Caesar Cipher

Encode and decode messages with the classic Caesar shift cipher, includes ROT13 and frequency analysis

Overview

The Caesar Cipher is one of the oldest and simplest encryption techniques, named after Julius Caesar who used it to communicate with his generals. This substitution cipher shifts each letter by a fixed number of positions in the alphabet (for example, with a shift of 3, A becomes D, B becomes E, and so on). While historically significant and educational, Caesar ciphers are trivially breakable with only 25 possible keys and should never be used for actual security.

Tips

  1. Try ROT13: Use a shift of 13 and notice the self-inverse property - encoding “HELLO” gives “URYYB”, and encoding “URYYB” returns “HELLO”
  2. Break with Brute Force: When you receive an encrypted message with unknown shift, try all 25 possible shifts to quickly find the readable plaintext
  3. Use Frequency Analysis: For longer messages, count which letters appear most often - the most common letter is likely “E”, helping you determine the shift value
  4. Understand the Weakness: Experiment with different shifts to see why only 25 possible keys makes this cipher trivially breakable and unsuitable for real security
  5. Explore Patterns: Notice how double letters remain double (LL becomes OO), word lengths stay the same, and spaces are preserved - these patterns make Caesar ciphers easy to crack