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Vigenère Cipher
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.
 
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] .
  
The Confederate Cipher Disc, used during the American Civil War, based on the Vigenère Cipher.

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.
 
Confederate Cipher Disc
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.
 
The Confederate Cipher Disc, used during the American Civil War, based on the Vigenère Cipher. The Confederate Cipher Disc, used during the American Civil War, based on the Vigenère Cipher. A replica of the Confederate Cipher Disc on top of its leather bag. A replica of the Confederate Cipher Disc on top of its leather bag.

 
Variations
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 (transposition 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.
 
Breaking the Vigenère Cipher
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 methematical 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.
 
Information on Wikipedia

References
  1. Wikipedia, Vigenère Cipher
    Retrieved September 2010.

Further information

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