lfarci / vigenere_cipher

Vigenere cipher tool.

Frequential analysis tool

Assignment submitted by Logan Farci (47923) and Anthony Farci (49737) for the Security course of Mr Absil (ABS) at HE2B ESI (2018-2019).

Setup

Once the project is cloned locally, run the following commands...

$ cd <path to the project directory>
$ make

Note: the code use the POSIX operating system API. This code only run Unix systems.

How to use

General ommand

./sec <cipher | uncipher | attack> <-vigenere | -caesar> <source> <destination> [your key]

Arguments

1. Action:
   • cipher in order to cipher source and store the result in destination.
   • uncipher in order to uncipher source and store the result in destination.
   • attack in order to make frequential analysis on source and store the result in destination.
2. Cipher type:
   • -vigenere in order to use Vigenère ciphering.
   • -caesar in order to use Caesar ciphering.
3. Source: the path to the file to read from.
4. Destination: the path to the file that will store the result of the treatment.
5. Key: is the key. It is a character string, each character corresponding to its position in the alphabet. A key of size 1 should be used with Caesar cipher. Also, when attacking the key optional (when given it is not treated).

Examples

Ciphering example

In order to cipher initial_message, use the following command:

./sec cipher -vigenere initial_message ciphered_message example

The ciphered content is stored in the ciphered_message file.

Note: ciphering in Vigenère or in Caesar will be treated the same way.

Unciphering example

In order to uncipher ciphered_message, use the following command:

./sec uncipher -vigenere ciphered_message unciphered_message example

The uciphered content is stored in the unciphered_message file.

Note: unciphering Vigenère or Caesar will be treated the same way.

Vigenère frequential analysis example

In order to break ciphered_message, use the following command:

./sec attack -vigenere ciphered_message broken_message

The uciphered content is stored in the broken_message file.

Caesar frequential analysis example

In order to break ciphered_message, use the following command:

./sec attack -caesar ciphered_message broken_message

The uciphered content is stored in the broken_message file.

Known bugs

This frequential analysis implementation is based on the fact that the letter 'e' is the most frequent in English. Trying to analyse and uncipher (without the key) a text that does not respect this assumption will result in gibberish.

Also, during the frequential analysis, if the key length divides the text in subtexts with a different most frequent character than 'e', the analysis won't be abble to find the right key.