# 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.
## Example

### Key

mercedes

### Equivalent Shifting Distance

13 5 18 3 5 4 5 19

### Plaintext

i am so cool

### Ciphertext

vespewtsptfo

## 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)