EverCrack Open Source Cryptanalysis

The Vigenere Cipher is a uniliteral, monographic, polyalphabetic

substitution cipher. It is the simplest form of polyalphabetic substitution

ciphers because it simply uses a series of rotated [Caesar] ciphers for the

set of cipher alphabets.

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Using a key, you encipher the alphabet by locating the plain letter to encypt

in the top column, then finding the row that begins with the key word letter,

then using the intersection of the two to locate the cipher letter.

Clear Message

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Cipher Message

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Cryptanalyzing a Vigenere Cipher can involve different approaches:

If you know it is a Vigenere Cipher you could stage a key attack.

In this case you would [with a computer] perform an IOC check on different key

lengths. This involves enumerating the IOC on every Nth letter [where N is the

length of the key to try]. When you have an IOC that matches the target lan-

guage then you have the proper key length. At this point you could try every

possible key [using the above table as the decryptor]. The symbol space of the

key consists of only the 26 letters of the alphabet, so a key of length 3 [as

in the above case] would only have 26 * 25 * 24 = 15, 600 keys - which could

be calculated in a second by an modern computer. The longer the key the more

difficult. If the key had a length of 6 there would be 165, 765, 600 - not

huge but substantially greater than the latter.

Another approach is looking for common polygraphs and measuring

the distance between them. Common polygraphs might encrypt to the same set of

cipher symbols [especially if the key word is short].

The Beaufort Cipher is uses the same encrypting table as the

Vigenere Cipher. The only difference is how it is used to encrypt and decrypt.

Instead of finding the intersection between the plain text letter [in the top

row] and the key word letter [down the left column] you find the plain text

letter [in the top row], travel down until you find the key word letter [in

that column] then travel to the first column on the left [using the letter

there as the cipher text letter]. This method simplifies the process.

Clear Message

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Cipher Message

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