Editing Frequency analysis (section)

== History and usage ==
[[File:Al-kindi-cryptanalysis.png|thumb|right|First page of [[Al-Kindi]]'s 9th century ''Manuscript on Deciphering Cryptographic Messages'']]

[[File:HurufUNCsort.png|thumb|right|[[Arabic Letter Frequency]] distribution]]

The first known recorded explanation of frequency analysis (indeed, of any kind of cryptanalysis) was given in the 9th century by [[Al-Kindi]], an [[Arab]] [[polymath]], in ''A Manuscript on Deciphering Cryptographic Messages''.<ref>[[Al-Kindi|Ibrahim A. Al-Kadi]] "The origins of cryptology: The Arab contributions", ''[[Cryptologia]]'', 16(2) (April 1992) pp. 97&ndash;126.</ref> It has been suggested that a close textual study of the [[Qur'an]] first brought to light that [[Arabic language|Arabic]] has a characteristic letter frequency.<ref>{{cite web|title=In Our Time: Cryptography|url=http://www.bbc.co.uk/programmes/p004y272|publisher=BBC Radio 4|access-date=29 April 2012}}</ref> Its use spread, and similar systems were widely used in European states by the time of the [[Renaissance]]. By 1474, [[Cicco Simonetta]] had written a manual on deciphering encryptions of [[Latin language|Latin]] and [[Italian language|Italian]] text.<ref name="isbn0-684-83130-9">{{ Cite book | last = Kahn | first = David L. | author-link = David Kahn (writer) | title = The codebreakers: the story of secret writing | publisher = Scribner | year=1996 | location = New York | isbn = 0-684-83130-9 }}</ref>

Several schemes were invented by cryptographers to defeat this weakness in simple substitution encryptions. These included:

* ''[[Homophonic substitution cipher|Homophonic substitution]]'': Use of ''homophones'' &mdash; several alternatives to the most common letters in otherwise monoalphabetic substitution ciphers. For example, for English, both X and Y ciphertext might mean plaintext E.
* ''[[Polyalphabetic cipher|Polyalphabetic substitution]]'', that is, the use of several alphabets &mdash; chosen in assorted, more or less devious, ways ([[Leone Alberti]] seems to have been the first to propose this); and
* ''[[Polygraphic substitution]]'', schemes where pairs or triplets of plaintext letters are treated as units for substitution, rather than single letters, for example, the [[Playfair cipher]] invented by [[Charles Wheatstone]] in the mid-19th century.

A disadvantage of all these attempts to defeat frequency counting attacks is that it increases complication of both enciphering and deciphering, leading to mistakes. Famously, a British Foreign Secretary is said to have rejected the Playfair cipher because, even if school boys could cope successfully as Wheatstone and Playfair had shown, "our attachés could never learn it!".

The [[rotor machine]]s of the first half of the 20th century (for example, the [[Enigma machine]]) were essentially immune to straightforward frequency analysis.
However, other kinds of analysis ("attacks") successfully decoded messages from some of those machines.<ref>{{Cite journal |last1=Kruh |first1=Louis |last2=Deavours |first2=Cipher |title=The Commercial Enigma: Beginnings of Machine Cryptography |date=January 2002 |url=http://www.tandfonline.com/doi/abs/10.1080/0161-110291890731 |journal=Cryptologia |language=en |volume=26 |issue=1 |pages=1–16 |doi=10.1080/0161-110291890731 |s2cid=41446859 |issn=0161-1194}}</ref>

[[File:Frecuencia de uso de letras en español.PNG|thumb|Letter frequency in Spanish]]
Frequency analysis requires only a basic understanding of the statistics of the plaintext language and some problem-solving skills, and, if performed by hand, tolerance for extensive letter bookkeeping. During [[World War II]], both the [[United Kingdom|British]] and the [[United States|Americans]] recruited codebreakers by placing [[crossword]] puzzles in major newspapers and running contests for who could solve them the fastest. Several of the ciphers used by the [[Axis powers]] were breakable using frequency analysis, for example, some of the consular ciphers used by the Japanese. Mechanical methods of letter counting and statistical analysis (generally [[IBM]] card type machinery) were first used in World War II, possibly by the US Army's [[Signals Intelligence Service|SIS]]. Today, the work of letter counting and analysis is done by [[computer]] [[software]], which can carry out such analysis in seconds. With modern computing power, classical ciphers are unlikely to provide any real protection for confidential data.