Interrupted Key

History

The Interrupted Key cipher was first described in the Oct 1935 edition of the American Cryptogram Association (ACA) magazine The Cryptogram and can be used with any of the Periodic substitution ciphers; Vigenère, Beaufort, Variant Beaufort and Porta. It differs to the Periodic ciphers in that the Interrupted Key cipher uses a keyword which is interrupted at random making it slightly more secure. In this implementation the keyword can be restarted at random or, less secure, at word divisions. If the keyword is restarted at random then the full keyword must be used at least once.

Description

Encipherment is performed by writing the plaintext with normal or random word divisions and above each group the keyword is written, repeating as necessary. The keyword is restarted from the beginning after each word division.

For example enciphering the Mark Twain quote: "If you tell the truth, you don't have to remember anything" with the keyword TWAIN could produce:

TW TWA TWAI TWA TWAIN, TWA TWA I TWAI TW TWAINTWA TWAINTWA

If you tell the truth, you don't have to remember anything

Encipherment follows the same procedure as the Periodic cipher to encipher the text. To encipher plaintext using the Vigenère cipher the key letter is found in the left column of the table and determines the row, and the plaintext letter is found in the top row and determines the column. The intersection of these reveals the cipher letter. So if the key is T and the plaintext letter is I the cipher letter is B.

Example

Type: Vigenère

Keyword: TWAIN

Key: TW TWA TWAI TWA TWAIN, TWA TWA I TWAI TW TWAINTWA TWAINTWA

Plaintext: If you tell the truth, you don't have to remember anything

Ciphertext: BB RKU MALT MDE MNUBU, RKU WKN'B AWVM MK KAMMZUAR TJYBUBJG