Editing Transposition cipher (section)

== Detection and cryptanalysis ==
Since transposition does not affect the frequency of individual symbols, simple transposition can be easily detected by the [[cryptanalysis|cryptanalyst]] by doing a frequency count. If the ciphertext exhibits a [[frequency distribution]] very similar to plaintext, it is most likely a transposition.

In general, transposition methods are vulnerable to [[anagram]]ming—sliding pieces of ciphertext around, then looking for sections that look like anagrams of words in English or whatever language the plaintext was written in, and solving the anagrams. Once such anagrams have been found, they reveal information about the transposition pattern, and can consequently be extended. Simpler transpositions often suffer from the property that keys very close to the correct key will reveal long sections of legible plaintext interspersed by gibberish. Consequently, such ciphers may be vulnerable to optimum seeking algorithms such as [[genetic algorithm]]s<ref>{{cite journal |last1=Matthews |first1=Robert A. J. |date=April 1993 |title=The Use of Genetic Algorithms in Cryptanalysis |journal=Cryptologia |volume=17 |issue=2 |pages=187–201 |doi=10.1080/0161-119391867863}}</ref> and [[Hill climbing|hill-climbing algorithms]].<ref>{{Cite journal |last1=Lasry |first1=George |last2=Kopal |first2=Nils |last3=Wacker |first3=Arno |date=2014-07-03 |title=Solving the Double Transposition Challenge with a Divide-and-Conquer Approach |url=http://www.tandfonline.com/doi/abs/10.1080/01611194.2014.915269 |journal=Cryptologia |language=en |volume=38 |issue=3 |pages=197–214 |doi=10.1080/01611194.2014.915269 |s2cid=7946904 |issn=0161-1194}}</ref><ref>{{Cite journal |last1=Lasry |first1=George |last2=Kopal |first2=Nils |last3=Wacker |first3=Arno |date=2016-07-03 |title=Cryptanalysis of columnar transposition cipher with long keys |url=http://www.tandfonline.com/doi/full/10.1080/01611194.2015.1087074 |journal=Cryptologia |language=en |volume=40 |issue=4 |pages=374–398 |doi=10.1080/01611194.2015.1087074 |s2cid=21179886 |issn=0161-1194}}</ref>

There are several specific methods for attacking messages encoded using a transposition cipher. These include:

# '''[[Known-plaintext attack]]:''' Using known or guessed parts of the plaintext (e.g. names, places, dates, numbers, phrases) to assist in reverse-engineering the likely order of columns used to carry out the transposition and/or the likely topic of the plaintext.
# '''[[Brute-force attack]]:''' If keys are derived from dictionary words or phrases from books or other publicly available sources, it may be possible to brute-force the solution by attempting billions of possible words, word combinations, and phrases as keys.
# '''Depth attack:''' If two or more messages of the same length are encoded with the same keys, the messages can be aligned and anagrammed until the messages show meaningful text in the same places, without needing to know the transposition steps that have taken place.
# '''Statistical attack:''' Statistics about the frequency of 2-letter, 3-letter, etc. combinations in a language can be used to inform a scoring function in an algorithm that gradually reverses possible transpositions based on which changes would produce the most likely combinations. For example, the 2-letter pair QU is more common than QT in English text, so a cryptanalyst will attempt transpositions that place QU together.

The third method was developed in 1878 by mathematician [[Edward S. Holden]] and [[New-York Tribune]] journalists [[John R. G. Hassard]] and William M. Grosvenor who managed to deciphere telegrams between the [[Democratic Party (United States)|Democratic Party]] and their operatives in the Southern states during the [[1876 presidential election]] and thus prove facts of [[vote buying]], influencing the [[1878–79 United States House of Representatives elections|1878-1879 congressional elections]].<ref>{{Cite web |title=[3.0] The Rise Of Field Ciphers |url=https://vc.airvectors.net/ttcode_03.html#m3 |access-date=2024-01-11 |website=vc.airvectors.net}}</ref>

A detailed description of the cryptanalysis of a German transposition cipher
can be found in chapter 7 of Herbert Yardley's "The American Black Chamber."

A cipher used by  the [[Zodiac Killer]], called "Z-340", organized into triangular sections with substitution of 63 different symbols for the letters and diagonal "knight move" transposition, remained unsolved for over 51 years, until an international team of private citizens cracked it on December 5, 2020, using specialized software.<ref name=":0">{{Cite web|date=2020-12-12|title=Zodiac Killer cipher is cracked after eluding sleuths for 51 years|url=https://arstechnica.com/information-technology/2020/12/zodiac-killer-cipher-is-cracked-after-eluding-sleuths-for-51-years/|access-date=2020-12-12|website=arstechnica.com|language=en-US}}</ref>