Alphabet Substitution Tables
Alphanumeric tables are one of the first great technical inventions in the field of cryptography, and it may not be an accident that they emerged with the advent of movable type. They took advantage of what the printing press did especially well — distributing permutations of individual letters and numbers in a square or rectangular grid — and they made the work of substituting plain-text with cipher-text easier, more accurate, and far more secure.
Best known for the twentieth century typeface that was named a after him, Palatino was a sixteenth century calligrapher with a gift for designing beautiful and legible letters. Among the model alphabets in his 1566 writing manual (seen at the top of the page) was this set of arbitrary symbols for use as ciphers — complete with “nulls," or meaningless figures that could be interspersed with meaningful ones to make decryption more difficult.
Disks and Volvelles
The cipher disk may be the most iconic image in the history of cryptography, placing a simple but powerful system of alphanumeric substitution into the palm of one's hand. In Renaissance books, the cipher disk was attached to the page with string or paper or parchment strips, and the different layers could be rotated independently to create new cipher alphabets. This moveable book technology was known as a volvelle.
Friedman's friend, the classicist and cryptanalyst Charles J. Mendelsohn, considered Giambattista della Porta the outstanding cryptographer of the Renaissance. He had a particularly solid grasp of the use of cipher disks, explaining how they can be converted into tables and vice versa. His books featured some of the period's most ornate volvelles, such as the one seen above.
William Friedman and his team at Riverbank were always on the lookout for ways to apply Renaissance codes to modern life. In 1920 they published an article for the Florists' Review on the famous trade slogan, "Say It With Flowers," suggesting that carefully designed bouquets could not only express general sentiments (such as love or sympathy) but also carry hidden messages. They provided an example from a seventeenth century German cryptography manual using a floral wreath and alphabetical key.
Music was another signifying system that could be used both to convey and to cover secret messages, with particular notes standing in for the different letters in the alphabet. The systems shown on the right made it possible for musical compositions to speak to people even when they did not contain words.
As in other examples, Friedman used the sheet music (arranged for solo mandolin) for Stephen Foster's famous song, "My Old Kentucky Home, Good Night" to show how, using Bacon's biliteral cipher, anything can be made to signify anything. By making notches in the stems of some notes and leaving others whole, he has turned the music into a sequence of a's and b's that read "Enemy advancing right / We march at daybreak."
Alongside the development of cryptography (whereby a message is scrambled using transposition or substitution), there was a parallel field called steganography (in which messages are kept intact but hidden within an innocent cover). An example of this can be seen in Johannes Balthasar Friderici's Cryptographia. At first glance, this image simply depicts a building under siege. But there are letters lurking in the window-panes. Variations in their shaded, unshaded, and dotted panels can be matched up with the alphabetical key on the next page, revealing a desperate message "Wir haben kein Pulver mehr"(We have have no more [gun]powder).
Popular scientist, Girolamo Cardano, was credited with an invention that is still in use today. In the so-called “Cardan grille,” a sheet with narrow rectangles of varying widths cut out of it is laid over a blank page and the secret message is written in the spaces. The sheet is then removed and the rest of the spaces are filled in with innocent—or even misleading—text. When the recipient lays the same grid over the paper in the same position, the hidden message is revealed.
Della Porta’s popular treatise on secret writing includes clear instructions for using a grille—or a membrane, as he describes it—with windows and gaps (fenestras et vacua). To make sure that the recipient knows where to place the overlay, four points should be drawn to mark the edges
Friedman's Cryptographic Christmas Card
Undoubtedly the most ingenious of the Friedmans’ annual cryptographic Christmas cards, this telegram came with a special kind of grille. Each 90-degree turn revealed a different hidden message, and the four directions yielded a rhyming quatrain:
“FOR CHRISTMAS GREETINGS IN 28
WE USE A MEANS QUITE UP TO DATE
A CRYPTOTELEPHOTGRAM HERE
BRINGS YOU WORD OF XMAS CHEER.”