Key Phrase
History
The Key Phrase cipher was first described by Helen Fouché Gaines (1888-1940) in the October 1937 edition of the ACA's magazine The Cryptogram and later included in her book Cryptanalysis: A Study of Ciphers and Their Solution, first published as Elementary Cryptanalysis, 1939. The Key Phrase is a simple substitution cipher whose key is based on a 26-letter key phrase which correspond to the plaintext alphabet and resulting in cipher letters representing multiple plaintext letters. To avoid ambiguity word divisions are maintained.
Description
To encipher using the Key Phrase cipher a key phrase of exactly 26-letters is used and placed under a straight alphabet. The plaintext is written retaining word divisions and each letter is substituted by the letter in the key phrase.
Example
To encipher using the Key Phrase cipher a key phrase of exactly 26-letters is used and placed under a straight alphabet. The plaintext is written retaining word divisions and each letter is substituted by the letter in the key phrase.
Key phrase: WHATSANOTHERWORDFORSYNONYM
Plaintext: The difference between fiction and reality? Fiction has to make sense.
Alphabet: abcdefghijklmnopqrstuvwxyz
Key phrase: WHATSANOTHERWORDFORSYNONYM
Plaintext: the difference between fiction and reality? fiction has to make sense.
Ciphertext: SOS TTAASOSOAS HSSOSSO ATASTRO WOT OSWRTSY? ATASTRO OWR SR WWES RSORS.
Solving
Solving methods: Hill Climbing and Dictionary keyword search.
After selecting the Keyphrase cipher, the minimum word length to search needs to be selected from the Setup drop down menu on the toolbar. Selecting a shorter word length will take longer to complete and may terminate prematurely.
Clicking on Solve will open a window to locate a tip or fix the key. This is optional but will improve solving speed if used.
Tip – Enter the tip or crib in the Tip field. A long tip will improve the searching speed.
Click on the Locate tip button to 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 confirming the location selected.
The tip location within the cipher will also be displayed in the Addition tip location textbox.
Additional ciphertext can be added in the Ciphertext textbox to help improve the solution.
Click OK to start searching for a solution.
It is recommended to use a small dictionary word list, such as EnglishSml, to improve solving speed.
