Around 50 BC Julius Caesar protected his military dispatches by sliding every letter three places down the alphabet. The outer disc below carries the plaintext alphabet; the raised inner disc carries the ciphertext alphabet. Choose a shift, type a message, and whatever lines up in the window at the top is the answer.
Drag the outer ring to turn the whole disc. Drag the inner disc to set the key — the shift controls follow it.
Shift right by 3 means A → D (Caesar's own key). Shift left by 3 means A → X. A left shift of k is identical to a right shift of 26 − k, which is why there are only 25 useful keys.
Only A–Z are shifted; spaces and punctuation pass straight through. Use ← → on the slider to watch the inner disc click one letter at a time.
Twenty-five keys is nothing: an attacker tries every one in seconds (a brute-force attack). Worse, English leans on E, T, A and O so heavily that the most common ciphertext letter almost always reveals the shift on its own (frequency analysis). Modern ciphers survive both attacks by having astronomically many keys and no letter-for-letter pattern to count.
Set the disc to the key shown, switch to Decrypt, and read each inner-ring letter back out to the outer ring. Ready for scoring and streaks? Move on to the Caesar Cipher Challenge.