Trifid

History

The Trifid cipher is a fractional substitution cipher and was developed by the French mathematician Felix Marie Delastelle (1840-1902) and published in his book Traité Élémentaire de Cryptographie (1902).

Description

To encipher text using the Trifid cipher a keyed alphabet is created from a keyword, with repeated letters being omitted, followed by the unused letters of the alphabet in alphabetical order. This is appended with a # symbol to form a 27-letter alphabet. The # symbol can be inserted at any position in the alphabet.

For example the keyword RALPHEMERSON is reduced to RALPHEMSON when repeated letters are removed. Appending unused alphabet letters and the # symbol produces the keyed alphabet:

RALPHEMSONBCDFGIJKQTUVWXYZ#

The keyed alphabet is written into a table with three additional rows containing all permutations of the numbers 1, 2 and 3 as shown in the following table:

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

R A L P H E M S O N B C D F G I J K Q T U V W X Y Z #

The plaintext is then divided into period length groups and the three numbers above each letter in the table is written below each plaintext letter. These digits are then read horizontally within each group and replaced by the letter represented by the numbers in the table to produce the ciphertext.

Example

Keyword: RALPHEMERSON

Period: 7

Plaintext: The only way to have a friend is to be one.

theonly waytoha veafrie ndistob eone

3111213 3133111 3112121 2221312 1121

1223113 2131321 2212132 1233131 2312

2233131 2212322 1322113 1112232 3313

QPTISKM UQPUBBW QBHPVWL FMBYQAJ AEEU

Plaintext: theonly waytoha veafrie ndistob eone

Ciphertext: QPTISKM UQPUBBW QBHPVWL FMBYQAJ AEEU

Solving

Solving methods: Hill Climbing and Dictionary keyword search.

After selecting the Trifid cipher, the cipher period must be selected from the Period drop down menu in the Cipher Settings ribbon group. The period range is obtained from the settings in the Options window.