Music cipher

Last updated
Giambattista della Porta's music cipher from De Furtivis Literarum Notis (1602). Porta Music Cipher.jpg
Giambattista della Porta's music cipher from De Furtivis Literarum Notis (1602).

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.

Contents

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. [1] 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 [2] or on relative scale degree names instead of absolute pitches. [3] [4] [5]

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 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"). [6] 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. [7] 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). [8] [9]

Diatonic substitution ciphers

Music cipher from "The Sermon Booklets of Friar Nicholas Philip" (1436). Philip Music Cipher.jpg
Music cipher from "The Sermon Booklets of Friar Nicholas Philip" (1436).

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), [10] 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. [11] A similar cipher appears in a 15th-century British anonymous manuscript [12] as well as in a much later treatise by Giambattista della Porta. [13]

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). [14] 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), [15] John Wilkins (1641), [16] Athanasius Kircher (1650), [17] Kaspar Schott (1655), [18] Philip Thicknesse (1722), [19] and even the British Foreign Office (ca. 1750). [20]

Chromatic substitution ciphers

Music Cipher attributed to Michael Haydn (1808) Michael Haydn Music Cipher.jpg
Music Cipher attributed to Michael Haydn (1808)

Music ciphers based on the chromatic scale provide a larger pool of note names to match with letters of the alphabet. Applying sharps and flats to the seven diatonic pitches yields twenty-one unique cipher symbols. Since this is obviously still less than a standard alphabet, chromatic ciphers also require either a reduced letter set or additional features (e.g., octave register or duration). Most chromatic ciphers were developed by composers in the 20th Century when fully chromatic music itself was more common. A notable exception is a cipher attributed to the composer Michael Haydn (brother of the more famous Joseph Haydn). [21] Haydn's algorithm is one of the most comprehensive with symbols for thirty-one letters of the German alphabet, punctuations (using rest signs), parentheses (using clefs), and word segmentation (using bar lines). However, because many of the pitches are enharmonic equivalents, this cipher can only be transmitted as visual steganography, not via musical sound. For example, the notes C-sharp and D-flat are spelled differently, but they sound the same on a piano. As such, if one were listening to an enciphered melody, it would not be possible to hear the difference between the letters K and L. Furthermore, the purpose of this cipher was clearly not to generate musical themes that could pass for normal music. The use of such an extreme chromatic scale produces wildly dissonant, atonal melodies that would have been obviously atypical for Haydn's time.

20th-century ciphers

Although chromatic ciphers did not seemed to be favored by cryptographers, there are several 20th-century composers who developed systems for use in their own music: Arthur Honegger, [22] Maurice Duruflé, [23] Norman Cazden, [24] Olivier Messiaen, [25] and Jacques Chailley. [26] Similar to Haydn's cipher, most likewise match the alphabet sequentially onto a chromatic scale and rely on octave register to extend to twenty-six letters. Only Messiaen's appears to have been thoughtfully constructed to meet the composer's aesthetic goals. Although he also utilized different octave registers, the letters of the alphabet are not mapped in scalar order and also have distinct rhythmic values. Messiaen called his musical alphabet the langage communicable, and used it to embed extra-musical text throughout his organ work Méditations sur le Mystère de la Sainte Trinité.

Compound motivic ciphers

Motivic music cipher by Johann Bucking (1804) Bucking Music Cipher.jpg
Motivic music cipher by Johann Bücking (1804)

In a compound substitution cipher, each single plaintext letter is replaced by a block of multiple cipher symbols (e.g., 'a' = EN or 'b' = WJU). Similarly, there are compound music ciphers in which each letter is represented by a musical motive with two or more notes. In the case of the former, the compound symbols are to make frequency analysis more difficult; in the latter, the goal is to make the output more musical. For example, in 1804, Johann Bücking devised a compound cipher which generates musical compositions in the form of a minuet in the key of G Major. [27] Each letter of the alphabet is replaced by a measure of music consisting of a stylistically typical motive with three to six notes. After the plaintext is enciphered, additional pre-composed measures are appended to the beginning and end to provide a suitable musical framing. A few years earlier, Wolfgang Amadeus Mozart appears to have employed a similar technique (with much more sophisticated musical motives), although more likely intended as a parlor game than an actual cipher. [28] [29] Since the compound symbols are musically meaningful motives, these ciphers could also be considered similar to codes.

