CM Bifid
History
The Conjugated Matrix Bifid, or CM Bifid, first appeared in the Jan-Feb 1960 edition of the American Cryptogram Association magazine The Cryptogram though it was thought to have been developed around 15 years previously. Based on the Bifid cipher it's a fractional substitution cipher which can be considered a combination of substitution and transposition.
Description
To encipher text using the CM Bifid cipher two 5 x 5 square matrices are created and filled with different keyed alphabets using all the letters of the alphabet, with J being replaced by I. If 6 x 6 square matrices are selected the 26 letters of the alphabet and numbers 0-9 are used with A followed by 1, B by 2, C by 3 and so on up to J followed by 0.
A keyed alphabet is created from a keyword, with repeated letters being omitted, followed by the unused letters of the alphabet in alphabetical order. The keyed alphabet is written into the matrix in a selected route, e.g. horizontal, diagonal or clockwise. For example the keyword MARKTWAIN is reduced to MARKTWIN when repeated letters are removed. Appending unused alphabet letters produces the following keyed alphabet:
MARKTWINBCDEFGHLOPQSUVWXYZ
The plaintext is then divided into period length groups and the two coordinates of the position of each letter in the first matrix is written below each letter. These digits are then read horizontally in pairs within each group and replaced by the letter in that position in the second matrix.
1 2 3 4 5 1 2 3 4 5
1 M A R K T 1 T W C I Q
2 W I N B C 2 O Y D K U
3 D E F G H 3 M E F L V
4 L O P Q S 4 S R G N X
5 U V X Y Z 5 A B H P Z
Example
Keyword 1: MARKTWAIN (Rows from top left)
Keyword 2: TOMSAWYER (Columns from top left)
Period: 7
Plaintext: It usually takes more than three weeks to prepare a good impromptu speech.
itusual lytakes moretha nthreew eekstop reparea goodimp romptus peech
2154514 4511134 1413131 2131332 3314144 1341131 3443214 1414154 43323
2515121 1452425 1232552 3553221 2245523 3232322 4221213 3213515 32255
OPARAAO XTCSXKU ICCTDUB OMFDZEO FIIRKZD CSCCDDY LGONYWC IIQGOVQ GEFYZ
Plaintext: itusual lytakes moretha nthreew eekstop reparea goodimp romptus peech
Ciphertext: OPARAAO XTCSXKU ICCTDUB OMFDZEO FIIRKZD CSCCDDY LGONYWC IIQGOVQ GEFYZ
Solving
Solving methods: Hill Climbing and Dictionary keyword search.
The CM Bifid cipher type requires a tip for the solving routine with a dictionary search. On selecting the Solve button the dialog box below will appear to enable the cipher period, tip and tip location to be set. On selecting OK the program will search for a solution.
Cipher period – Select the period of the cipher.
Tip – Enter the tip in the Tip field. Spaces and punctuation will be ignored.
Click on the Locate tip button will populate the Tip location list with possible locations for the tip.
Tip location – Select the location of the tip from the list then press OK. When a tip location is selected the ciphertext and plaintext positions are shown below the Tip location.
If the CM Bifid cipher is rerun the previous setting for the tip will be displayed.
Show table and possible keys – Selecting this option will open a new window, when the solving routine is finished, to display the common rows and columns of the CM Bifid squares based on the entered tip and location selection and a list of possible keywords and key squares found. If a solution isn’t found the table can be useful for seeing the possible keys and solving the cipher by hand.
If at least one of the keywords is known this can be entered into the Enter key field on the main window separating them by commas and leaving unknown keywords blank. E.g. “CHALLENGE,SPACEFLIGHT” or “CHALLENGE,”. The order of keywords is ciphertext square key and plaintext square key.
