Homophonic

History

A Homophonic substitution cipher replaces letters with numbers or symbols and each letter can have one or more replacements but each number or symbol always stands for the same letter. There are many variations of the Homophonic cipher and the one implemented here follows the method used by the American Cryptogram Association (ACA) and was first described in the Dec 1945 - Jan 1946 edition of the ACA's magazine The Cryptogram in an article written by ACA member Frances A. Harris (S-TUCK).

Description

In this variation of the Homophonic cipher four alphabets are used with the letter J being replaced by I to form 25-letter alphabets. The letters of each alphabet are then numbered 01-00 with the first being numbered 01-25, the second 26-50, the third 51-75 and the last alphabet 76-00 with the starting position of each alphabet determined by a 4-letter keyword. Each alphabet is numbered so that the lowest number in each set starts below the respective letter of the keyword.

For example using the key word SLOW the first alphabet is numbered S-01, T-02, U-03 and so on up to Q-24 and R-25. The second alphabet is numbered L-26, M-27, N-28 up to K-50 and this is repeated for the next two rows. In the final row 100 is numbered as 00 and the following table is produced:

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

09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 01 02 03 04 05 06 07 08

41 42 43 44 45 46 47 48 49 50 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

63 64 65 66 67 68 69 70 71 72 73 74 75 51 52 53 54 55 56 57 58 59 60 61 62

80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 76 77 78 79

To create the cipher each plaintext letter is replaced by one of the numbers, chosen at random, in the same column in the table.

Example

Keyword: SLOW

Plaintext: He who laughs last, thinks slowest.

Plaintext: h e w h o l a u g h s l a s t t h i n k s s l o w e s t

Ciphertext: 70 84 76 48 29 73 41 03 86 70 55 90 80 01 98 34 70 71 28 18 55 97 26 22 76 67 33 34

Solving

Solving method: Brute force search.

All possible solutions are tested.

If another type of Homophonic cipher it may be possible to try the Numbered Key cipher type. With this type a letter based cipher would need to be converted to numbers in the range 00-99 where each number replaces a single letter.