Security Tasks - Encryption & Decryption Algorithms

A comprehensive Java implementation of classical and modern encryption/decryption algorithms developed for the Computer and Network Security course at the Faculty of Computer and Information Sciences, Ain Shams University.

🔐 Implemented Algorithms

This repository includes implementations of the following cryptographic algorithms:

Classical Ciphers

1. Caesar Cipher - A simple substitution cipher that shifts letters by a fixed number of positions
2. Monoalphabetic Cipher - A substitution cipher with a fixed substitution for each letter
3. Playfair Cipher - A digraph substitution cipher using a 5×5 key square
4. Hill Cipher - A polyalphabetic cipher using matrix multiplication
5. Railfence Cipher - A transposition cipher that arranges plaintext in a rail-like pattern
6. Columnar Cipher - A transposition cipher that rearranges plaintext based on column permutations

Advanced Ciphers

7. AutoKey Cipher - An extension of the Vigenère cipher that uses the plaintext as part of the key
8. Repeating Key Cipher - A polyalphabetic cipher using a repeating key (Vigenère)

Modern Encryption Algorithms

9. AES (Advanced Encryption Standard) - A symmetric block cipher using 128, 192, or 256-bit keys
10. DES (Data Encryption Standard) - A symmetric block cipher using 56-bit keys
11. ElGamal - A public-key cryptosystem based on the discrete logarithm problem
12. Diffie-Hellman Key Exchange - A method for securely establishing shared keys over an insecure channel

📁 Project Structure

security-tasks/
├── pom.xml                          # Maven configuration file
├── README.md                        # This file
└── src/
    ├── main/java/
    │   ├── org/example/
    │   │   └── Main.java           # Main entry point
    │   └── Security/               # All algorithm implementations
    │       ├── AES.java
    │       ├── AutoKey.java
    │       ├── CaeserCipher.java
    │       ├── ColumnarCipher.java
    │       ├── DES.java
    │       ├── DiffieHellman.java
    │       ├── ElGamal.java
    │       ├── HillCipher.java
    │       ├── MonoalphabeticCipher.java
    │       ├── PlayfairCipher.java
    │       ├── Railfence.java
    │       └── RepeatingKey.java
    └── test/java/                   # Unit tests for each algorithm
        ├── AESTest.java
        ├── AutoKeyTest.java
        ├── CaeserCipherTest.java
        ├── ColumnarCipherTest.java
        ├── DESTest.java
        ├── DiffieHellmanTest.java
        ├── ElgamalTest.java
        ├── HillCipherTest.java
        ├── MonoalphabeticCipherTest.java
        ├── PlayfairCipherTest.java
        ├── RailfenceTest.java
        └── RepeatingKeyTest.java

🚀 Getting Started

Prerequisites

• Java Development Kit (JDK) 17 or higher
• Maven 3.6.0 or higher
• Git (optional, for cloning the repository)

Installation

1. Clone the repository:

   git clone https://github.com/Bonus-Hunters/security-tasks.git
   cd security-tasks

2. Build the project:

   mvn clean compile

🏗️ Building and Running

Compile the project:

mvn clean compile

Run tests:

mvn test

Run a specific test class:

mvn test -Dtest=CaeserCipherTest

Generate project documentation:

mvn javadoc:javadoc

💻 Usage Examples

Each algorithm class provides encrypt() and decrypt() methods with specific parameters:

Caesar Cipher

CaeserCipher cipher = new CaeserCipher();
String encrypted = cipher.encrypt("HELLO", 3);
String decrypted = cipher.decrypt(encrypted, 3);

Playfair Cipher

PlayfairCipher cipher = new PlayfairCipher();
String encrypted = cipher.encrypt("HELLO", "SECRET");
String decrypted = cipher.decrypt(encrypted, "SECRET");

AES Encryption

AES aes = new AES();
String encrypted = aes.encrypt("Hello, World!", "MySecretKey123456");
String decrypted = aes.decrypt(encrypted, "MySecretKey123456");

DES Encryption

DES des = new DES();
String encrypted = des.encrypt("Hello!!!!", "MySecret");
String decrypted = des.decrypt(encrypted, "MySecret");

Diffie-Hellman Key Exchange

DiffieHellman dh = new DiffieHellman();
// Alice and Bob establish a shared secret key
long aliceShared = dh.calculateSharedSecret(alicePrivate, bobPublic, p);
long bobShared = dh.calculateSharedSecret(bobPrivate, alicePublic, p);

ElGamal Encryption

ElGamal elGamal = new ElGamal();
long[] encrypted = elGamal.encrypt(message, p, g, publicKey);
long decrypted = elGamal.decrypt(encrypted, p, privateKey);

Some algorithms provide an Analyze method to find the secret key used for encryption.

🧪 Testing

The project includes comprehensive unit tests for all implemented algorithms using JUnit 5. To run all tests:

mvn test

Tests are located in src/test/java/ and cover:

• Encryption and decryption correctness
• Edge cases and special inputs
• Algorithm-specific requirements

📝 Notes

• Classical ciphers (Caesar, Playfair, Hill, etc.) typically work with uppercase alphabetic characters
• Modern encryption algorithms (AES, DES) support various character sets and typically use Base64 encoding for output
• Some algorithms may have specific key requirements (length, format, etc.) - refer to individual class documentation
• All implementations are for educational purposes and should not be used for production security

📄 License

This project is provided for educational purposes as part of the Computer and Network Security course at Ain Shams University.