Running Key

Description

A variation of the Periodic ciphers known as the Running Key cipher, is a polyalphabetic cipher using the Vigenère, Beaufort, Variant Beaufort and Porta tables but has a very long non-repeating key, derived from its plaintext, to improve its security.

To encipher a Running Key cipher the plaintext is divided in half and written in two rows, one above the other. The top row is used as the key and the lower one the plaintext. Using one of the periodic tables the key letter is found in the left column of the table to determine the row, and the plaintext letter in the top row to determine the column. The intersection of these reveals the cipher letter. So, using the Vigenère type, if the key is W and the plaintext letter is S the cipher letter is O.

The solution of the Running Key was first described by the U.S. Government Cryptographer William F. Friedman (1891-1969) in his publication Methods for the Solution of Running-Key Ciphers (Riverbank Laboratories. Publication), 1918. 

Example

Encipher the following quote from British comedian Bob Monkhouse. "When I die, I want to go peacefully like my grandfather did – in his sleep. Not yelling and screaming like the passengers in his car."

Key:        whenidieiwanttogopeacefullylikemygrandfatherdidinhis

Plaintext:  sleepnotyellingandscreaminglikethepassengersinhiscar

Ciphertext: OSIRXQWXGALYBGUGBSWCTIFGTYEWQUIFFKGAFVJNZLVJLVKQFJIJ

Vigenère Cipher

OSIRX QWXGA LYBGU GBSWC TIFGT YEWQU IFFKG AFVJN ZLVJL VKQFJ IJ

Beaufort Cipher

WEARH KGPQI LYPUS UZOOC PAVSX CIAAA AHJYY AFPZN NXNBF FEAFV SZ

Variant Beaufort Cipher

EWAJT QULKS PCLGI GBMMY LAFID YSAAA ATRCC AVLBN NDNZV VWAVF IB

Porta Cipher

AZTLP WPNUH SICAL TIBNO XTSBP RIQZP TVIVK NEZUT DWZIU OTZEV VK