Delastelle Trifid Cipher

The Delastelle trifid cipher uses three 9-character grids (for 27 distinct characters in total) and an integer $ N $ (usually 5 or 7).

Example: Encrypt the message SECRET, with $ N = $ 5 and grids

Often, a keyword is used to generate a disordered alphabet with 27 characters (the Latin alphabet accompanied by another symbol like _ replacing any non-alphabetic character)

Step 1: For each character, search for it in grids and note are triplet of 3 corresponding digits (grid, line, column)

Example: S is in grid 3, line 1, column 1, its triplet is 311

Step 2: Write the triplets in columns, in groups of $ N $ columns next to each other and read each group in rows.

Example:

Reading group 1: 31121,12132,12332, group 2: 312

Step 3: Cut out each sequence of digits read in triplet group of 3 digits corresponding to (grid, line, column) and note the corresponding letter. These letters constitute the encrypted message.

Example: 311,211,213,212,332,312 corresponds to SJLKZT

Decryption is very similar to encryption, the difference is in step 2.

Example: Decrypt the message SJLKZT, with $ N = $ 5 and grids

Step 1: identical to encryption

Step 2: Take the triplets in groups of $ N $ and write them in $ N $-length lines below each other then read each group in columns.

Example: 311,211,213,212,332,312 is written

Reading group 1: 311,122,113,233,123, group 2: 3,1,2

Step 3: Identical to encryption

Example: 311,122,113,233,123,312 corresponds to the plain message SECRET

The message is theoretically composed of not more than 27 distinct characters.

The $ N $ number quickly changes the encrypted message, for better encryption, it is advisable to take a value of $ N $ first with 3.

Félix Marie Delastelle described this encryption in 1902 in his book Traité Élémentaire de Cryptographie