Coding by Vigenère II

Blaise de Vigenère

The Vigenère cipher is a method of text protection by encrypting, based on an alphabet and a series of Caesar ciphers in the tabula recta, than on the letters in keyword. It is a simple form of polyalphabetic substitution.

Here is reserved the standard form of Vigenère cipher, but with the Alphabet that consists of a series of ten digits and 26 lowercase than uppercase of the English letters. Letters out of the Alphabet are ignored, i.e. not encoded. Letters of the keywords should be in the Alphabet.

Encryption is the process of scrambling or enciphering data so it can be read only by someone with the means to return it to its original state. The conversion of encrypted data into its original form is called Decryption. It decodes the encrypted information so that an authorized user can only decrypt the data because decryption requires a secret key or password. Keyword is a word that acts as the password of the code.

2. Alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz (Letters)


How it works

In a Caesar cipher, each letter of the alphabet is shifted along some number of places. In Vigenere cipher we have sequence of Caesar's alphabet each shifted by one letter to the previous and formed the tabula recta. You enter a plaintext, and a keyword that has been repeated into keytext, so it matches the length of the plaintext. The n-th letter of plaintext is enciphered using the alphabet in the row that start by the same letter. Than the letter in plaintext is changed by the letter in keytext, and so on until you got ciphertext. Decryption is performed backward, 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.

Blaise de Vigenère (1523 - 1596) was a French diplomat. At the age of 26 he was sent to Rome on a two-year diplomatic mission. Firstly, his interest in cryptography was practical and part of diplomatic work. Later, at the age of 39 with enough money Vigenere concentrated on a life of study. Than he examined in detail the ideas of his predecessors in work as we know today as Vigenere cipher.

The Vigenère cipher is well known, easy to understand and implement. It is used to be le chiffre indéchiffrable, e.g. the unbreakable cipher. Unlike the standard Vigenère code which works with uppercase letters of the English alphabet only, here are added the small letters and decimal digits. This encoder/decoder is for my private use.

The first successful attack on the Vignere cipher had developed Charles Babbage in 1854, but in 1863 Friedrich Kasiski was the first to publish it. In the Vignere cipher the same letter can be coded by different characters, depending on the position and keyword. That is the most powerful point of the cipher. For instance, if the keyword has four different signs, like keyword "king", than one of the most often letters in English, e.g. letter "e" could be coded by four different signs. Similarly, one of the most often word in English, e.g. "the" could be coded by four different sets of three signs. Searching for these repetitions Charles Babbage has developed the method for decoding of Vegnere cipher.

Tabula recta

1. Capital letters

2. Lowercase and uppercase

3. Letters and numerals