Slidefair

History

The Slidefair cipher was invented by Helen Fouché Gaines (1888-1940) and described in her book Cryptanalysis: A Study of Ciphers and Their Solution, first published as Elementary Cryptanalysis, 1939.

Description

The Slidefair cipher can be used with the Vigenère, Beaufort or Variant Tables and encryption is performed in letter pairs and a keyword defines the period and alphabets used. If the plaintext contains an odd number of letters a null letter, such as X, is added at the end of the text to complete the last letter pair.

In each table the key letter is found in the left column of the table and determines the table row or alphabet used. The first plaintext letter of each pair is located in the top table row to determine the column and the second letter is found in the table on the same row as the key letter. These are considered to be on a diagonal and the two letters in the opposite corners form the ciphertext pair with the top letter used first. If the two plaintext letters are in the same column then the letters immediately to the right of the plaintext letters are used as the ciphertext.

Example

Type: Vigenère Slidefair

Keyword: Slidefair

Plaintext: If you do a job too well, you’ll get stuck with it.

Key: S L I D E F A I R S L I D E F A I R S

Plaintext: if yo ud oa jo bt oo we ll yo ul lg et st uc kw it hi tx

Ciphertext: NA DJ VC XR KN OG PP XF UC WQ AF YT QH PW XZ WK LQ RY FL

S L I D E F A I R

if yo ud oa jo bt oo we ll

NA DJ VC XR KN OG PP XF UC

yo ul lg et st uc kw it hi

WQ AF YT QH PW XZ WK LQ RY

tx

FL

Encipher the following quote "If you do a job too well, you’ll get stuck with it.” using the keyword SLIDEFAIR gives:

Vigenère Slidefair Cipher

if yo ud oa jo bt oo we ll yo ul lg et st uc kw it hi tx

NA DJ VC XR KN OG PP XF UC WQ AF YT QH PW XZ WK LQ RY FL

Beaufort Slidefair Cipher

if yo ud oa jo bt oo we ll yo ul lg et st uc kw it hi tx

NK XN FO DP QV ME MM EM GG EU AR CX KZ LM DL EQ PA JK VZ

Variant Slidefair Cipher

if yo ud oa jo bt oo we ll yo ul lg et st uc kw it hi tx

XQ ZN LM DL SF YW PP MO CU GG WJ OD WB XO HP WK BA ZQ PB