Playfair Cipher: Encryption, Decryption, Algorithm, Advantages, and Limitations

Algorithm for Playfair Cipher

1. Key Preparation:
- Create a 5×5 matrix using a keyword.
- Include each letter of the alphabet only once, omitting 'J' (treat 'I' and 'J' as the same).

2. Plaintext Preparation:
- Divide the plaintext into digraphs (pairs of two letters).
- Insert a filler character (e.g., 'X') between identical letters in a pair (e.g., "BALLOON" → "BA LX LO ON").
- If the plaintext has an odd number of letters, add a filler at the end.

3. Encryption Rules:
- For letters in the same row: Replace each letter with the letter to its immediate right (wrap around to the start if necessary).
- For letters in the same column: Replace each letter with the letter immediately below it (wrap around to the top if necessary).
- For letters in different rows and columns: Form a rectangle, and replace each letter with the one in its row but in the other letter's column.

4. Decryption:
- Reverse the encryption rules to recover the plaintext.


Advantages of Playfair Cipher
1. Pairs of Letters: Encrypts digraphs (pairs of letters) instead of single letters, making it more secure than monoalphabetic ciphers.
2. Simplicity: Easy to implement manually with paper and pencil.
3. No Need for Numbers or Special Characters: Only the alphabet is used, avoiding complications with additional symbols.
4. Improved Security: Reduces frequency analysis attacks due to the use of digraphs.

Disadvantages of Playfair Cipher
1. Limited Security: Vulnerable to frequency analysis of digraphs if enough ciphertext is available.
2. Key Dependency: The key matrix must be shared securely, which can be a challenge.
3. Not Suitable for Computers: Lacks the robustness needed for digital encryption due to simplicity.
4. Plaintext Size Increases: Adding filler characters ('X' or 'Z') for identical letters or odd-length plaintext increases message size.