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Vigenère Cipher Disk

Alberti
Leon Battista Alberti
(1404-1472)
Vigenere
Blaise de Vigenère
(1523-1596)

The Vigenère cipher disk was named for Blaise de Vigenère, even though it was invented in 1467 by Leon Battista Alberti, 56 years before Vigenère was born. This was the beginning of a long tradition of cipher devices being credited to the wrong inventor.


Alberti is considered the Father of Western Cryptology. He not only invented the Vigenère cipher disk, he also invented polyalphabetic ciphers, which he called "worthy of kings" and claimed was unbreakable. This was the start of another tradition of cipher inventors, claiming the invincibility of their invention, only to be proven wrong in every case except the one-time pad. At least in Alberti's case, he had the concurrence of no less an authority than Scientific American magazine in 1917, 450 years later!


A polyalphabetic cipher uses more than one monoalphabetic cipher arrangement within each message. The more cipher arrangements used, the stronger the cipher. This may seem to be a straightforward solution to thwart letter frequency analysis, so why did this solution take 500 years? The problem with polyalphabetic ciphers is the complexity of using multiple cipher arrangements and the high probability of introducing human errors. The beauty and power of the Vigenère cipher disk was the ability to make polyalphabetic ciphers user friendly.


Two other methods to make polyalphabetic ciphers easier to use are the Playfair cipher and the electro-mechanical rotor ciphers, which will be described in upcoming chapters.


The Vigenère disk is made up of 2 concentric disks with the alphabet written on the outer disk in the normal sequence and the inner disk has the alphabet usually in reverse order or any other order. The inner disk rotates. The outer disk usually represents the plaintext and the inner disk represents the enciphered text.


Vigenere Disk
Alberti cipher disk
(note the numbers 1-4)

Alberti recommended that the sender and receiver agree on an index letter, for example, “g” to set the initial position for the disks. So "A" on the outer ring is aligned with the "g" on the inner ring (as shown on the picture). The first letter of the message, say "T", is aligned with the "i" on the inner ring. So the first letter of the encipherment is "i", then the next letters are enciphered. This is still a monoalphabetic cipher until you get to his next important step, which is to rotate the disk after 3 or four words by writing a capital letter in the message text, say "P", which is aligned with the "s". So by spinning the disk around to align the "A" and "s", an entirely new substitution alphabet is generated.


A later method of using the Vigenère disk is to use a keyword or phrase, so that the disk is rotated after each letter of a message based on that keyword or phrase.


The significance of polyalphabetic ciphers is that each letter is represented by several different letters in the ciphered message. Every time the disk is rotated during the encipherment of a single message, a new cipher alphabet is created. Letter frequency analysis will no longer work. This was an astounding insight and solution, which can be baffling to decipher, but the cipher is not unbreakable. The method used to break the cipher is to decipher the keyword along with the ciphertext and to exploit the recurrence of the keywords with letter frequency analysis. If several messages are sent with the same keyword, then letter frequency analysis will work on the first letter in each message, the second letter, and so on. This solution was first published in 1863 by Friedrich Kasiski.


Alberti made a third remarkable invention in cryptology, enciphered code. The difference between a code and a cipher is the cipher changes a message on a letter-by-letter basis while a code will substitute a few numbers or a word for different words or phrases.


Alberti had the numbers 1-4 on his disk and used codes from 11 to 4444 to replace entire phrases. For example, 324 may mean "the ships are ready to sail." Rather than send 324 as part of the message, he recommended using the cipher disk to encipher the code! This represents a very strong cipher system, which would be difficult to decipher even if you captured the codebook. Codes became popular, but it was 400 years later, at the end of the 19th century, before the practice of enciphering codes was widely adopted.


Blaise de Vigenère had nothing to do with the famous cipher disk that bears his name. He did, however, make a major contribution to cryptology by inventing autokey. Like Alberti, he recommended the sender and receiver agree to an index letter as the starting position of the cipher disk. After the first letter is enciphered, he then proposed using the first plaintext letter as the next letter to align as the index letter. Continue in this manner through the entire message, changing the disk rotation after every letter of the message.


This is an advantage over repeating keywords, because a cryptanalyst would not be able to exploit the cyclic nature of the keyword. Also, this avoids the problem of changing and distributing keywords. Autokey is a cipher strategy still in use during modern times. It is ironic that even though Vigenère does not deserve credit for his namesake, the credit for his stronger method of encipherment is largely ignored.





Vigenère Cipher Disks

from the Cipher History Museum

Confederate Cipher Disk (Replica) – 1862

CHM

“CSA SS” is inscribed on the Confederate cipher disk, which means Confederate States of America and Secret Service. It was designed by Francis LeBarre and the 2 disks have the alphabet in the normal sequence around their circumference.


