Music cipher

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.