Skip to content

Caesar Cipher

The Secret Decoder Ring

Julius Caesar sent secret messages to his generals by shifting each letter. With a shift of 3:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

"HELLO" becomes "KHOOR". To decrypt, shift back by 3.

This is the simplest cipher -- a secret decoder ring that every kid can understand. It's also easy to break (only 26 possible shifts), which teaches us why we need stronger methods.

How It Works in PyCrypt

from pycrypt.caesar import encrypt, decrypt

secret = encrypt("HELLO WORLD", shift=3)
# "KHOOR ZRUOG"

original = decrypt(secret, shift=3)
# "HELLO WORLD"

Why It's Weak

There are only 26 possible shifts. An attacker can try all 26 in under a second. Real encryption needs keys that are much, much longer.

What We Test

  • Encrypting shifts letters correctly.
  • Decrypting reverses the shift.
  • Non-letter characters (spaces, numbers) are left unchanged, and so are letters from other alphabets, like À.
  • Wrapping works (Z + 1 = A).

Next Up

The Caesar cipher shifts letters. What if we worked with individual bits instead? Head to XOR Cipher.