Editing Caesar cipher (section)

==History and usage==
{{See also|History of cryptography}}
[[File:Bust of Julius Caesar from History of the World (1902).png|thumbnail|The Caesar cipher is named for [[Julius Caesar]], who used an alphabet where decrypting would shift three letters to the left.]]
The Caesar cipher is named after [[Julius Caesar]], who, according to [[Lives of the Twelve Caesars|Suetonius]], used it with a shift of three (A becoming D when encrypting, and D becoming A when decrypting) to protect messages of military significance.<ref>{{cite book|author=[[Suetonius]]|url=http://thelatinlibrary.com/suetonius/suet.caesar.html#56|title= Vita Divi Julii |chapter=56.6|lang=la}}</ref> While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier.<ref>{{cite web|title=Cracking the Code|url=https://www.cia.gov/news-information/featured-story-archive/2007-featured-story-archive/cracking-the-code.html|website=Central Intelligence Agency|access-date=21 February 2017|archive-url=https://web.archive.org/web/20201226065538/https://www.cia.gov/news-information/featured-story-archive/2007-featured-story-archive/cracking-the-code.html|archive-date=26 December 2020}}</ref><ref>{{cite book |title=[[The Code Book]] |last=Singh |first=Simon |author-link=Simon Singh |year=2000 |publisher=Anchor |isbn=0-385-49532-3 |pages=[https://archive.org/details/codebook00simo/page/289 289-290] }}</ref>
{{Blockquote|"If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others."|[[Suetonius]]|''[[Life of Julius Caesar]]'' 56}}
His nephew, [[Augustus Caesar|Augustus]], also used the cipher, but with a right shift of one, and it did not wrap around to the beginning of the alphabet:
{{Blockquote|"Whenever he wrote in cipher, he wrote B for A, C for B, and the rest of the letters on the same principle, using AA for Z."|[[Suetonius]]|''[[Life of Augustus]]'' 88}}
Evidence exists that Julius Caesar also used more complicated systems,<ref>{{cite journal |last=Reinke |first=Edgar C. |date=December 1962 |title=Classical Cryptography |journal=The Classical Journal |volume=58 |issue=3 |page=114 }}</ref> and one writer, [[Aulus Gellius]], refers to a (now lost) treatise on his ciphers:
{{Blockquote|"There is even a rather ingeniously written treatise by the grammarian Probus concerning the secret meaning of letters in the composition of Caesar's epistles."|[[Aulus Gellius]]|''Attic Nights 17.9.1–5''}}

It is unknown how effective the Caesar cipher was at the time; there is no record at that time of any techniques for the solution of simple substitution ciphers. The earliest surviving records date to the 9th-century works of [[Al-Kindi]] in the [[Arab]] world with the discovery of [[frequency analysis]].<ref>{{cite book |title=[[The Code Book]] |last=Singh |first=Simon |author-link=Simon Singh |year=2000 |publisher=Anchor |isbn=0-385-49532-3 |pages=[https://archive.org/details/codebook00simo/page/14 14–20] }}</ref>

A piece of text encrypted in a [[Hebrew language|Hebrew]] version of the Caesar cipher not to be confused with [[Atbash]], is sometimes found on the back of Jewish [[mezuzah]] scrolls. When each letter is replaced with the letter before it in the [[Hebrew alphabet]] the text translates as "[[YHWH]], our God, YHWH", a quotation from the main part of the scroll.<ref>{{cite book |last1=Eisenberg |first1=Ronald L. |title=Jewish Traditions |date=2004 |publisher=Jewish Publication Society |location=Philadelphia |isbn=9780827610392 |pages=582 |edition=1st}}</ref><ref>{{cite book |last1=Sameth |first1=Mark |title=The Name : a history of the dual-gendered Hebrew name for God |date=2020 |publisher=Wipf & Stock |location=Eugene, Oregon |isbn=9781532693830 |pages=5–6}}</ref>

