PlayFair Cipher

Python implementation of the PlayFair cipher, a classical digraph substitution cipher developed in 1854.

Overview

The PlayFair cipher encrypts pairs of letters using a matrix generated from a keyword. It was one of the first practical digraph substitution ciphers and was notably used in military communications.

This repository contains two implementations:

• 5×5 Traditional: Classic PlayFair with 25 letters (I and J share position)
• 6×6 Extended: Modern variant supporting alphanumeric text (A-Z, 0-9)

Installation

git clone https://github.com/YonAndualem/PlayFair-Cipher.git
cd PlayFair-Cipher

No external dependencies required for core functionality. GUI requires tkinter (usually included with Python).

Usage

5×5 Traditional Version

Graphical Interface

python3 main.py gui

Command Line Interface

python3 main.py cli

Run Tests

python3 main.py test

Programmatic Usage

from src.cipher import PlayFairCipher

cipher = PlayFairCipher("KEYWORD")
ciphertext = cipher.encrypt("HELLO WORLD")
plaintext = cipher.decrypt(ciphertext)

6×6 Extended Version

Graphical Interface

python3 main6x6.py gui

Command Line Interface

python3 main6x6.py cli

Run Tests

python3 main6x6.py test

Programmatic Usage

from src.cipher6x6 import PlayFairCipher6x6

cipher = PlayFairCipher6x6("CRYPTO2026")
ciphertext = cipher.encrypt("PASSWORD123")
plaintext = cipher.decrypt(ciphertext)

Algorithm

5×5 Matrix Generation

1. Remove duplicate letters from keyword
2. Fill remaining spaces with alphabet (A-Z, excluding J)
3. Arrange in 5×5 grid

Example with keyword "MONARCHY":

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

6×6 Matrix Generation

1. Remove duplicate characters from keyword
2. Fill remaining spaces with unused letters (A-Z)
3. Add remaining digits (0-9)
4. Arrange in 6×6 grid

Example with keyword "CRYPTO2026":

C R Y P T O
2 6 A B D E
F G H I J K
L M N Q S U
V W X Z 0 1
3 4 5 7 8 9

Encryption Rules

For each pair of letters:

1. Same Row: Replace with letters to the right (wrap around)
2. Same Column: Replace with letters below (wrap around)
3. Rectangle: Swap the columns of each letter

Decryption

Apply inverse operations:

• Same row: shift left
• Same column: shift up
• Rectangle: swap columns (same operation)

Project Structure

PlayFair-Cipher/
├── src/
│   ├── cipher.py       # 5×5 core algorithm
│   ├── cipher6x6.py    # 6×6 extended algorithm
│   ├── gui/
│   │   ├── app.py      # 5×5 graphical interface
│   │   └── app6x6.py   # 6×6 graphical interface
│   └── cli/
│       ├── demo.py     # 5×5 command-line interface
│       └── demo6x6.py  # 6×6 command-line interface
├── tests/
│   ├── test_cipher.py     # 5×5 test suite
│   └── test_cipher6x6.py  # 6×6 test suite
├── main.py             # 5×5 application entry point
├── main6x6.py          # 6×6 application entry point
└── README.md

Features

5×5 Traditional Version

• Core Implementation: Clean, well-documented cipher algorithm
• GUI Application: Interactive visual interface with real-time encryption
• CLI Tool: Command-line demonstration and interactive mode
• Test Suite: Comprehensive unit tests
• Historical Accuracy: Implements the original PlayFair cipher

6×6 Extended Version

• Alphanumeric Support: Encrypt messages containing both letters and numbers
• 36-Character Matrix: All 26 letters (A-Z) + 10 digits (0-9)
• No Information Loss: No character merging (unlike J→I in 5×5)
• Modern Use Cases: Perfect for passwords, codes, and alphanumeric messages
• Same Interfaces: GUI, CLI, and programmatic API

Comparison: 5×5 vs 6×6

Feature	5×5 Traditional	6×6 Extended
Characters	25 (A-Z, no J)	36 (A-Z, 0-9)
Digit Support	❌ No	✅ Yes
Character Merging	J → I	None
Historical	✅ Original	Modern variant
Use Cases	Text only	Alphanumeric

Technical Details

5×5 Text Preprocessing

• Converts to uppercase
• Replaces J with I
• Removes non-alphabetic characters
• Splits into digraphs (letter pairs)
• Inserts 'X' between duplicate letters
• Pads with 'X' if odd length

6×6 Text Preprocessing

• Converts to uppercase
• Removes non-alphanumeric characters
• Splits into digraphs (character pairs)
• Inserts 'X' between duplicate characters
• Pads with 'X' if odd length

Matrix Properties

5×5:

• 5×5 grid containing 25 letters (I and J share a position)
• Generated deterministically from keyword
• Unique letter positioning ensures reversible encryption

6×6:

• 6×6 grid containing 36 characters (26 letters + 10 digits)
• Generated deterministically from keyword
• All alphanumeric characters supported
• No character sharing or merging

Examples

5×5 Traditional

from src.cipher import PlayFairCipher

# Create cipher
cipher = PlayFairCipher("SECRET")

# Encrypt
encrypted = cipher.encrypt("MEET AT NOON")
print(encrypted)

# Decrypt
decrypted = cipher.decrypt(encrypted)
print(decrypted)

6×6 Extended

from src.cipher6x6 import PlayFairCipher6x6

# Create cipher
cipher = PlayFairCipher6x6("CRYPTO2026")

# Encrypt with digits
encrypted = cipher.encrypt("PASSWORD123")
print(encrypted)

# Decrypt
decrypted = cipher.decrypt(encrypted)
print(decrypted)

# Verbose mode (shows steps)
cipher.encrypt("AGENT007", verbose=True)

Testing

Run the test suites to verify implementations:

5×5 Version:

python3 main.py test

6×6 Version:

python3 main6x6.py test

Tests cover:

• Matrix generation
• Position mapping
• Text preparation
• Encryption/decryption reversibility
• Individual cipher rules
• Edge cases
• Alphanumeric support (6×6 only)

Security Note

This is a classical cipher for educational and historical purposes. It is not secure for modern cryptographic use and is vulnerable to:

• Frequency analysis
• Known-plaintext attacks
• Chosen-plaintext attacks

License

MIT License

References

• Wheatstone, Charles (1854). Original cipher design
• Playfair, Lyon (1854). Promotion and popularization
• Historical military usage in Boer War and WWI

Contributing

Contributions welcome. Please ensure:

• Code follows existing style
• Tests pass
• New features include tests
• Documentation is updated