Seriated Playfair

History

The Seriated Playfair cipher was described by Helen Fouché Gaines (1888-1940) in her book Cryptanalysis: A Study of Ciphers and Their Solution, first published as Elementary Cryptanalysis, 1939 and is a more secure variation of the Playfair cipher which was invented by the English physicist and inventor Sir Charles Wheatstone (1802-1875) in 1854. The main difference is that the plaintext is divided into periodic groups of letters and seriated, or arranged, into two rows. Each vertical letter pair is then enciphered using the same rules as the Playfair cipher.

Description

A 5 x 5 square matrix is created by filling it with a keyed alphabet using all the letters of the alphabet, with J being replaced by I. If a 6 x 6 square matrix is selected the 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 key word, with repeated letters being omitted, followed by the unused letters of the alphabet in alphabetic order. For example the keyword SERIATEDPLAYFAIR is reduced to SERIATDPLYF when repeated letters are removed. Appending unused alphabet letters produces the keyed alphabet:

SERIATDPLYFBCGHKMNOQUVWXZ

The keyed alphabet is written into the matrix in a selected route, e.g. horizontal, diagonal or clockwise.

Encryption is performed by separating the plaintext into pairs of letters or digraphs. If a pair contains identical letters a null letter, such as X, is inserted between them. If the plaintext contains an odd number of letters a null letter is also added at the end of the text to complete the last letter pair. The two letters of each pair are located in the matrix and are enciphered using one of the following rules:

  1. If the two letters are in the same row take the two letters to the right of the plaintext letters with the last column wrapping around to the first column. So letters in the last column are replaced by letters in the first column.

  2. If the two letters are in the same column take the two letters below the plaintext letters with the last row wrapping around to the first row. So letters in the last row are replaced by letters in the first row.

  3. If the two letters are on different rows and columns treat them as being in the corners of a rectangle and take the two letters in the other corners, beginning with the letter on the same row as the first plaintext letter.

S E R I A

T D P L Y

F B C G H

K M N O Q

U V W X Z

Example

Keyword: SERIATEDPLAYFAIR

Period: 7

Plaintext: Babbage's Rule: "No man's cipher is worth looking at unless the inventor has himself solved a very difficult cipher" -- The Codebreakers by David Kahn

babbage manscip rthlook lessthe rhashim vedaver ultciph odebrea ah

sruleno heriswo ingatun invento selfsol ydiffic erxthec kersbyk nx

FSFGSCI QSWEFRL IPFYKKM GREEPFI IBITFLO ZDLSURP VPLFADF QBRFEAS RG

EIVDROM BRPARXN AKHILXO LMURKYM EAYKAXD DBEHBAN SIUPGRG MDIECDQ QZ

Ciphertext: FSFGSCI EIVDROM QSWEFRL BRPARXN IPFYKKM AKHILXO GREEPFI LMURKYM IBITFLO EAYKAXD ZDLSURP DBEHBAN VPLFADF SIUPGRG QBRFEAS MDIECDQ RGQZ

Solving

Solving methods: Hill Climbing and Dictionary keyword search.

Both 5x5 and 6x6 Seriated Playfair solvers are implemented. The square size is determined automatically depending on whether the cipher contains any numbers. After selecting the Seriated Playfair cipher, the cipher period must be selected from the Period drop down menu in the Cipher Settings ribbon group. Only valid periods will appear in the menu.