Playfair Cipher - Part 1

Session Outcomes

• Understand the basic working of Playfair Cipher
• Learn about diagrams and their generation with and without filler characters

Classical Encryption Techniques

1. Substitution Technique
2. Transposition Technique

Focus: Substitution Technique (Playfair Cipher)

Introduction to Playfair Cipher

• Also known as Playfair Square or Wheatstone Playfair Cipher
• Type: Manual symmetric encryption technique
  • Symmetric encryption: Same key used for both encryption and decryption
  • Example: Sender encrypts with a key, and the receiver decrypts using the same key
• First literal diagram substitution technique
• Invented: 1854 by Charles Wheatstone
• Named after Lord Playfair for promoting its use

Characteristics of Playfair Cipher

• Multi-letter encryption technique
  • Unlike single-letter techniques (e.g., Caesar cipher)
  • Example: Letter 'D' can be encrypted to different characters at different occurrences
• Improves security compared to classical ciphers

Diagrams Approach

• Plain text letters are split into diagrams (pairs of letters)
• Key: A keyword used to construct a 5x5 matrix
  • Example: Using "monarchy" as the key

Constructing the 5x5 Matrix

1. Fill matrix with keyword letters (no repeating letters)
   • Example:
     • M O N A R
     • C H Y
2. Fill remaining letters of the alphabet, combining I and J
   • Resulting matrix example:
     • M O N A R
     • C H Y B D
     • E F G I/K L
     • M N O P Q
     • R S T U V
     • W X Y Z

Rules for Encryption

1. Creating Diagrams

   • Split plaintext into pairs of letters (diagrams)
   • Example: "hello" becomes HE LL O

2. Inserting Filler Character

   • Insert a filler character (e.g., 'X') if repeating letters are found or if the last letter is left alone
   • Example:
     • "attack" → AT TA CK
     • "balloon" → BA LX LO ON

3. Encryption Logic

   • Same Column: Move down (wrap around if at bottom)
   • Same Row: Move right (wrap around if at end)
   • Rectangle: Swap letters

Summary of Diagram Creation

• Create diagrams from plaintext
• Handle repeating letters with filler characters
• Ensure all letters are included in diagrams

Conclusion

• Basic working of Playfair cipher understood
• Diagrams generation rules covered
• Next presentation: Encryption using Playfair cipher

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Notes

• Remember the importance of the filler character and its agreed selection by sender and receiver.
• Explore more examples in the next session.