Tridigital

History

First published in the Jul-Aug 1959 edition of American Cryptogram Association (ACA) magazine The Cryptogram the Tridigital cipher is a substitution cipher developed by ACA member OWLEYES, which replaces letters of the alphabet with single digit numbers.

Description

Encipherment is performed by creating a keyed alphabet from a key word, with repeated letters being omitted and followed by the unused letters of the alphabet in alphabetical order. This is written into a 3x10 table leaving the last column blank. Above the table is written a numerical key using the numbers 0-9. This can be based on a 10-letter keyword with the order of the numbers determined by the alphabetical order of the key word letters. For example the key word OSCARWILDE is appended with unused alphabet letters to produce the keyed alphabet:

OSCARWILDEBFGHJKMNPQTUVXYZ

The keyword TRAMPOLINE is used to create the numerical key:

0915874362

The keyed alphabet is written into a table leaving the last column empty and the numerical key is written above this as shown in the following table:

T R A M P O L I N E

0 9 1 5 8 7 4 3 6 2

O S C A R W I L D -

E B F G H J K M N -

P Q T U V X Y Z - -

The ciphertext is produced by replacing each letter of the plaintext with the digit above it in the same column with the last digit used as a word separator.

Example

Key word #1: 0915874362 (TRAMPOLINE)

Key word #2: OSCARWILDE

Plaintext: Some cause happiness wherever they go others whenever they go.

    Keys: 

    TRAMPOLINE

    0915874362

    OSCARWILD-

    EBFGHJKMN-

    PQTUVXYZ--

Plaintext:  Some cause happiness wherever they go others whenever they go

Ciphertext: 9030215590285004609927808080821804250201808927806080821804250

Solving

Solving method: Hill-climbing and Dictionary search.

After selecting the Tridigital cipher, select the number representing the word separator from the Setup drop down menu. Only valid word separators will be shown in the menu. The program will search the dictionary for plaintext words matching the given ciphertext words from the longest words to the shorter ones. To reduce the program runtime and the number of solutions found the program will only search for words five characters long or greater. In most occasions the solution can be found from a partial solution.

If no solution is found it’s likely due to the wrong word separator being selected or a search word not being present in the dictionary files.

If solving a cipher constructed from a keyword grid of English words, a small dictionary list of keywords can be used such as the EnglishSml word list as this reduces the number of words checked.

A keyword can be determined by taking the keyword found and performing a keyword search using the Key Square finder in the Word Search tab. Copy the partial keyword into the ‘Common columns’ field and select the 9x3 Grid option.