How does the Caesar cipher work?

Each letter is shifted by a fixed number of positions in the alphabet — e.g., with shift 3, A becomes D.

* The Caesar cipher: every letter shifted by a fixed amount (+3). *

The Caesar cipher (used by Julius Caesar around 50 BC) is a substitution cipher:

• Encryption: shift each letter forward by the key value
  • Key = 3: A → D, B → E, C → F, ..., Z → C
• Decryption: shift each letter backward by the same amount

Mathematically (using $\bmod 26$ arithmetic):

• Encrypt: $c = (m + r) \bmod 26$
• Decrypt: $m = (c - r) \bmod 26$

Where $m$ = plaintext letter, $c$ = ciphertext letter, $r$ = key (shift amount).

Why it's trivially breakable: There are only 26 possible keys (shifts 0-25). An attacker can try all of them in seconds — this is called a brute force attack. Additionally, frequency analysis instantly reveals the shift by looking at which letter appears most often (likely 'E' in English).

Go deeper:

• Caesar cipher - the shift-substitution cipher, with modular formula and cryptanalysis.
• dCode Caesar Cipher - encode/decode and brute-force all 26 shifts interactively.