Polybius square music cipher designed by Friedrich von Ottingen-Wallerstein (ca. 1600) Ottingen-Wallerstein Music Cipher.jpg
Polybius square music cipher designed by Friedrich von Öttingen-Wallerstein (ca. 1600)

Friedrich von Öttingen-Wallerstein proposed a different type of compound music cipher modeled after a polybius square cipher. [30] Öttingen-Wallerstein used a 5x5 grid containing the letters of the alphabet (hidden within the names of angels). Instead of indexing the rows and columns with coordinate numbers, he used the solfege syllables Ut, Re, Mi Fa, and Sol (i.e., the first five degrees of a diatonic scale). Each letter, therefore, becomes a two-note melodic motive. This same cipher appears in treatises by Gustavus Selenus (1624) [31] and Johann Balthasar Friderici (1665) [32] (but without credit to the earlier version of Öttingen-Wallerstein).

Music ciphers with keys

Music Cipher Wheel from anonymous 18th-century manuscript, Port-Lesney, France Music Cipher Wheel (ca. 1750).jpg
Music Cipher Wheel from anonymous 18th-century manuscript, Port-Lesney, France

Because Öttingen-Wallerstein's cipher uses relative scale degrees, rather than fixed note names, it is effectively a polyalphabetic cipher. The same enciphered message could be transposed to a different musical key––with different note names––and still retain the same meaning. The musical key literally becomes a cipher key (or cryptovariable), because the recipient needs that additional information to correctly decipher the melody. Öttingen-Wallerstein inserted rests as cipherkey markers to indicate when a new musical key was needed to decrypt the message.

Francesco Lana de Terzi used a more conventional text-string cryptovariable, to add security to a very straightforward 'Porta-style' music cipher (1670). [33] Similar to a Vigenère cipher, a single-letter cipher key shifts the position of the plaintext alphabet in relation to the sequence musical cipher symbols; a multi-letter key word shifts the musical scale for each letter of the text in a repeating cycle.

A more elaborate cipherkey algorithm was found in an anonymous manuscript in Port-Lesney, France, most likely from the mid-18th century. [34] The so-called 'Port-Lesney' music cipher uses a mechanical device known as an Alberti cipher disk [35] There are two rotating disks: the outer disk contains two concentric rings (one with time signatures and the other with letters of the alphabet); the inner disk has a ring of compound musical symbols, and a small inner circle with three different clef signs. The disks are rotated to align the letters of the alphabet with compound musical symbols to encrypt the message. When the melody is written out on a music staff, the corresponding clef and time signature are added to the beginning to indicate the cipher key (which the recipient aligns on their disk to decipher the message). This particular music cipher was apparently very popular, with a dozen variations (in French, German, and English) appearing throughout the 18th and 19th centuries. [36] [37] [38] [39]

Solfa cipher interface Solfa Cipher interface.jpg
Solfa cipher interface

The more recent Solfa Cipher [40] combines some of the above cryptovariable techniques. As the name suggests, Solfa Cipher uses relative solfege degrees (like Öttingen-Wallerstein) rather than fixed pitches, which allows the same encrypted message to be transposable to different musical keys. Since there are only seven scale degrees, these are combined with a rhythmic component to create enough unique cipher symbols. However, instead of absolute note lengths (e.g., quarter note, half note, etc.) that are employed in most music ciphers, Solfa Cipher uses relative metric placement. This type of tonal-metric [41] cipher makes the encrypted melody both harder to break and more musically natural (i.e. similar to common-practice tonal melodies). [42] To decrypt a cipher melody, the recipient needs to know in which musical key and with what rhythmic unit the original message was encrypted, as well as the clef sign and metric location of the first note. The cipher key could also be transmitted as a date by using Solfalogy, a method of associating each unique date with a tone and modal scale. [43] To further confound interceptors, the transcribed sheet music could be written with a decoy clef, key signature, and time signature. The musical output, however, is a relatively normal, simple, singable tune in comparison to the disjunct, atonal melodies produced by fixed-pitch substitution ciphers.

