MAT200 Cryptography Library

A comprehensive Java library implementing various classical and modern cryptographic algorithms, developed for the MAT200 course at Northeastern University.

Overview

This library provides implementations of multiple cipher algorithms ranging from simple classical ciphers to modern RSA encryption. Each cipher includes both encryption and decryption capabilities with an interactive console interface for testing and demonstration.

Supported Ciphers

Unit 1 - Basic Ciphers

• Bacon Cipher - Binary encoding using A/B patterns
• Space Break Cipher - Simple space removal encoding
• Every Nth Letter Cipher - Extract patterns by position
• Reverse Cipher - Character order reversal

Unit 2 - Transposition Ciphers

• Transposition Cipher - Column-based rearrangement with numeric or word keys

Unit 3 - Substitution Ciphers

• Caesar Cipher - Alphabet shifting with numeric or keyword-based shifts
• Multiplicative Cipher - Modular multiplication-based substitution
• Affine Cipher - Combination of multiplicative and additive ciphers

Unit 4 - Polyalphabetic Ciphers

• Vigenère Cipher - Keyword-based polyalphabetic substitution

Final Project - Modern Cryptography

• RSA Cipher - Public-key cryptography with key generation and file I/O

Getting Started

Prerequisites

• Java 17 or higher
• Maven 3.6 or higher

Installation

1. Clone the repository:

git clone https://github.com/snxethan/MAT200-LIBRARY.git
cd MAT200-LIBRARY

2. Build the project:

mvn compile

3. Run the interactive console:

mvn exec:java -Dexec.mainClass="NEU.ET39.MAT200.Main"

Usage

Interactive Console

The main application provides an interactive menu system to test all cipher implementations:

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    runConsole(scanner);
    scanner.close();
}

Using Individual Ciphers

Caesar Cipher Example

// Encrypt with shift
String encrypted = CaesarCipher.encryptByShift("HELLO WORLD", 3);
// Result: "KHOOR ZRUOG"

// Decrypt with shift
String decrypted = CaesarCipher.decryptByShift("KHOOR ZRUOG", 3);
// Result: "HELLO WORLD"

// Encrypt with keyword
String encrypted = CaesarCipher.encryptByKeyWord("HELLO", "SECRET");
// Uses keyword to generate substitution alphabet

Vigenère Cipher Example

// Encrypt with keyword
String encrypted = VigenereCipher.encrypt("ATTACK AT DAWN", "LEMON");
// Result: "LXFOPV EF RNHR"

// Decrypt with keyword
String decrypted = VigenereCipher.decrypt("LXFOPV EF RNHR", "LEMON");
// Result: "ATTACK AT DAWN"

RSA Cipher Example

RSACipher rsa = new RSACipher();

// Save keys to file
rsa.savePublicKeyOnly("public_key.txt");
rsa.savePrivateKeyOnly("private_key.txt");

// Load and encrypt
BigInteger[] publicKey = RSACipher.loadPublicKeyFromTxt("public_key.txt");
BigInteger encrypted = rsa.encryptBlock("Hello RSA!", publicKey[0], publicKey[1]);

// Load and decrypt
BigInteger[] privateKey = RSACipher.loadPrivateKeyFromTxt("private_key.txt");
rsa.loadPrivateKey(privateKey[0], privateKey[1]);
String decrypted = rsa.decryptBlock(encrypted);

Testing

The project includes JUnit tests. Run them with:

mvn test

Project Structure

src/
├── main/java/NEU/ET39/MAT200/
│   ├── Main.java                 # Interactive console interface
│   ├── AffineCipher.java         # Affine cipher implementation
│   ├── BaconCipher.java          # Bacon cipher implementation
│   ├── CaesarCipher.java         # Caesar cipher implementation
│   ├── EveryNthCipher.java       # Every nth letter cipher
│   ├── MultiplicativeCipher.java # Multiplicative cipher implementation
│   ├── RSACipher.java            # RSA cipher implementation
│   ├── ReverseCipher.java        # Reverse cipher implementation
│   ├── SpaceBreakCipher.java     # Space break cipher implementation
│   ├── TranspositionCipher.java  # Transposition cipher implementation
│   └── VigenereCipher.java       # Vigenère cipher implementation
└── test/java/
    └── BaconCipherTest.java      # Unit tests

Features

• Interactive Console: Menu-driven interface for all ciphers
• File I/O Support: RSA cipher supports key file generation and loading
• Multiple Input Methods: Support for both programmatic and console input
• Comprehensive Error Handling: Input validation and error messages
• Educational Output: Displays cipher alphabets and transformation steps

Technical Details

RSA Implementation

• Key Length: 1024-bit keys for security
• Key Generation: Uses SecureRandom for cryptographically secure random numbers
• File Format: Keys saved in readable text format
• Block Processing: Handles message size limitations

Classical Cipher Features

• Case Preservation: Maintains original character casing
• Non-alphabetic Character Handling: Preserves spaces and punctuation
• Alphabet Display: Shows plaintext/ciphertext alphabet mappings
• Input Validation: Checks for valid keys and parameters

Course Alignment

This library is structured to align with the MAT200 curriculum:

• Unit 1: Basic encoding and simple transformations
• Unit 2: Transposition-based cryptography
• Unit 3: Substitution cipher theory and implementation
• Unit 4: Advanced polyalphabetic methods
• Final Project: Modern public-key cryptography

Contributing

This is a course project repository. For educational purposes, please:

1. Fork the repository
2. Create a feature branch
3. Make your changes
4. Add appropriate tests
5. Submit a pull request

License

This project is developed for educational purposes as part of the MAT200 course at Northeastern University.

Author(s)

• Ethan Townsend (snxethan)