Chris Spackman's NeoCities Page

Caesar Cipher

The Caesar cipher is one of the oldest known ciphers. It works by shifting each letter in the message a fixed number of positions in the alphabet. It is named after Julius Caesar, who reportedly used a shift of 3 to communicate with his generals.

The Decoder Ring Analogy

Imagine two rings of the alphabet, one inside the other. You rotate the inner ring by a certain number of positions — that is your key. To encrypt, find each letter on the outer ring and replace it with the letter lined up on the inner ring. To decrypt, do the reverse.

Try It

Result

Letter-by-Letter

How It Works

Each letter in the plaintext is replaced by a letter a fixed number of positions down the alphabet. With a shift of 3:

• A becomes D
• B becomes E
• Z wraps around to C

The formula is: ciphertext letter = (plaintext letter + shift) mod 26. The "mod 26" means we wrap around when we go past Z.

Why It Is Easy to Break

The Caesar cipher has only 26 possible keys (shifts 0 through 25). An attacker can simply try all 26 — this is called a brute-force attack. Click "Show All 26 Shifts" above to see exactly how easy this is.

Things to Try

• Encrypt a message with shift 13 (ROT13), then encrypt the result again with shift 13. What happens? Why?
• What is the difference between a shift of 0 and a shift of 26?
• Encrypt "HELLO" with several different shifts. How many different outputs can you get?
• Try encrypting a message, then decrypting the result with the same shift. Do you get your original message back?