Related Research Articles

<span class="mw-page-title-main">Baudot code</span> Pioneering five-bit character encodings

The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use before ASCII. Each character in the alphabet is represented by a series of five bits, sent over a communication channel such as a telegraph wire or a radio signal by asynchronous serial communication. The symbol rate measurement is known as baud, and is derived from the same name.

In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication channel or storage in a storage medium. An early example is an invention of language, which enabled a person, through speech, to communicate what they thought, saw, heard, or felt to others. But speech limits the range of communication to the distance a voice can carry and limits the audience to those present when the speech is uttered. The invention of writing, which converted spoken language into visual symbols, extended the range of communication across space and time.

<span class="mw-page-title-main">Encryption</span> Process of converting plaintext to ciphertext

In cryptography, encryption is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Despite its goal, encryption does not itself prevent interference but denies the intelligible content to a would-be interceptor.

<span class="mw-page-title-main">Musical notation</span> Visual representation of music

Musical notation is any system used to visually represent music. Systems of notation generally represent the elements of a piece of music that are considered important for its performance in the context of a given musical tradition. The process of interpreting musical notation is often referred to as reading music.

In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters, pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution process to extract the original message.

<span class="mw-page-title-main">ROT13</span> Simple encryption method

ROT13 is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the Latin alphabet.

In music, solfège or solfeggio, also called sol-fa, solfa, solfeo, among many names, is a mnemonic used in teaching aural skills, pitch and sight-reading of Western music. Solfège is a form of solmization, though the two terms are sometimes used interchangeably.

In computer programming, Base64 is a group of binary-to-text encoding schemes that transforms binary data into a sequence of printable characters, limited to a set of 64 unique characters. More specifically, the source binary data is taken 6 bits at a time, then this group of 6 bits is mapped to one of 64 unique characters.

<span class="mw-page-title-main">Dorabella Cipher</span> Enciphered letter written by English composer Edward Elgar

The Dorabella Cipher is an enciphered letter written by composer Edward Elgar to Dora Penny, which was accompanied by another dated July 14, 1897. Penny never deciphered it and its meaning remains unknown.

The numbered musical notation is a cipher notation system used in mainland China, Taiwan, Hong Kong, and to some extent in Japan, Indonesia, Malaysia, Australia, Ireland, the United Kingdom, the United States and English-speaking Canada. It dates back to the system designed by Pierre Galin, known as Galin-Paris-Chevé system. It is also known as Ziffernsystem, meaning "number system" or "cipher system" in German.

<span class="mw-page-title-main">Pigpen cipher</span> Type of substitution cipher

The pigpen cipher is a geometric simple substitution cipher, which exchanges letters for symbols which are fragments of a grid. The example key shows one way the letters can be assigned to the grid.

The trifid cipher is a classical cipher invented by Félix Delastelle and described in 1902. Extending the principles of Delastelle's earlier bifid cipher, it combines the techniques of fractionation and transposition to achieve a certain amount of confusion and diffusion: each letter of the ciphertext depends on three letters of the plaintext and up to three letters of the key.

<span class="mw-page-title-main">Delphic Hymns</span> Musical compositions from Ancient Greece

The Delphic Hymns are two musical compositions from Ancient Greece, which survive in substantial fragments. They were long regarded as being dated c. 138 BC and 128 BC, respectively, but recent scholarship has shown it likely they were both written for performance at the Athenian Pythaids in 128 BC. If indeed it dates from ten years before the second, the First Delphic Hymn is the earliest unambiguous surviving example of notated music from anywhere in the Western world whose composer is known by name. Inscriptions indicate that the First Delphic Hymn was written by Athenaeus, son of Athenaeus, while Limenius is credited as the Second Delphic Hymn's composer.

Armenian chant is the melismatic monophonic chant used in the liturgy of the Armenian Apostolic Church and the Armenian Catholic Church.

<span class="mw-page-title-main">BATCO</span> British paper cryptographic system

BATCO, short for Battle Code, is a hand-held, paper-based encryption system used at a low, front line level in the British Army. It was introduced along with the Clansman combat net radio in the early 1980s and was largely obsolete by 2010 due to the wide deployment of the secure Bowman radios. BATCO consists of a code, contained on a set of vocabulary cards, and cipher sheets for superencryption of the numeric code words. The cipher sheets, which are typically changed daily, also include an authentication table and a radio call sign protection system.

