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Vigenère Cipher


Alberti
Leon Battista Alberti

Leon Battista Alberti (1404-1472) is considered the Father of Western Cryptology and was the real inventor of the "Vigenère" cipher disk in 1467, 56 years before Vigenère was even born. He also invented polyalphabetic ciphers, which he called "worthy of kings" and claimed was unbreakable. This was the start of a long tradition of cipher makers claiming the invincibility of their inventions, only to be proven otherwise in every case except one, the one-time pad cipher. At least in Alberti's case, he had the concurrence of no less an authority than Scientific American magazine 450 years later!

His invention of the cipher disk allowed the more complex polyalphabetic cipher to be user friendly. He recommended that the sender and receiver agree on an index letter, for example, "2" on the outer ring which would be aligned with the "a" on the inner ring. The first letter of the message, say "t", would be aligned with the "G" on the outer ring (as shown on the picture). So the first letter of the encipherment is "G", then using that arrangement of the disks, 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 will now be aligned with the "2". So by spinning his disk around to align the "p" and "2", an entirely new substitution alphabet is generated.


Alberti Cipher Disk
Alberti cipher disk
note the use of the numbers 1-4

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, since each letter will be enciphered to several different letters. This was an astounding insight and solution, which can be baffling to decipher, but the cipher is not unbreakable.

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 system will substitute a code for whole words or phrases. Some code systems have been very ingenious including several codes to mean the same thing, null codes, codes for specific people, places, etc. Some codebooks run to tens of thousands of entries and can be slow to use but are very difficult to break if the codebook has not been compromised.

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 with all the major countries of Europe, but it was 400 years later, at the end of the 19th century, before the practice of enciphering codes was widely adopted.


Vigenère
Blaise de Vigenère

Blaise de Vigenère (1523-1596) had nothing to do with the famous cipher disk that bears his name. He did, however, make a major contribution to cryptology by inventing the 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, then use the first plaintext letter as the next letter to align with the index letter. Continue this 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. Autokey is a cipher strategy still in use during modern times.


Civil War Cipher Disk
Civil War Confederate cipher disk,
one of 5 in existence

Pictured is one of only 5 remaining Confederate cipher disks used in the Civil War and is very similar to the original cipher described by Alberti. "Complete Victory" was one of only three keywords used throughout the war. The other keywords were "Manchester Bluffs" and "Come Retribution". Capturing several messages with the same keyword allows the cryptanalyst to use letter frequency analysis on the first letter of each message, the second letter, etc. Also, they can then determine the length of the keyword and decipher the keyword as an aid to deciphering the message. The Union army quickly deciphered the Confederate messages, with tragic results for the South.

The only theoretically unbreakable cipher is the one-time pad, which could be used in combination with the Vigenère cipher. The one-time pad is a long list of random letters, so the key would be the length of the message and the key itself would not contribute to the deciphering effort. The important point of this is the "one-time" use, since a repeated use of the pad would open it to letter frequency analysis. The disadvantage is that it is rather cumbersome to use and if captured by the enemy the solution is trivial.




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Mystery Cipher
See detailed pictures of the mystery cipher wheel


This mystery cipher wheel, dated 1942, has a partial alphabet on one wheel and the digits 0-9 on the other. It may have been used to send code or other information only requiring numbers, such as weather or targeting information. This wheel is the same as the Vigenère cipher wheel, with one alphabet exchanged for numbers. This was probably used as a polyalphabetic cipher machine.