Vigenere Cipher: Description

Vigenere ciphers are simple poly-alphabetic ciphers derived from Caesar ciphers. Instead of shifting each plain text character by the same number, as with a Caesar cipher, characters at different positions might be shifted by different numbers. Concretely, one picks an orderes n-Tuple of Caesar ciphers. A character at position p in the plaintext is encrypted using the (p mod n) + 1th cipher in the tuple. For convenience one encodes such a tuple as a word by mapping a Caesar cipher with shifting distance 1 to a, shifting distance 2 to b, and so on. Vigenere ciphers form a group where the keylength of the product of two vigenere ciphers is the lcm of the keylengths of the factor keys. Hence super-encipherment makes limited sense.




Equivalent Shifting Distance

13 5 18 3 5 4 5 19


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Breaking Vigenere Cipher

Vigenere ciphers are weak ciphers. They can be broken by just looking at a cipher-text and studying common bigrams and trigrams in the cipher-text and their distance from each other.

In our approach we will concentrate on trigrams.

Last Update: 19.04.96 (Format: DD.MM.YY)