<span class="mw-page-title-main">Symphony No. 3 (Penderecki)</span> Symphony by Krzysztof Penderecki

The Symphony No. 3 is a symphony in five movements composed between 1988 and 1995 by Krzysztof Penderecki. It was commissioned and completed for the centenary of the Munich Philharmonic. Its earliest version, Passacaglia and Rondo, premiered at the International Music Festival Week in Lucerne, Switzerland, on August 20, 1988. It was performed by the Lucerne Festival Orchestra and conducted by Penderecki. The full symphony premiered in Munich on 8 December 1995, performed by the Munich Philharmonic, again conducted by the composer.

A musical cryptogram is a cryptogrammatic sequence of musical symbols 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 musically translated versions of their own or their friends' names as themes or motifs in their compositions. These are not really rigorous cipher algorithms in the formal sense, but more like musical monograms. The methods used historically by composers were either too incomplete or too simplistic to meaningfully encrypt long text messages. There is a separate history of music ciphers utilizing music notation to encode messages for reasons of espionage or personal security that involved encryption and/or steganography.

<span class="mw-page-title-main">B</span> 2nd letter of the Latin alphabet

B, or b, is the second letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is bee, plural bees.

Méditations sur le Mystère de la Sainte Trinité is a work for organ by the French composer Olivier Messiaen. It was composed from 1967 to 1969 at Messiaen's house in Pétichet.

The Aristocrat Cipher is a type of monoalphabetic substitution cipher in which plaintext is replaced with ciphertext and encoded into assorted letters, numbers, and symbols based on a keyword. The formatting of these ciphers generally includes a title, letter frequency, keyword indicators, and the encoder's nom de plume. The predecessor to these ciphers stems from the Caesar Cipher around 100. The Aristocrat Cipher also used a transposition of letters to encrypt a message.