The Confederates used only 4 keywords throughout the war, “Manchester Bluff,” “Complete Victory,” “Come Retribution” and “In God we trust.” This helped the Union to easily decipher the Confederate messages. During the Civil War, most messages were sent by courier, so intercepting a message was rare compared to intercepting radio traffic. Only 5 of these disks are known to exist, one at the Smithsonian, two at the Museum of the Confederacy in Richmond, VA and two in private hands. One sold in Nov. 2014 for $20,000.


Wheatstone Cryptograph - c.1860

CHM

Charles Wheatstone invented the clock or progressive cipher disk independently of the original 1817 invention by Decius Wadsworth. It is ironic this type of cipher bears his name, but his later invention of the stronger digraphic cipher is known as the Playfair cipher, named after Wheatstone’s friend who encouraged its use by the British military.


The outer disk has 27 letter positions, including a blank, while the inner disk has 26 mixed letters. These letter wheels remain fixed and the clock style hands rotate. Letters are enciphered and deciphered by rotating the large hand clockwise. The hands are geared 27 to 26 so the small hand advances or progresses one letter position for each rotation of the large hand. This progression means there is a new coding algorithm after every rotation of the large hand, so coding and decoding can be accomplished without the use of keywords.


Improvements in Cryptographic Instruments, UK Patent - 1896

CHM

Richard Harte of Croydon, England was issued UK patent #16,730 in 1896 for an improvement to the Vigenère cipher disk. His patent added 3 pointers to the inner disk which allows the users to choose 3 additional cipher letters (for a total of 4) by a pre-arranged key.


These pages are from a bound volume of 100 patents printed by Her Majesty’s Stationery Office (HMSO) and have the impression stamp of the Manchester Library. HMSO is responsible for all the Crown copyrights in the UK.


Mystery Cipher Disk – 1942

CHM

This is a 2-wheel mystery cipher device with letters on the circumference of one wheel and numbers on the other. The face of Disk 1 is inscribed, "TO THOSE WHO WAIT" and "1942". Disk 2 is inscribed "I MISS HER SMILE" and the reverse side has the logo of an "H" inside a circle.


The disks are substantial but rather crudely inscribed. The numbers are evenly spaced but some letters do not lined up with the numbers. The Husky factory in Wisconsin used the circle H logo from 1935-1945. Sears bought Husky and the Craftsman tools manufactured in the old Husky factory have the same circle H logo as the mystery cipher disk. Since the entire alphabet is not present, this disk was most likely used to encipher numbers, such as for codes or targeting locations.


Vigenère Wheel Games - 1934-50

CHM  CHM
CHM

The Code-a-Graph code wheel is from a child’s game and dated 1934. This variation of the Vigenère disk allows the selection of 3 different letters for each rotation of the wheel. The Radio Orphan Annie Secret Society cipher badge is from the radio show, which ran from 1931-42. A different cipher badge was made for each year from 1935-40 to send secret messages, usually giving a preview for the next week’s show. This is an example of the first badge made in 1935.


The “Code-Maker” slide rule was part of a game which uses the same encipherment as a Vigenère disk, but in a linear format. It was made by Fun Inc. in the 1940-50s.


Reverse Vigenère Wheel – 1920-50s

CHM

The reverse Vigenère cipher wheel, sometimes called a reverse Caesar cipher wheel, is milled out of a solid piece of brass. It has an outer alphabet ring in the normal sequence and a rotating inner ring with the alphabet in reverse order. The inner ring is attached to a ruler, which can be fixed to any of the letters on the inner ring.


The advantage to having the inner disk in reverse order is that encoding and decoding can use the same procedures, even when the alphabet is shifted using the moveable ruler.


“Regensburg Verschlusselung” is inscribed on the reverse side. Regensburg is the city in Germany where this was manufactured and Verschlüsselung is German for encryption.


US KAL-55B “Whiz Wheel” Authenticator - 1960s

CHM

The US KAL-55B or “Whiz Wheel” is a Vigenère style cipher device using up to 3 letters per cell on the outer wheel and numbers on the inner wheel. It was used during the Vietnam War for authentication of the sender before sending in air support or medevac helicopters. Also, coordinates would be encrypted with this wheel to prevent disclosing locations to the enemy. The wheel was changed daily, this one is labeled “Day 18.” The back of the card gives a full description of how to encipher a message or geographic location using the wheel and to send short, encrypted messages.


Copyright © 2016 by Ralph Simpson - Creative Commons Attribution - ShareAlike 4.0 International License