Musical cryptogram

A musical cryptogram is a cryptogrammatic sequence of musical notes, a sequence which can be taken to refer to an extra-musical text by some 'logical' relationship, usually between note names and letters. The most common and best known examples result from composers using ciphered versions of their own or their friends' names as themes or motifs in their compositions. Much rarer is the use of music notation to encode messages for reasons of espionage or personal security called steganography.

Because of the multitudinous ways in which notes and letters can be related, detecting hidden ciphers and proving accurate decipherment is difficult.

From the initial assignment by Western music theorists of letter names to notes in the 9th century it became possible to reverse the procedure and assign notes to the letters of names. However, this does not seem to have become a recognized technique until the Baroque period. From the mid-19th century it has become quite common. Sporadic earlier encipherments used solmization syllables.

It is believed that this method was first used by Josquin des Prez in his Missa Hercules Dux Ferrarie. It was named Soggetto cavato by the later theorist Zarlino. Under this scheme the vowel sounds in the text are matched to the vowel sounds of the solmization syllables of Guido of Arezzo (where 'ut' is the root, which we now call 'do'). Thus the Latin name of the dedicatee 'Hercules Dux Ferrarie' (Ercole d'Este, Duke of Ferrara) becomes re-ut-re-ut-re-fa-mi-re, which translates as D-C-D-C-D-F-E-D in modern notation with C as 'ut'. This is used as the cantus firmus of the mass setting. Josquin's method was imitated by several of his contemporaries and successors, including Adrian Willaert and Costanzo Festa.

...
Wikipedia