Vigenère's Cypher and next steps of the project

The Vigenère cypher is also a very simple cypher and is based on Caesar's one. In fact, it is basically a Caesar cypher but on every letter of the message. Therefore, the key is no longer just a shift but a whole word.

For every character in the message is applied a different shift. For example, let's crypt the message "hello world" with the key "secret": ZINCS PGVNU

Even though this is very simple and symmetric cypher, the is no way to decypher a message directly because the key can be any combination of letter and of any length.

This algorithm remained uncracked for three centuries!

There are two methods of cracking it: the first is called the "Babbage and Kasiski" method, and the second, invented by commander Bazeries, consists of basically guessing some key words. Let's start with the latter.

Suppose you know a word of the message which could be at least as long as the key used to crypt the message. We also suppose the key used to crypt the message is a word and not a random letter combination. Now, what we have to do is decrypt the message with the word we think is in there. So, if that word is 8 characters long, we decrypt the first 8 characters of the message with that word. If the result is not satisfying, we repeat the process 1 characters further. And so on until the result sounds good.

An illustration of the process can be found on apprendre-en-ligne.net.


Let's now talk about the project. Due to our major exams this week, we haven't taken the time to work on the project. What is good though is that, as Emeric said in an earlier post, the structure of the program is defined and operational. Now, all there is left is to code the graphical buttons (using Swing) for each algorithm and program each algorithm to decipher a message. That second step is pretty easy since most of the ciphers used are symmetric.

See you later!