Chapter Notes: Sending Secret Messages

Ciphers and codes

• In this unit, students will learn about ciphers and how to use simple codes to write and decode secret messages.
• Students will learn:
  • About ciphers: Understanding the concept and purpose of ciphers in securing information.
  • How to write messages using a simple code: Creating and decoding messages using various coding techniques.
• Networks and information security:
  • Networks are used daily for tasks such as sending emails, messaging, accessing files, and online shopping via the World Wide Web.
  • Information sent over a network can be intercepted by unauthorized individuals, posing risks to sensitive data.
  • Examples of sensitive information include:
    • Home address
    • Birthday
    • Bank account details
  • Unauthorized access to such information can lead to theft or harm, such as financial loss.
  • Ciphers are used to protect information by making it unreadable to those without the decryption key.
• Ciphers:
  • A cipher is a set of instructions or rules used to transform plain text into ciphertext.
  • Plain text: The original message, composed of letters or numbers, that is easily readable and understandable by anyone.
  • Ciphertext: The encoded message, which may consist of letters, numbers, or symbols, and is unreadable without the cipher.
  • The cipher rules are shared only between the sender and the intended recipient, allowing them to decode the ciphertext back into plain text.
• Example: Flip text cipher
  • A simple cipher where the letters in a sentence are written backwards.
  • Example:
    • Plain text: "hide the gems"
    • Ciphertext: "smeg eht edih"
  • The recipient uses the reverse rule (reading backwards) to convert ciphertext back to plain text.
• Keywords:
  • Cipher: A set of rules to convert plain text into ciphertext.
  • Plain text: A set of letters, words, or numbers that anyone can understand.
  • Ciphertext: Text that only those with the cipher can understand.

Codes

Creating and decoding codes

• Codes are a method of creating ciphertext by replacing letters or words in a message with different letters, words, or symbols.
• Codes have been used historically to keep messages secret, ensuring only intended recipients can understand them.
• Types of codes:
  • Fruit code:
    • Each letter of the alphabet is represented by a different type of fruit.
    • Example: The word "hello" is coded as a sequence of fruits corresponding to the letters h, e, l, l, o.
    • A secret message written in fruit code requires the recipient to know the fruit-to-letter mapping to decode it.
  • Animal code:
    • Each letter of the alphabet is represented by a different animal.
    • Example: The word "pet" is coded as a sequence of animals for p, e, t.
    • Example for "pet cat": A sequence of animals representing p, e, t, c, a, t.
  • Letters to numbers:
    • Each letter of the alphabet is assigned a number (e.g., A=01, B=02, ..., Z=26).
    • Uses two-digit numbers to represent each letter for consistency.
    • Example: The word "hello" is coded as:
      • h = 08
      • e = 05
      • l = 12
      • l = 12
      • o = 15
      • Resulting code: 08 05 12 12 15
  • Letters to letters:
    • Each letter of the alphabet is mapped to another letter, often using a reverse alphabet (a=Z, b=Y, ..., z=A).
    • Example:
      • Plain text: "hello"
      • Ciphertext: "svool" (h=s, e=v, l=o, l=o, o=l)
• Did you know?
  • The ancient Greeks used codes to send secret messages, demonstrating the historical significance of ciphers and codes.

What can you do?

• Explain what ciphers are, including their purpose and how they transform plain text into ciphertext.
• Write and decode messages using simple codes, such as fruit, animal, letter-to-number, and letter-to-letter codes.