Caesar Cipher Encryptor

Tags

Caesar Cipher Encryptor

Category

Strings

Difficulty

Easy

Problem Statement

Given a non-empty string of lowercase letters and a non-negative integer representing a key, shift every letter in the string by key positions in the alphabet, wrapping around from z back to a. Return the new encrypted string.

Intuition

Each letter in the alphabet can be mapped to a number (a=0, b=1, ..., z=25). Shifting a letter by key positions is equivalent to adding key to its numeric value and taking the result modulo 26 to handle wrapping. This transforms the problem into simple arithmetic.

Approach

  1. Reduce the key modulo 26 to handle keys larger than the alphabet size.
  2. Initialize an empty result list.
  3. For each character in the input string:
    • Compute the new character code: (charCode - 'a' + key) % 26 + 'a'.
    • Append the new character to the result.
  4. Join the result list into a string and return it.

Pseudocode

function caesarCipherEncryptor(string, key):
    key = key % 26
    result = []

    for char in string:
        newCharCode = (charCode(char) - charCode('a') + key) % 26
        newChar = characterFromCode(newCharCode + charCode('a'))
        result.append(newChar)

    return join(result, "")

Time & Space Complexity

  • Time: O(n) where n is the length of the string. Each character is processed exactly once.
  • Space: O(n) for the output string.

Key Insights

  • Always reduce the key modulo 26 first to avoid unnecessary full rotations through the alphabet.
  • The modulo operation handles the wrap-around from z to a elegantly.
  • Building the result in a list and joining at the end is more efficient than repeated string concatenation in many languages.

Edge Cases

  • Key of 0: the output is identical to the input.
  • Key of 26 (or any multiple of 26): the output is identical to the input since a full rotation returns to the original letter.
  • Very large key values: handled correctly by the modulo operation.
  • Single character string: works correctly with the same logic.

Linked mentions