Computer Network Security (F20CN)

Coursework 1

This repository contains the code and report for Coursework 1. The coursework consists of 2 tasks related to cryptography and network security.

Task 1: Verifying Key Length Guesses: Vigenere Cipher

This task is written in Python and implements a function that guesses the key length of a ciphertext encrypted with the Vigenere cipher.

Tasks:

• Write a Python program
  • Input a ciphertext (encrypted with a Vigenere cipher) and guess of one or more possible key lengths
  • Output
    • Value of the Index of Coincidence (IC) for each key length guess
    • Indication of which key length guess is most likely to be correct
      • This will be the key length guess where the IC value is closest to the known value of the IC for English language text
• Testing
  • Find a plaintext example of English text (at least 1000 words in length)
  • Encrypt it with the Vigenere cipher at https://rumkin.com/tools/cipher/vigenere with the Cipher key equal to the first 5 characters of your surname
  • The results for the key length guesses should be 4, 5, and 6.

IC formula

$$ IC = \frac{\sum_{i=1}^{26}f_i * (f_i-1)}{L * (L-1)} $$

where $f_i$ is the frequency of the $i^{\text{th}}$ letter of the alphabet in the column, and $L$ is the total number of letters in the column.
Note that $IC_{\text{english}} \approx 0.0686$.

Task 2: Verifying Key Length Guesses: Substitution Cipher

Tasks:

• Write a Shell Script
  • Find the
    • Password
    • Remainder of the plaintext
  • Call the openssl command and check the resulting file
  • Create a loop to read each line and use it as a password? In bash