CSEC Classic Cryptography Workshop

A hands-on introduction to breaking the Vigenère cipher through cryptanalysis in Python.

Prerequisites

• Python 3.8+ - If not installed, see: https://www.python.org/downloads/
• Jupyter Notebook - Will be installed in setup, official installation instructions can be found here: https://jupyter.org/install
• Basic Python knowledge (reading the reference sheet from this document)
• macOS, Windows, or Linux

Workshop Structure

This workshop teaches cryptanalysis by implementing attacks on the Vigenère cipher. All work is done in a Jupyter notebook where you implement the cryptanalysis techniques.

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Quick Start

Important:

Unless you want to have Jupyter Notebook installed on your computer locally or learn how to use Google Colab, you can skip this step (Jupyter Notebook installation steps) entirely and just use the link below:

Online Jupyter Notebook: https://jupyter.org/try-jupyter/lab/

Install Jupyter Notebook

# Install Jupyter
pip3 install jupyter

# Verify installation
jupyter --version

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Launch Workshop

# Clone the workshop repo
git clone https://github.com/CSEC-President/csec-classic-crypto-workshop

# Navigate to workshop directory
cd /path/to/workshop

# Launch Jupyter
jupyter notebook

This opens your browser at http://localhost:8888

Open vigenere_workshop.ipynb in the file browser and follow the instructions.

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After Setup

In Jupyter:

• Shift+Enter runs current cell
• Cells run in order from top to bottom
• Kernel → Restart to reset everything

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Basic Python Reference

Essential Python for the workshop:

Strings and Characters

# String operations
text.upper()                    # Convert to uppercase
text.lower()                    # Convert to lowercase
''.join(list_of_chars)          # Join list into string
text.split()                    # Split into words

# Character operations
ord('A')                        # Get ASCII value (65)
chr(65)                         # Get character from ASCII ('A')
char.isalpha()                  # Check if letter

# Looping through string
for char in text:
    print(char)

for i, char in enumerate(text):
    print(f"Position {i}: {char}")

Lists

# Lists
my_list = [1, 2, 3]
my_list.append(4)              # Add to end
my_list[0]                     # First element
len(my_list)                   # Length

# List comprehension
squares = [x**2 for x in range(10)]
filtered = [x for x in my_list if x > 2]

# Slicing
my_list[start:end:step]        # Slice with step
my_list[::3]                   # Every 3rd element

Dictionaries

# Creating dictionaries
my_dict = {'a': 1, 'b': 2}

# Accessing
my_dict['a']                   # Get value
my_dict.get('a', 0)           # Get with default
my_dict.keys()                 # All keys
my_dict.values()               # All values

# Looping
for key, value in my_dict.items():
    print(f"{key}: {value}")

# Finding max by value
max(my_dict, key=my_dict.get)

Counter (for frequency analysis)

from collections import Counter

# Count frequencies
freq = Counter("hello world")
freq['l']                      # Returns 3
freq.most_common(3)            # Top 3 most common

# Usage example
text = "ABCABC"
letter_freq = Counter(text)
most_common_letter = letter_freq.most_common(1)[0][0]

Math Operations

# Basic math
a % b                          # Modulo (remainder)
a // b                         # Integer division

# GCD 
from math import gcd
gcd(12, 8)                     # Greatest common divisor

# GCD of multiple numbers
from functools import reduce
result = reduce(gcd, [12, 8, 16])

Common Patterns

# Filter non-alphabetic characters
clean = ''.join(c for c in text if c.isalpha())

# Get every nth character (for splitting by key position)
group = text[start::key_length]

# Count occurrences
text.count('ABC')

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Troubleshooting

Jupyter Issues

Problem: jupyter: command not found

python3 -m jupyter notebook

Problem: Port 8888 already in use

jupyter notebook --port 8889

Problem: Kernel keeps dying

• Restart the kernel: Kernel → Restart

Problem: Cell output is missing

• Click "Kernel → Restart & Run All"

Python Issues

Problem: Import errors

# Check Python version (should be 3.8+)
python3 --version

Problem: Functions not found

• Make sure you ran all previous cells in order
• Restart kernel and run from beginning

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Alternative: Using Google Colab

To use without installing Jupyter locally:

1. Go to https://colab.research.google.com/
2. File → Upload notebook
3. Upload vigenere_workshop.ipynb
4. Run cells normally

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Disclaimer

Ethical Guidelines: Never attempt to decrypt communications without authorization. Report vulnerabilities responsibly. Use knowledge to understand security, not to violate it.

Your Responsibility: You are solely responsible for your actions. The instructor and workshop organizers accept no responsibility for:

• Hardware issues, data loss, or system instability
• Unauthorized decryption of communications or breaking into systems
• Legal consequences of misuse or any malicious use of techniques learned

By participating, you acknowledge understanding these terms and agree to use this knowledge ethically and legally.

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Additional Resources

• CryptoHack - Interactive cryptography CTF-style challenges
• Cryptopals Challenges - Practical cryptography exercises
• Khan Academy: Cryptography - Modern Cryptography section
• CyberChef - Useful toolkit for cryptoanalysis
• "Introduction to Modern Cryptography" by Jonathan Katz
• Elementary Group Theory for Programmers - Mathematical foundations for modern cryptography

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Authors: Clarence Lam, Sasha Zyuzin

Good luck! 🔐