Playfair Cipher Encryption & Decryption in Java

Introduction

This Java program implements the Playfair Cipher, a classical encryption technique, to encrypt and decrypt messages using a key. The encrypted text is saved in a file (encrypted.txt), and it can be decrypted back to its original form using the same key.

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How the Playfair Cipher Works

The Playfair Cipher encrypts text using a 5x5 matrix of letters derived from a keyword. Here’s how it works:

1. Creating the Key Square:

   • The given keyword (e.g., SECRET) is placed into a 5x5 grid.
   • The rest of the alphabet (excluding J, which is replaced with I) fills the remaining spaces.

   Example key square for SECRET: S E C R T A B D F G H I K L M N O P Q U V W X Y Z

pgsql Copy Edit

Preparing the Text:

• Converts text to uppercase and removes spaces and special characters.
• If a letter pair repeats (e.g., HELLO → HE LX LO), an X is inserted.
• If the text length is odd, an X is appended.

Encryption Rules:

• Same row: Each letter moves right (wrap around if needed).
• Same column: Each letter moves down (wrap around if needed).
• Rectangle rule: Swap letters diagonally within the matrix.

Decryption Rules:

• Similar to encryption but move left (for rows) and up (for columns).

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Files & Code Structure

1️⃣ Main.java

Handles user input and controls encryption & decryption.

2️⃣ Encrypt.java

• Generates the 5x5 key square from the given key.
• Encrypts the input text using Playfair rules.

3️⃣ Decrypt.java

• Uses the same key square.
• Decrypts the message from encrypted.txt.

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How to Run the Program

1️⃣ Set Up the Project

mkdir PlayfairCipher cd PlayfairCipher touch Main.java Encrypt.java Decrypt.java README.md

bash Copy Edit Copy and paste the Java files into the directory.

2️⃣ Compile the Java Files

javac Main.java Encrypt.java Decrypt.java

shell Copy Edit

3️⃣ Run the Program

java Main

pgsql Copy Edit

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Usage Instructions

1. Encryption:

   • Choose 1 (Encrypt a message).
   • Enter a key (e.g., SECRET).
   • Enter the message (e.g., HELLO WORLD).
   • The encrypted text is saved to encrypted.txt.

2. Decryption:

   • Choose 2 (Decrypt a file).
   • Enter the same key used for encryption.
   • The program will output the decrypted message.

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Example Output

Encryption Process

Playfair Cipher Encryption & Decryption

Encrypt a message

Decrypt a file Choose an option: 1 Enter the key: SECRET Enter the message to encrypt: HELLO WORLD Encrypted text saved to encrypted.txt

pgsql Copy Edit (The encrypted text is stored in encrypted.txt.)

Decryption Process

Playfair Cipher Encryption & Decryption

Encrypt a message

Decrypt a file Choose an option: 2 Enter the key: SECRET Decrypted message: HELXLOXWORLDX

yaml Copy Edit (Note: X is used for repeated letters and padding.)

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Playfair Cipher Rules Recap

Condition	Encryption	Decryption
Same Row	Move right	Move left
Same Column	Move down	Move up
Rectangle	Swap diagonally	Swap diagonally

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Notes & Improvements

✅ Works with any keyword for encryption.
✅ Automatically removes spaces & special characters.
✅ Handles repeated letters by inserting X.
✅ Supports file-based encryption & decryption.

🔹 Future Improvements:

• Allow users to choose a custom file name.
• Improve handling of special characters.

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Author

Wallace Scott
🛠️ Ethical Hacker, Software Engineer, and Security Enthusiast.

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This README provides everything needed to understand, set up, and run the Playfair Cipher encryption & decryption in Java! 🚀 Let me know if you want any modifications.

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