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

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.

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

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).

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.

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.

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.

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.

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.