In the 19th century, the personal advertisements section in newspapers would sometimes be used to exchange messages encrypted using simple cipher schemes. [[David Kahn (writer)|David Kahn]] (1967) describes instances of lovers engaging in secret communications enciphered using the Caesar cipher in ''[[The Times]]''.<ref>{{cite book |title=The Codebreakers |last=Kahn |first=David |author-link=David Kahn (writer) |year=1967 |isbn=978-0-684-83130-5 |pages=775–6 }}</ref> Even as late as 1915, the Caesar cipher was in use: the Russian army employed it as a replacement for more complicated ciphers which had proved to be too difficult for their troops to master; German and Austrian [[Cryptanalysis|cryptanalysts]] had little difficulty in decrypting their messages.<ref>{{cite book |title=The Codebreakers |last=Kahn |first=David |author-link=David Kahn (writer) |year=1967 |isbn=978-0-684-83130-5 |pages=631–2 }}</ref>
[[File:Confederate cipher disk.jpg|alt=Caesar cipher can be constructed into a disk with outer rotating wheel as plain text and the inner fixed wheel as cipher text. Both outer and inner plates should have alphabets in the same direction. |thumb|Caesar cipher translated to a disk has both outer and inner plates having alphabets in the same direction and not the reverse as seen in [[:File:CipherDisk2000.jpg|CipherDisk2000]].]]
Caesar ciphers can be found today in children's toys such as [[secret decoder ring]]s. A Caesar shift of thirteen is also performed in the [[ROT13]] [[algorithm]], a simple method of obfuscating text widely found on [[Usenet]] and used to obscure text (such as joke punchlines and story [[Spoiler (media)|spoilers]]), but not seriously used as a method of encryption.<ref>{{cite book |title=Cryptology Unlocked |url=https://archive.org/details/Cryptology_Unlocked |last=Wobst |first=Reinhard |year=2001 |publisher=Wiley |isbn=978-0-470-06064-3 |page=20 }}</ref>

The [[Vigenère cipher]] uses a Caesar cipher with a different shift at each position in the text; the value of the shift is defined using a repeating keyword.<ref>{{cite book |title=The Codebreakers |last=Kahn |first=David |author-link=David Kahn (writer) |year=1967 |pages=148–149 |isbn=978-0-684-83130-5 }}</ref> If the keyword is as long as the message, is chosen at [[randomly|random]], never becomes known to anyone else, and is never reused, this is the [[one-time pad]] cipher, proven unbreakable. However the problems involved in using a random key as long as the message make the one-time pad difficult to use in practice. Keywords shorter than the message (e.g., "[[Vigenère cipher#History|Complete Victory]]" used by the [[Confederate States of America|Confederacy]] during the [[American Civil War]]), introduce a cyclic pattern that might be detected with a statistically advanced version of frequency analysis.<ref>{{cite book |title=The Codebreakers |last=Kahn |first=David |author-link=David Kahn (writer) |year=1967 |pages=398–400 |isbn=978-0-684-83130-5 }}</ref>

In April 2006, fugitive [[Sicilian Mafia|Mafia]] boss [[Bernardo Provenzano]] was captured in [[Sicily]] partly because some of his messages, clumsily written in a variation of the Caesar cipher, were broken. Provenzano's cipher used numbers, so that "A" would be written as "4", "B" as "5", and so on.<ref>{{cite news |first=John |last=Leyden |title=Mafia boss undone by clumsy crypto |url=https://www.theregister.co.uk/2006/04/19/mafia_don_clueless_crypto/ |work=[[The Register]] |date=2006-04-19 |access-date=2008-06-13 }}</ref>

In 2011, Rajib Karim was convicted in the United Kingdom of "terrorism offences" after using the Caesar cipher to communicate with Bangladeshi Islamic activists discussing plots to blow up [[British Airways]] planes or disrupt their IT networks.  Although the parties had access to far better encryption techniques (Karim himself used [[Pretty Good Privacy|PGP]] for data storage on computer disks), they chose to use their own scheme (implemented in [[Microsoft Excel]]), rejecting a more sophisticated code program called [[Mujahedeen Secrets]] "because 'kaffirs', or non-believers, know about it, so it must be less secure".<ref>{{cite news |title=BA jihadist relied on Jesus-era encryption |url=https://www.theregister.co.uk/2011/03/22/ba_jihadist_trial_sentencing/ |work=[[The Register]] |date=2011-03-22 |access-date=2011-04-01 }}</ref>