Understanding the Playfair Cipher Basics

Aug 31, 2024

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

Notes

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