Vigenere Cipher

Tam

Basic knowledge

Simple substitution cipher

The most famous example: the Caesar cipher.

Example: Shift one character.

Simple substitution cipher is

each 'H' of plain text become the same charactor.

Vigenere Cipher

plain text: 'CODE'

key: 'ARM'

cipher text: 'CFPE'

Using Computer

We assume the below:

$ P_i $ : The i-th charactor of plain text

$ K_i $ : The i-th charactor of key

$ C_i $ : The i-th charactor of ciphered text

Encrypt:

Decrypt:

Kasisky Test

Babbage found the below:

1. Search for strings of the same characters.
2. Count the intervals between the strings.
3. Calculate the common factors of these numbers.
   For example: we found the intervals number are 9, 63, 180, we can calculate the common factors of these numbers are 3 or 9.
4. We can guess the length of the key is 3 or 9.
5. We run it through frequency analysis.

Finally, this is the same as the Caesar cipher with intervals of 3 or 9.

Charles Babbage

• The father of computer.
• discover the steam calculator "Babbage machine".

autokey cipher

Basic Idea

Continue to Enigma...