https://github.com/cr2007/f20cn-cw1

Coursework on Vigenere Cipher and Known-Plaintext Attack
https://github.com/cr2007/f20cn-cw1

computer-network-security key-length openssl plaintext-attack python shell-scripting streamlit symmetric-cipher vigenere-cipher

Last synced: 7 months ago
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Coursework on Vigenere Cipher and Known-Plaintext Attack

• Host: GitHub
• URL: https://github.com/cr2007/f20cn-cw1
• Owner: cr2007
• Created: 2023-10-23T17:42:03.000Z (almost 2 years ago)
• Default Branch: main
• Last Pushed: 2025-03-09T12:21:51.000Z (7 months ago)
• Last Synced: 2025-03-09T13:25:32.970Z (7 months ago)
• Topics: computer-network-security, key-length, openssl, plaintext-attack, python, shell-scripting, streamlit, symmetric-cipher, vigenere-cipher
• Language: Typst
• Homepage: https://cr2007.github.io/f20cn-cw1-web/
• Size: 201 KB
• Stars: 0
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

          
# Computer Network Security (F20CN)

[![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/cr2007/F20CN-CW1)

[![Streamlit App](https://static.streamlit.io/badges/streamlit_badge_black_white.svg)](https://f20cn-cw1.streamlit.app)



  

  



## 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:

- [X] Write a Python program

  - [X] Input a ciphertext (encrypted with a Vigenere cipher) and guess of one or more possible key lengths

  - [X] Output

    - [X] Value of the Index of Coincidence (IC) for each key length guess

    - [X] 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

- [X] Testing

  - [X] Find a plaintext example of English text (at least 1000 words in length)

  - [X] 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

  - [X] 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