Editing Trifid cipher (section)

{{Short description|Fractionated cipher}}
The '''trifid cipher''' is a [[classical cipher]] invented by [[Félix Delastelle]] and described in 1902.<ref>Delastelle, pp. 101–3.</ref> Extending the principles of Delastelle's earlier [[bifid cipher]], it combines the techniques of [[Transposition cipher#Fractionation|fractionation]] and [[Transposition cipher|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.

The trifid cipher uses a table to ''fractionate'' each plaintext letter into a [[trigram]],<ref>Hence the name ''trifid'', which means "divided into three parts" (''Oxford English Dictionary'').</ref> mixes the constituents of the trigrams, and then applies the table in reverse to turn these mixed trigrams into ciphertext letters. Delastelle notes that the most practical system uses three symbols for the trigrams:<ref>Delastelle, p. 101: "Afin de pouvoir fragmenter les lettres en trois parties…"</ref><blockquote>In order to split letters into three parts, it is necessary to represent them by a group of three signs or numbers. Knowing that ''n'' objects, combined in trigrams in all possible ways, give ''n'' × ''n'' × ''n'' = ''n''<sup>3</sup>, we recognize that three is the only value for ''n''; two would only give 2<sup>3</sup> = 8 trigrams, while four would give 4<sup>3</sup> = 64, but three give 3<sup>3</sup> = 27.</blockquote>