaabb
E = aabaa
F = aabab
G = aabba
H = aabbb
I/J = abaaa
K = abaab
L = ababa
M = ababb
N = abbaa
O = abbab
P = abbba
Q = abbbb
R = baaaa
S = baaab
T = baaba
U/V = baabb
W = babaa
X = babab
Y = babba
Z = babbb
This was a startlingly early form of binary code. But more than that, it had major implications for the history and theory of encryption. In developing his biliteral cipher, Bacon realized that A and B were effectively arbitrary. Any pair of signifiers — two colors, a pair of musical notes, or a doubled gesture of the hand — could provide the basis for a binary system of ciphered communication. In addition to anticipating binary code, Bacon also anticipated Morse code, which represents individual letters as long or short pulses of light, electricity, or sound. Bacon concluded with palpable surprise that “anything could signify anything” [“omnia per omnia”]:
And by this Art a way is opened, whereby a man may expresse and signifie the intentions of his mind, at any distance of place, by objects which may be presented to the eye, and accommodated to the eare … as by Bells [and] Trumpets, by Lights and Torches … and any instruments of like nature (quoted in Sherman 2010).
As Bacon almost predicts here, binary code now forms the mathematical basis for our society; heaps of information are enciphered as mere 1s and 0s — everything signified by almost nothing. But stranger signifiers have found expression in less familiar cryptographic systems. The British aristocrat and writer Lady Mary Wortley Montagu (1689–1762) described a method for disguising messages in flower arrangements, a kind of floristic cryptography practiced in eighteenth-century18th-century Istanbul that she observed during her husband’s appointment as British ambassador to the Ottoman Porte. Recalling Bacon’s own enthusiasm for the cryptic potential of everyday things, Montagu wrote:
There is no colour, no flower, no weed, no fruit, herb, pebble, or feather, that has not a verse belonging to it; and you may quarrel, reproach, or send letters of passion, friendship, or civility, or even of news, without ever inking your fingers (Montagu 1763, 4–5).
The visual language of early cryptography was clearly a mixed and sweeping one: flowers, musical notes, torches, obscure alphabets, and arcane symbols. While few if any of these systems have survived to influence modern encryption, the cipher disc can claim a long afterlife. The underlying mechanics of the Enigma machine are more advanced, but no different: rotating discs establish fleeting links between plaintext and ciphertext. What early cipher discs show us, then, is that the history of cryptology is woven through with old ideas, technological iterations, and reinventions.
Bibliography
Sawday, Jonathan. 2007. Engines of the Imagination: Renaissance Culture and the Rise of the Machine. New York: Routledge.
Grafton, Anthony. 2002. Leon Battista Alberti: Master Builder of the Italian Renaissance. Harvard: Harvard University Press.
Kahn, David. 1996. The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. New York: Scribner.
Akkerman, Nadine, and Pete Langman. 2024. Spycraft: Tricks and Tools of the Dangerous Trade from Elizabeth I to the Restoration. New Haven: Yale University Press.
Saiber, Arielle. 2017. Measured Words: Computation and Writing in Renaissance Italy. Toronto: University of Toronto Press.
Williams, Kim, Lionel March, and Stephen R. Wassell, eds. 2010. The Mathematical Works of Leon Battista Alberti. Basel: Springer Basel. https://doi.org/10.1007/978-3-0346-0474-1.
Sherman, William H. 2009. Used Books : Marking Readers in Renaissance England. Philadelphia: University of Pennsylvania Press.
———. 2010. “How to Make Anything Signify Anything: William F. Friedman and Birth of Modern Cryptanalysis.” Cabinet 40 (Winter 2010–-2011).
Montagu, Mary Wortley. 1763. Letters of the Right Honourable Lady M–y W—y M—-e; Written, during Her Travels in Europe, Asia and Africa, to Persons of Distinction, Men of Letters, &c. in Different Parts of Europe. 2nd edition. Vol. 3. London: Printed for T. Becket and P.A. De Hondt.
This essay is an edited excerpt from Rare Books & Ancestral Machines: A Handbook to the Posner Center for Special Collections at Carnegie Mellon University (Carnegie Mellon University Press, 2025).
