Trifid cipher
Fractionated cipher / From Wikipedia, the free encyclopedia
The trifid cipher is a classical cipher invented by Félix Delastelle and described in 1902.[1] 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.
The trifid cipher uses a table to fractionate each plaintext letter into a trigram,[2] 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:[3]
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 = n3, we recognize that three is the only value for n; two would only give 23 = 8 trigrams, while four would give 43 = 64, but three give 33 = 27.