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Simple polyalphabetic cipher
The Vigenére Cipher is a manual encryption method
that is based on a variation of the
Caesar Cipher.
It works by applying a series of different Caesar Ciphers on the
plaintext, based on the letters of a so-called keyword.
It is in fact a simple form of polyalphabetic substitution.
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The Vigenère Cipher exists in different forms, such as a rectangular matrix
with 26 shifted alphabets (tabula recta) and as two concentric discs with a
full alphabet each.
The letters of the keyword determine how many places the inner disc should
be shifted.
During the course of history, the Vigenère Cipher has been reinvented many
times. It was falsely attributed to Blaise de Vigenère as it was originally
described in 1553 by Giovan Battista Bellaso in his book La cifra del.
Sig. Giovan Battista Bellaso
[1]
.
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The first well-documented description of a polyalphabetic cipher however,
was made around 1467 by Leon Battista Alberti. The Vigenère Cipher is
therefore sometimes called the Alberti Disc or Alberti Cipher.
In 1508, Johannes Trithemius invented the so-called tabula recta
(a matrix of shifted alphabets) that would later be a critical component
of the Vigenère Cipher.
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A good example of the Vigenère Cipher is the
Confederate Cipher Disk
that was used as a field cipher during the American Civil War (1861-1865).
It was usually supplied in a small leather bag (see below).
CSA stands for Confederate States of America, and
SS means Secret Service.
Only five original Confederate Cipher Discs are known to have survived.
The image above shows a reasonable reproduction that is
available from the gift shop at the National Cryptologic Museum (NCM),
the NSA's public museum in Fort Meade (Maryland, USA).
They are also frequently offered on Ebay.
Check the images below for more details of this replica.
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The Vigenère Cipher exists in many different forms and flavours.
Although it was initially implemented as a rectangular table,
the most common application is as a cipher disc.
In most cases, the outer disc and the (smaller) inner disc both contain the
alphabet in the usual order.
Sometimes, however, the alphabet on the inner disc was printed
in reverse order (Reverse Caesar Cipher)
and sometimes it contained a scrambled alphabet, making it a
substitution cipher.
Furthermore, the application of the disc varies. It can be used as a simple shift
cipher by shifting a fixed number of positions, or as an advanced alphabetic
or polyalphabetic substitution cipher, by using a key word or phrase to
determine the number of shifts.
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Breaking the Vigenère Cipher
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The Vigenère Cipher is very easy to understand and often appears to
beginners to be unbeatable. Over the years, even reputable authors,
mathematicians and magazines have claimed the cipher to be unbreakable
(e.g. in Scientific American in 1917).
This is not true however. The cipher can be broken by a variety
of hand and mathematical methods. It was first broken by Charles Babbage
and later by Kasiski, who published the technique he used.
In 1920, the famous American Army cryptographer William F. Friedman
developed the so-called Friedman test (a.k.a. the Kappa test).
The primary weakness of the Vigenère Cipher is the principle of the
repeating key. If a cryptanalist can work out the length of the key,
he can treat the ciphertext as a number of interwoven Caesar Ciphers,
which can all individually be broken. Both Kasiski and Friedman have
developed a mathematical test to determine the length of the key.
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© Crypto Museum. Created: Saturday 14 August 2010. Last changed: Friday, 26 May 2023 - 21:03 CET.
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