References

  1. ↑ Code, David Løberg. 2023. "Can musical encryption be both? A survey of music-based ciphers." Cryptologia Volume 47 - Issue 4, p.320.
  2. ↑ Friderici, Johann Balthasar. 1665. Cryptographia, p.181.
  3. ↑ Öttingen-Wallerstein, Friedrich von. c.1600. Steganographia comitis.
  4. ↑ Sudre, François. 1866. Langue Musicale Universelle.
  5. ↑ Solfa Cipher. 2013. https://solfa-co.de
  6. ↑ Terzi, Francesco Lana de. 1670. Prodromo, ouero, Saggio di alcune inuentione nuoue, premesso all'arte maestra
  7. ↑ New York Public Library, Manuscripts and Archives Division. 1916. "My Old Kentucky Home, Good Night" The New York Public Library Digital Collections. https://digitalcollections.nypl.org/items/bd9b1e30-8607-0131-ac72-58d385a7b928
  8. ↑ Thicknesse, Philip. 1772. A treatise on the art of decyphering, and of writing in cypher. With an harmonic alphabet. W. Brown.
  9. ↑ Cazden, Norman. 1961. "How to Compose Non-Music," Perspectives of New Music, Vol. 5, No. 2, 287-296
  10. ↑ Écorcheville, Jules. 1909. "Homage à Joseph Haydn," Revue musicale S.I.M. Société internationale de musique. Online encoder: https://wmich.edu/mus-theo/ciphers/ecorcheville.html
  11. ↑ Philip, Nicholas. 1436. "The Sermon Booklets of Friar Nicholas Philip" MS Lat. th.d.I, Bodleian Library, Oxford. Online encoder: https://wmich.edu/mus-theo/ciphers/philip.html
  12. ↑ Bernard, Francis. c.1400. Sloan MS 351, British Library.
  13. ↑ Porta, Giambattista della. 1602. De Furtivis Literarum Notis.
  14. ↑ Porta Music Cipher, online encoder: https://wmich.edu/mus-theo/ciphers/porta.html
  15. ↑ Schwenter, Daniel. 1622. Steganologia & Steganographia Aucta. Online encoder: https://wmich.edu/mus-theo/ciphers/schwenter.html
  16. ↑ Wilkins, John. 1641. Mercury, or The Secret and Swift Messenger. Online encoder: https://wmich.edu/mus-theo/ciphers/wilkins.html
  17. ↑ Kircher, Athanasius. 1650. Musurgia Universalis.
  18. ↑ Schott, Kaspar. 1655. Schola Steganographica.
  19. ↑ Thicknesse, Philip. 1772. Online encoder: https://wmich.edu/mus-theo/ciphers/thicknesse.html
  20. ↑ Schooling, John Holt. 1896. “Secrets in Cipher I-IV”, Pall Mall Magazine, viii , 119-29 Online encoder: https://wmich.edu/mus-theo/ciphers/bfo.html
  21. ↑ Rettensteiner, Werigand. 1808. Biographische Skizze von Michael Haydn. Online encoder: https://wmich.edu/mus-theo/ciphers/haydn.html
  22. ↑ Honegger, Arthur. 1928. Homage à Albert Roussel, H.69. Editions Salabert. Online encoder: https://wmich.edu/mus-theo/ciphers/honegger.html
  23. ↑ Duruflé, Maurice. 1942. Prélude et fugue sur le nom d'Alain, Op. 7. Online encoder: https://wmich.edu/mus-theo/ciphers/durufle.html
  24. ↑ Cazden, Norman. 1961. "How to Compose Non-Music," Perspectives of New Music, Vol. 5, No. 2, 287-296. Online encoder: https://wmich.edu/mus-theo/ciphers/cazden.html
  25. ↑ Messiaen, Olivier. 1969. Méditations sur le mystère de la Sainte Trinité. Online encoder: https://wmich.edu/mus-theo/ciphers/messiaen.html
  26. ↑ Chailley, Jacques. 1981. "Anagrammes Musicales Et "langages Communicables"." Revue De Musicologie 67, no. 1: 69-80. https://wmich.edu/mus-theo/ciphers/chailley.html.
  27. ↑ Bücking, Johannn. J. 1804. Anweisung zur geheimen Correpondenz. Heinrich Georg Albreht. Online encoder: https://wmich.edu/mus-theo/ciphers/bucking.html
  28. ↑ Mozart, Wolfgang Amadeus. 1787. Music manuscript MS 253-01, Bibliothèque Nationale de France. Online encoder: https://wmich.edu/mus-theo/ciphers/mozart.html
  29. ↑ Reuter, Christoph. 2013. "Namadeus – Play Your Name with Mozarts Game (KV 516f). Musicpsychologie, Bd.23, 154-159.
  30. ↑ Öttingen-Wallerstein, Friedrich von. c.1600. Steganographia comitis. Online encoder: https://wmich.edu/mus-theo/ciphers/oettingen.html
  31. ↑ Selenus, Gustavus. 1624. Cryptomenytices et Cryptographiae libri IX
  32. ↑ Friderici, Johann Balthasar. 1665. Cryptographia.
  33. ↑ Terzi, Francesco Lana de. 1670. Prodromo, ouero, Saggio di alcune inuentione nuoue, premesso all'arte maestra. Online encoder: https://wmich.edu/mus-theo/ciphers/terzi.html
  34. ↑ Chailley, Jacques. 1981. "Anagrammes Musicales Et "langages Communicables"." Revue De Musicologie 67, no. 1: 69-80. Online encoder: https://wmich.edu/mus-theo/ciphers/port-lesney.html
  35. ↑ Alberti, Leon Battista. 1467. De Cifris. Biblioteca Nazionale Marciana. Cod. Marc. Lat. XIV 32 (4702) f. 1r. (sec. XVI).
  36. ↑ Guyot, Edme Gilles and Guillaume Germain Guyot. 1769. Recreations Sur Les Nombres. Gueffier.
  37. ↑ Hooper, William. 1794. 'Rational Recreations'. B. Law and Son
  38. ↑ Klüber, Johann Ludwig. 1808. Kryptographik. Cottaschen. Online encoder: https://wmich.edu/mus-theo/ciphers/kluber.html
  39. ↑ Arnold, George. 1862. 'The Magician's Own Book'. New York: Dick & Fitzgerald.
  40. ↑ Solfa Cipher: https://solfa-co.de
  41. ↑ Prince, Jon and Mark Schmuckler. 2014. "The Tonal-Metric Hierarchy: A Corpus Analysis," Music Perception, 31(3), 254-270.
  42. ↑ Code (2023), p.352.
  43. ↑ "Solfalogy". music-co.de/solfalogy.

Sources

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.