In cryptography, a music cipher is an algorithm for the encryption of a plaintext into musical symbols or sounds. Music-based ciphers are related to, but not the same as musical cryptograms. The latter were systems used by composers to create musical themes or motifs to represent names based on similarities between letters of the alphabet and musical note names, such as the BACH motif, whereas music ciphers were systems typically used by cryptographers to hide or encode messages for reasons of secrecy or espionage.
Types There are a variety of different types of music ciphers as distinguished by both the method of encryption and the musical symbols used. Regarding the former, most are simple substitution ciphers with a one-to-one correspondence between individual letters of the alphabet and a specific musical note. There are also historical music ciphers that utilize homophonic substitution (one-to-many), polyphonic substitution (many-to-one), compound cipher symbols, and/or cipher keys; all of which can make the enciphered message more difficult to break. Regarding the type of symbol used for substitution, most music ciphers utilize the pitch of a musical note as the primary cipher symbol. Since there are fewer notes in a standard musical scale (e.g., seven for diatonic scales and twelve for chromatic scales) than there are letters of the alphabet, cryptographers would often combine the note name with additional characteristics––such as octave register, rhythmic duration, or clef––to create a complete set of cipher symbols to match every letter. However, there are some music ciphers which rely exclusively on rhythm instead of pitch or on relative scale degree names instead of absolute pitches.
Musical steganography
Music ciphers often have both cryptographic and steganographic elements. Simply put, encryption is scrambling a message so that it is unreadable; steganography is hiding a message so no one knows it is even there. Most practitioners of music ciphers believed that encrypting text into musical symbols gave it added security because, if intercepted, most people would not even suspect that the sheet music contained a message. However, as Francesco Lana de Terzi notes, this is usually not because the resulting cipher melody appears to be a normal piece of music, but rather because so few people know enough about music to realize it is not ("ma gl'intelligenti di musica sono poci"). A message can also be visually hidden within a page of music without actually being a music cipher. William F. Friedman embedded a secret message based on Francis Bacon's cipher into a sheet music arrangement of Stephen Foster's "My Old Kentucky Home" by visually altering the appearance of the note stems. Another steganographic strategy is to musically encrypt a plaintext, but hide the message-bearing notes within a larger musical score that requires some visual marker that distinguishes them from the meaningless null-symbol notes (e.g., the cipher melody is only in the tenor line or only the notes with stems pointing down).
The cipher manuscript from Agostino Amadi there is a musical score in 41v with a pseudo-letter ciphered in it, which is an imaginary letter that Venice writes to Charles V. Italian historian Paolo Preto. "...The emperor sent to prince Gritti, with whom he had been familiar for a long time, a music score that looked like a madrigal....The prince summoned Willaert and the other musicians and asked them to play the melody sent to them by emperor Charles V. When Willaert and the others carefully studied the score, they were unable to play it and confessed they could not understand it."
Diatonic substitution ciphers
Diatonic music ciphers utilize only the seven basic note names of the diatonic scale: A, B, C, D, E, F, and G. While some systems reuse the same seven pitches for multiple letters (e.g., the pitch A can represent the letters A, H, O, or V), most algorithms combine these pitches with other musical attributes to achieve a one-to-one mapping. Perhaps the earliest documented music cipher is found in a manuscript from 1432 called "The Sermon Booklets of Friar Nicholas Philip." Philip's cipher uses only five pitches, but each note can appear with one of four different rhythmic durations, thus providing twenty distinct symbols. A similar cipher appears in a 15th-century British anonymous manuscript as well as in a much later treatise by Giambattista della Porta. In editions of the same treatise (De Furtivis Literarum Notis), Porta also presents a simpler cipher which is much more well-known. Porta's music cipher maps the letters A through M (omitting J and K) onto a stepwise, ascending, octave-and-a-half scale of whole notes (semibreves); with the remainder of the alphabet (omitting V and W) onto a descending scale of half notes (minims). Since alphabetic and scalar sequences are in such close step with each other, this is not a very strong method of encryption, nor are the melodies it produces very natural. Nevertheless, one finds slight variations of this same method employed throughout the 17th and 18th centuries by Daniel Schwenter (1602), John Wilkins (1641), Athanasius Kircher (1650), Kaspar Schott (1655), Philip Thicknesse (1722), and even the British Foreign Office (ca. 1750).
… excerpt ends here. Continue reading the full article.
