Caesar Cipher

Caesar Cipher Problem?

Julius Caesar protected his confidential information by encrypting it using a cipher.

• Caesar's cipher shifts each letter by a number of letters
• If the shift takes you past the end of the alphabet
  • rotate back to the front of the alphabet
• Example: rotation by 3, wxyz would map to zabc

Original alphabet:      abcdefghijklmnopqrstuvwxyz
Alphabet rotated +3:    defghijklmnopqrstuvwxyzabc

Original alphabet:      abcdefghijklmnopqrstuvwxyz
Alphabet rotated +2:    cdefghijklmnopqrstuvwxyzab

a -> c
b -> d
c -> e
d -> f
-    -
A -> C
B -> D

Caesar Cipher Assumptions/Questions

• will the string already be lowercase or mixed casing?
• can the number to shift by be negative?
• the cipher only encrypts letters
  • symbols, such as spaces and special characters remain unencrypted

Javascript Caesar Cipher

/***
 * Algorithm Complexity/Big O Notation
 * O(n) | Linear Time Complexity
 * n is length of string passed in
 ***/

function caesarCipher(str, num) {
  // used to ensure we don't loop more than we need to
  num = num % 26;

  // we will handle casing below for now force lowercase
  const lowerCaseString = str.toLowerCase();
  const alphabet = "abcdefghijklmnopqrstuvwxyz".split("");
  let newString = "";

  // loop over each character in the string
  for (let i = 0; i < lowerCaseString.length; i++) {
    const currentLetter = lowerCaseString[i];

    // if not in alphabet it's a special character or space
    if (alphabet.indexOf(currentLetter) < 0) {
      // just add it to the newString
      newString += currentLetter;

      // and exit for loop for this iteration
      continue;
    }

    const currentIndex = alphabet.indexOf(currentLetter);

    // get new cipher index
    let newIndex = currentIndex + num;

    // if it's at the end of alphabet loop back to beginning
    if (newIndex > 25) {
      newIndex = newIndex - 26;
    }

    // if it's at the beginning of alphabet loop to end
    if (newIndex < 0) {
      newIndex = 26 + newIndex;
    }

    if (str[i] === str[i].toUpperCase()) {
      // if it was originally uppercase
      // add the uppercase cipher letter to string
      newString += alphabet[newIndex].toUpperCase();
    } else {
      // else add the cipher letter to string
      newString += alphabet[newIndex];
    }
  }

  return newString;
}

caesarCipher("Abc Xyz", 3); // $ "Def Abc"
caesarCipher("a-b-c", -3); // $: "x-y-z"
caesarCipher("xyz", 29); // $: "abc"

Additional Caesar Cipher Resources