From Academic Kids

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The Vigenčre cipher is a method of encryption that uses a series of different Caesar ciphers based on the letters of a keyword. It is a simplified version of the more general polyalphabetic substitution cipher, invented by Alberti circa 1465.
The invention of the Vigenčre cipher was misattributed to Blaise de Vigenčre in the 19th century; it was originally described by a Giovan Batista Belaso in his 1553 book La cifra del. Sig. Giovan Batista Belaso.
This cipher is wellknown because while it is easy to understand and implement, it often appears to beginners to be unbreakable. Consequently, many programmers have implemented obfuscation or encryption schemes in their applications which are essentially Vigenčre ciphers, only to have them broken by the first cryptanalyst who comes along.
Contents 
Description
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In a Caesar cipher, each letter of the alphabet is shifted along some number of places; for example, in a Caesar cipher of shift 3, A would become D, B would become E and so on. The Vigenčre cipher consists of using several Caeser ciphers in sequence with different shift values.
To encipher, a table of alphabets can be used, termed a tabula recta, Vigenčre square, or Vigenčre table. It consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers. At different points in the encryption, an encipherer uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword.
For example, suppose the encipherer wishes to encrypt a plaintext:
 ATTACKATDAWN
The encipherer chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON":
 LEMONLEMONLE
The first letter of the plaintext, A, is enciphered using the alphabet in row L, which is the first letter of the key. This is done by looking at the letter in row L and column A of the Vigenčre square, namely L. Similarly, for the second letter of the plaintext, the second letter of the key is used; the letter at row E and column T is X. The rest of the plaintext is enciphered in a similar fashion:
Plaintext:  ATTACKATDAWN 
Key:  LEMONLEMONLE 
Ciphertext:  LXFOPVEFRNHR 
Decryption is performed by finding the position of the ciphertext letter in a row of the table, and then taking the label of the column in which it appears as the plaintext. For example, in row L, the ciphertext L appears in column A, which taken as the first plaintext letter. The second letter is decrypted by looking up X in row E of the table; it appears in column T, which is taken as the plaintext letter.
Vigenčre can also be viewed algebraically. If the letters A–Z are taken to be the numbers 0–25, and addition is performed modulo 26, then Vigenčre encryption can be written,
 <math>C_i \equiv P_i + K_i \pmod{26},<math>
and decryption,
 <math>P_i \equiv C_i  K_i \pmod{26}.<math>
Cryptanalysis
Main article: Kasiski examination
The method described below was not in fact invented by Kasiski but instead by Charles Babbage; its attribution to Kasiski is a common misconception.
The idea behind the Vigenčre cipher is like that of all polyalphabetic ciphers — to make frequency analysis more difficult. Frequency analysis is the practice of decrypting a message by counting the frequency of ciphertext letters, and equating it to the letter frequency of normal text. For instance if P occurred most in a ciphertext whose plaintext is in English one could suspect that P corresponded to E, because E is the most frequently used letter in English. Using the Vigenčre cipher, E can be enciphered as any of several letters in the alphabet at different points in the message thus defeating simple frequency analysis.
Noted author and mathematician Charles Ludwidge Dodgson (aka Lewis Carroll) called this cipher unbreakable in his 1868 piece "The Alphabet Cipher" in a children's magazine. In 1917, the Vigenčre was described as "impossible of translation" in the respected science magazine Scientific American. Despite this reputation, however, the cipher can be broken.
The weakness lies with the fact that the key is relatively short and constantly repeated: as a result, common words like "the" will likely be encrypted using the same key letters, leading to repeated groups in the ciphertext. Look at this example:
abcdefabcdefabcdefabcdefabcdefabc crypto is short for cryptography.
The encrypted text here will not have repeated sequences that correspond to repeated sequences in the plaintext. However, if the key length is different, as in this example:
abcdeabcdeabcdeabcdeabcdeabcdeabc crypto is short for cryptography.
Then a Kasiski examination can be used.
The cipher of Blaise de Vigenčre
Vigenčre actually invented a stronger cipher: an autokey cipher. The name "Vigenčre cipher" became associated with this polyalphabetic cipher instead. In fact the two ciphers were often confused, and both were sometimes called "le chiffre indéchiffrable", or "the unbreakable cipher". For nearly 300 years this cipher was thought to be unbreakable, but Charles Babbage and Friedrich Kasiski independently found a way to break it the middle of the 19th century. Babbage actually broke the much stronger autokey cipher, while Kasiski is generally credited with the first published solution to fixedkey polyalphabetic ciphers.
External links
 The Vigenère Cipher (http://www.murky.org/cryptography/archives/2004/09/vigenre_1.html) as discussed on The Beginner's Guide to Cryptography (http://www.murky.org/cryptography/index.shtml)
 Basic Cryptanalysis (http://www.bbc.co.uk/dna/h2g2/alabaster/A613135) at H2G2
 Java Vigenere (http://it.geocities.com/teutoburgo/java/indexJavaEn.html) applet with source code (GNU GPL)
Classical cryptography edit (https://search.academickids.com:443/encyclopedia/index.php?title=Template:Classical_cryptography&action=edit) 
Ciphers: ADFGVX  Affine  Atbash  Autokey  Bifid  Book  Caesar  Foursquare  Hill  Permutation  Pigpen  Playfair  Polyalphabetic  Reihenschieber  Running key  Substitution  Transposition  Trifid  Twosquare  Vigenčre

Cryptanalysis: Frequency analysis  Index of coincidence 
Misc: Cryptogram  Polybius square  Scytale  Straddling checkerboard  Tabula recta 