Computer Security Algorithms in Java

Java implementations of classical and modern cryptographic algorithms in a single repository, with accompanying test cases.

Contents

• src/main/java/:

  • RSA.java – Public‑key cryptosystem (key generation, encryption, decryption)
  • DES.java – Symmetric block cipher implementation (key schedule, S‑boxes, Feistel rounds)
  • ElGamal.java – Public‑key cryptosystem based on discrete logarithms (key generation, encryption, decryption)
  • DiffieHellman.java – Key‑exchange protocol (prime modulus generation, shared secret computation)
  • ColumnarCipher.java – Classical transposition cipher (keyed columnar transposition)
  • MonoalphabeticCipher.java – Simple substitution cipher (letter‑to‑letter mapping)
  • PlayfairCipher.java – Digraph substitution cipher (5×5 key square)

• src/test/java/:

  • RSATest.java
  • DESTest.java
  • ElGamalTest.java
  • DiffieHellmanTest.java
  • ColumnarCipherTest.java
  • MonoalphabeticCipherTest.java
  • PlayfairCipherTest.java

• README.md – This documentation

• .gitignore – Standard Java ignore rules

Description

This repository demonstrates how to implement various cryptographic algorithms from scratch in Java. Each implementation in src/main/java/ contains a standalone class with its own main method, input parsing, and algorithm logic. Unit tests in src/test/java/ verify correctness of encryption, decryption, and key exchange operations for each algorithm.

Prerequisites

• Java Development Kit (JDK) 8 or higher
• JUnit 5 or a compatible test runner
• (Optional) An IDE such as IntelliJ IDEA or Eclipse
• Basic familiarity with command‑line tools and unit testing

Setup

1. Clone this repository:

   git clone https://github.com/YourUsername/Computer-Security-Algorithms.git
   cd Computer-Security-Algorithms

2. Compile main classes:

   javac src/main/java/*.java -d out/production

   This produces .class files under out/production.

3. Compile test classes:

   javac -cp .;path/to/junit5.jar;out/production src/test/java/*.java -d out/test

   Replace path/to/junit5.jar with your local JUnit 5 JAR path. Output .class files go under out/test.

Running Algorithms

Use the compiled classes in out/production. For example:

java -cp out/production RSA <prime_p> <prime_q> <plaintext>

Replace <ClassName> and arguments accordingly:

RSA
DES
ElGamal
DiffieHellman
ColumnarCipher
MonoalphabeticCipher
PlayfairCipher

Each class’s main method parses input parameters, performs encryption/decryption (or key exchange), and prints results to standard output.

Example Usage

• RSA (generate keys, encrypt, decrypt):

  java -cp out/production RSA 61 53 1234

  • 61 and 53 are prime numbers.
  • 1234 is the integer plaintext.
  • Output: public key (n, e), ciphertext, and decrypted plaintext.

• DES (encrypt plaintext string):

  java -cp out/production DES "HELLOWORLD" "0123456789ABCDEF"

  • "HELLOWORLD" is plaintext.
  • "0123456789ABCDEF" is 64-bit key in hex.
  • Output: hexadecimal ciphertext.

• ElGamal (encrypt/decrypt):

  java -cp out/production ElGamal 467 2 1234

  • 467 is prime modulus.
  • 2 is generator.
  • 1234 is integer plaintext.
  • Output: public key (p, g, h), ciphertext pair (c1, c2), decrypted message.

• DiffieHellman (key exchange):

  java -cp out/production DiffieHellman 467 5 45 67

  • 467 is prime modulus.
  • 5 is generator.
  • 45 and 67 are private keys for parties A and B.
  • Output: shared secret for both parties.

• ColumnarCipher (classical transposition):

  java -cp out/production ColumnarCipher "ATTACKATDAWN" "4312567"

  • "ATTACKATDAWN" is uppercase plaintext.
  • "4312567" is numeric key.
  • Output: ciphertext string.

• MonoalphabeticCipher (simple substitution):

  java -cp out/production MonoalphabeticCipher "HELLOWORLD" "QWERTYUIOPASDFGHJKLZXCVBNM"

  • Substitution alphabet is a 26-letter permutation of A–Z.
  • Output: encrypted text.

• PlayfairCipher (digraph substitution):

  java -cp out/production PlayfairCipher "HELLOWORLD" "KEYPHRASE"

  • "KEYPHRASE" builds the 5×5 key square.
  • Output: ciphertext digraphs.

Refer to each .java file’s header comments for precise argument formats and behavior.

Running Tests

Ensure JUnit 5 is on your classpath. From project root:

java -cp out/production;out/test;path/to/junit5.jar org.junit.platform.console.ConsoleLauncher --scan-classpath

This command discovers and runs all tests in src/test/java/.

Alternatively, import the project into IntelliJ IDEA or Eclipse, add JUnit 5 as a library, and run each *Test.java class directly from the IDE.

File Structure

Computer-Security-Algorithms/
├── src/
│   ├── main/
│   │   └── java/
│   │       ├── ColumnarCipher.java
│   │       ├── DES.java
│   │       ├── DiffieHellman.java
│   │       ├── ElGamal.java
│   │       ├── MonoalphabeticCipher.java
│   │       ├── PlayfairCipher.java
│   │       └── RSA.java
│   └── test/
│       └── java/
│           ├── ColumnarCipherTest.java
│           ├── DESTest.java
│           ├── DiffieHellmanTest.java
│           ├── ElGamalTest.java
│           ├── MonoalphabeticCipherTest.java
│           ├── PlayfairCipherTest.java
│           └── RSATest.java
├── README.md
└── .gitignore