The Trifid cipher combines substitution with transposition and fractionation. It was invented by Felix Delastelle. Delastelle was a Frenchman who invented several ciphers including the bifid, trifid, and the four-square ciphers.
Trifid is very similar to Bifid, except that instead of a 5 by 5 keysquare (in bifid) we use a 3 by 3 by 3 key cube.
The Algorithm
The key is in the form of 3 squares:
key = EPSDUCVWYM.ZLKXNBTFGORIJHAQ
square 1 square 2 square 3
1 2 3 1 2 3 1 2 3
1 E P S 1 M . Z 1 F G O
2 D U C 2 L K X 2 R I J
3 V W Y 3 N B T 3 H A Q
As an example, we will encipher the text DEFEND THE EAST WALL OF THE CASTLE. The first step means locating the plaintext letters in the squares above, D is in square 1, row 2, column 1, so D becomes 121. In the same manner, E becomes 111. If we write down the numbers corresponding to each letter vertically, it becomes:
D E F E N D T H E E A S T W A L L O F T H E C A S T L E .
1 1 3 1 2 1 2 3 1 1 3 1 2 1 3 2 2 3 3 2 3 1 1 3 1 2 2 1 2
2 1 1 1 3 2 3 3 1 1 3 1 3 3 3 2 2 1 1 3 3 1 2 3 1 3 2 1 1
1 1 1 1 1 1 3 1 1 1 2 3 3 2 2 1 1 3 1 3 1 1 3 2 3 3 1 1 2
At the moment this is still a substitution cipher and fairly easy to break. The next step is to use a 'period', which is a number usually 5 - 20, which is part of the key material agreed on by both sender and receiver. If we take a period of 5,
DEFEN DTHEE ASTWA LLOFT HECAS TLE.
11312 12311 31213 22332 31131 2212
21113 23311 31333 22113 31231 3211
11111 13111 23322 11313 11323 3112
we group the numbers. We now read off the numbers in each group horizontally, and do the substitution back to letters using the original keysquare.
113122111311111 123112331113111 312133133323322 223322211311313
S U E F E C P H S E G Y Y J I X I M F O
311313123111323 221232113112
F O C E J L B S P
Which means DEFEND THE EAST WALL OF THE CASTLE. is enciphered to SUEFE CPHSE GYYJI XIMFO FOCEJ LBSP using the key square above and a period of 5.
PINKY_DUST
