Vigenere

This program was write during my studies in ENSTA a French engineering school. It consist in a cryptanalysis of a Vigenere cypher. It was first write in septembrer 2012.

Use

In order to use the programme for decrypt FILE :

make   
./vigenere [FILE]    

In the case that there is no argument, we decrypt "my_chiffre.txt".
The file must have a particular format :
1. The file must have no blank beetween letter
2. each letter must be Upper letter

Present files

• Chiffre.txt : It is an extract of "Deux cavalier de l'orage" from Jean Giono encrypted with the key 'JEANG'.

• Chiffre_Pr.txt : It a text encrypt with the key 'PROUST'. It is an extract of "Du coté de chez Swann, A la Recherche du temps perdu" from Marcel Proust.

• Chiffre_Sw.txt : It is the same text but encrypt with the key 'MARCO'.

• my_chiffre.txt : It is an extract of "Toast Funebre" from Stephane Mallarme encrypted with the key 'MALLARME'

• Vigenere_Singh.txt : It is "L'albatros, Spleen et idéal, les Fleurs du mal " a poem from Charles Baudelaire encrypted with the key 'SCUBA'

Inside the code (see commentary for more informations) :

• First of all we read the text and load it in memory.
• Then we compute the coincidence for i substrings incrementing i until the compute coincidence correspond to the French one. That give us the key length.
• Then we seek the offset between each key's letter by computing the mutual coincidence.
• Then we seek the first key letter by "moving" the most frequent letter in the first substring to the MOST_USE_LETTER. That give us the all key.
• Finally we just decrypt the initial encrypted text with the key and print it in the console.

Note

I don't even know why I don't use git all the line in this project. Maybe because I start it in class and I was not really focus on git but on the technical inside.