Caesar Cipher - JavaScript Implementation

Overview

The Caesar Cipher is one of the simplest and most famous encryption techniques. It shifts each letter in the plaintext by a fixed number of positions down the alphabet.

Key Concepts:

• Shift (Key): The number of positions to move each letter (typically 1-25)
• Plaintext: The original, unencrypted message
• Ciphertext: The encrypted message
• Symmetry: The same shift is used for both encryption and decryption (but in opposite directions)

How It Works

Encryption Process:

For each character in the message:

1. If it's a letter, shift it by the key amount
2. If the shift goes past 'Z' (or 'z'), wrap around to the beginning of the alphabet
3. If it's not a letter (numbers, spaces, punctuation), leave it unchanged

Example with Shift = 3:

A → D
B → E
C → F
...
X → A
Y → B
Z → C

Decryption Process:

Simply reverse the process by shifting backwards by the same amount.

Algorithm Explanation

CharCode Approach:

The implementation uses ASCII character codes:

• Uppercase 'A' = 65, 'Z' = 90
• Lowercase 'a' = 97, 'z' = 122

Step-by-step for encryption:

1. Get the character's position in the alphabet: position = char.charCodeAt(0) - 'A'.charCodeAt(0)

   • This converts 'A' → 0, 'B' → 1, ..., 'Z' → 25

2. Add the shift: newPosition = (position + shift) % 26

   • Using modulo (%) ensures wrapping (e.g., 25 + 3 = 28 % 26 = 2)

3. Convert back to character: String.fromCharCode(newPosition + 'A'.charCodeAt(0))

   • This adds the code back to get the actual character

Function Reference

encrypt(message, shift)

• Parameters:
  • message (string): Text to encrypt
  • shift (number): Positions to shift (1-25, but any number works due to modulo)
• Returns: Encrypted text
• Example: encrypt('Hello', 3) → 'Khoor'

decrypt(encrypted, shift)

• Parameters:
  • encrypted (string): Encrypted text
  • shift (number): Original shift value
• Returns: Decrypted text
• Example: decrypt('Khoor', 3) → 'Hello'

bruteForceDecrypt(encrypted)

• Parameters:
  • encrypted (string): Encrypted text with unknown shift
• Returns: Array of 26 objects with all possible decryptions
• Example: Try all shifts automatically to find the correct one

Security Notes

⚠️ Important: The Caesar Cipher is NOT secure for real-world use:

• Only 26 possible shifts, easy to crack with brute force
• Pattern analysis can reveal the shift
• Modern encryption uses complex algorithms (AES, RSA, etc.)

Use this only for:

• Learning cryptography concepts
• CTF (Capture The Flag) challenges
• Simple educational purposes

Running the Examples

node demo.js

This will show various examples of encryption, decryption, and brute force attacks.