Skip to main content

Caesar Cipher

The Caesar cipher is a substitution cipher that shifts each letter a fixed number of positions in the alphabet. Named after Julius Caesar, who reportedly used a shift of 3, it is the simplest classical cipher and the basis for ROT13. While trivially breakable today, it teaches the core concepts of encryption: plaintext, ciphertext, key, and substitution.

Input

Encrypted

Share this tool
Security

About the Caesar Cipher Encoder & Decoder

The Caesar cipher is a substitution cipher that shifts each letter a fixed number of positions in the alphabet. Named after Julius Caesar, who reportedly used a shift of 3, it is the simplest classical cipher and the basis for ROT13. While trivially breakable today, it teaches the core concepts of encryption: plaintext, ciphertext, key, and substitution.

How to use it

  1. Enter text to encrypt or decrypt.
  2. Set the shift amount (1–25). Caesar himself used 3; ROT13 uses 13.
  3. Choose encoding direction: encrypt (shift forward) or decrypt (shift backward).
  4. Use brute-force mode to see all 25 possible shifts — useful for decoding without knowing the key.

Formula & methodology

Encryption: C = (P + shift) mod 26. Decryption: P = (C − shift + 26) mod 26. ROT13 is self-inverse: ROT13(ROT13(x)) = x. Letter frequency analysis: most common ciphertext letter likely corresponds to E (12.7%), T (9.1%), A (8.2%) in English — enables trivial cracking.

Common use cases

  • Introduction to cryptography concepts in computer science courses
  • ROT13: traditional Usenet/forum practice for hiding spoilers
  • Puzzle design: escape rooms, treasure hunts, puzzles
  • Demonstrating why simple substitution ciphers are insecure
  • Historical study of Roman and wartime cryptography

Frequently asked questions

Easily — try all 25 possible shifts (brute force takes seconds). For longer texts, frequency analysis: count letter frequencies in the ciphertext and match to known English letter frequencies (E=12.7% is most common). The shift that maps the most frequent ciphertext letter to E is almost certainly correct. Any substitution cipher with a fixed alphabet is breakable this way.
ROT13 is a Caesar cipher with shift 13. Since the English alphabet has 26 letters, applying ROT13 twice returns the original text — it is its own inverse. ROT13 is widely used in online communities to hide spoilers, punchlines, or offensive content that readers must consciously decrypt. It offers zero security — it is purely a courtesy mechanism.

Related tools

Credit Card Validator (Luhn) MD5 Hash Generator Password Crack Time Estimator Password Generator Password Strength Checker SHA Hash Generator

Related tools

All Tools →
Text

Find and Replace Text
Text

Character Counter
Text

Strikethrough Text Generator
Text

NATO Phonetic Alphabet
Text

Anagram Generator
Text

Reverse Text Tool

Embed this tool on your site

Free for personal and commercial use. Just copy the snippet below.