🔐 Caesar Cipher — Class Notes

High school-friendly notes on substitution ciphers, encryption, decryption, and simple algorithms.

Cryptography Basics Encryption / Decryption Modular Arithmetic Beginner Algorithms

🎯 Learning Objectives

  • Understand what a Caesar Cipher is
  • Explain encryption vs decryption
  • Apply a shift cipher manually
  • Write a basic Caesar Cipher algorithm
  • Recognize strengths and weaknesses of simple encryption

🧠 What is a Caesar Cipher?

A Caesar Cipher is one of the oldest and simplest encryption techniques. It is a type of substitution cipher.

Key Idea: Each letter is shifted a certain number of places down the alphabet. 
A → D
B → E
C → F

📜 History

  • Named after Julius Caesar
  • Used to send secret military messages
  • Simple but important in the history of cryptography

🔑 Key Vocabulary

Encryption

Turning readable text into secret code.

HELLO → KHOOR

Decryption

Turning coded text back into readable text.

KHOOR → HELLO

Plaintext

Original readable message.

Ciphertext

Encrypted (hidden) message.

Shift (Key)

Number of letters moved forward or backward.

🔄 How the Caesar Cipher Works

Step 1 — Choose a Shift Value

Shift = 3

Step 2 — Shift the Alphabet

Alphabet:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

Shifted by 3:

DEFGHIJKLMNOPQRSTUVWXYZABC

Step 3 — Replace Letters

Example: encrypt HELLO with shift 3

H → K
E → H
L → O
L → O
O → R

Encrypted = KHOOR

🔁 Wrap Around Rule

When shifting past Z, wrap back to A.

Z + 3 = C
Y + 3 = B

✏️ Manual Practice Example

Encrypt DOG with shift 1:

D → E
O → P
G → H

Encrypted = EPH

💻 Algorithm (Simple Steps)

Encryption Algorithm

  1. Read message
  2. Choose shift value
  3. For each letter:
    • Convert to alphabet position
    • Add shift
    • Wrap around if needed
  4. Convert back to letter
  5. Display encrypted text

👨‍💻 Java Logic Concept (High-Level)

This formula shifts uppercase letters and wraps using modulo:

char shifted = (char)((original - 'A' + shift) % 26 + 'A');
Why it works: Convert letter → number, add shift, use % 26 to wrap, convert back.

🔓 Decryption

Decryption shifts letters backwards.

decryptShift = 26 - shift

Or simply subtract shift instead of adding.

🔍 Strengths

  • Easy to understand
  • Good introduction to cryptography
  • Helps learn algorithms and modular arithmetic

⚠️ Weaknesses

  • Very easy to break
  • Only 25 possible shifts
  • Vulnerable to brute-force attacks
Important: Caesar Cipher is educational, not secure.

🧪 Real Cybersecurity Connection

Modern encryption uses:

  • Complex math
  • Large keys
  • Algorithms like AES or RSA
Caesar Cipher is considered educational, not secure.

📊 Example Table

Plain Shift Cipher
A+3D
B+3E
C+3F

🧩 Challenge Question

If Encrypted = ZRUOG and Shift = 3, what is the original message?