caesarCipher.js

/**
 * Caesar Cipher - Encryption and Decryption
 * 
 * The Caesar Cipher is a simple substitution cipher where each letter in the plaintext
 * is shifted a fixed number of places down or up the alphabet.
 * 
 * Example: With a shift of 3, 'A' becomes 'D', 'B' becomes 'E', etc.
 */

/**
 * Encrypts a message using Caesar Cipher
 * @param {string} message - The text to encrypt
 * @param {number} shift - The number of positions to shift (1-25)
 * @returns {string} - The encrypted text
 */
function encrypt(message, shift) {
  let encrypted = '';
  
  // Normalize shift to 0-25 range
  shift = shift % 26;
  
  for (let i = 0; i < message.length; i++) {
    const char = message[i];
    
    // Check if character is uppercase letter
    if (char >= 'A' && char <= 'Z') {
      // Get position in alphabet (0-25)
      const position = char.charCodeAt(0) - 'A'.charCodeAt(0);
      // Apply shift and wrap around using modulo
      const newPosition = (position + shift) % 26;
      // Convert back to character
      encrypted += String.fromCharCode(newPosition + 'A'.charCodeAt(0));
    }
    // Check if character is lowercase letter
    else if (char >= 'a' && char <= 'z') {
      // Get position in alphabet (0-25)
      const position = char.charCodeAt(0) - 'a'.charCodeAt(0);
      // Apply shift and wrap around using modulo
      const newPosition = (position + shift) % 26;
      // Convert back to character
      encrypted += String.fromCharCode(newPosition + 'a'.charCodeAt(0));
    }
    // Non-alphabetic characters remain unchanged
    else {
      encrypted += char;
    }
  }
  
  return encrypted;
}

/**
 * Decrypts a Caesar Cipher message
 * @param {string} encrypted - The encrypted text
 * @param {number} shift - The shift value used for encryption
 * @returns {string} - The decrypted text
 */
function decrypt(encrypted, shift) {
  // Decryption is just encryption with negative shift
  return encrypt(encrypted, -shift);
}

// Export for use in other modules
if (typeof module !== 'undefined' && module.exports) {
  module.exports = { encrypt, decrypt };
}