Caesar Shift — Bench tool
Published 30 Aug 2026 ·
The oldest alphabet cipher most people meet first: every letter moves a fixed number of places down the alphabet. It takes about ninety seconds to learn and about four minutes to break — which is exactly why it is worth studying first.
- Era
- Antiquity
- Type
- Substitution
- Key space
- 25 shifts
- Level
- Starter
IWhat this tool does
Take the sentence ATTACK AT DAWN and push every letter three places forward: A becomes D, T becomes W, and the message leaves the room as DWWDFN DW GDZQ.
Nothing about the letters changed except their labels. The word lengths survive, the punctuation survives, and any letter that repeated in the plaintext repeats in the ciphertext at exactly the same distance. ATTACK has a doubled T at positions 2 and 3; DWWDFN has a doubled W at positions 2 and 3.
That survival of shape is what makes the cipher readable to an attacker. The ciphertext is not a new language, it is the same language wearing a uniform.
| Plaintext marker | Shifted marker | What still shows through |
|---|---|---|
| doubled letters | doubled letters | word shape |
| one-letter words | one-letter words | A and I stay obvious |
| word lengths | word lengths | sentence rhythm |
| letter frequency | rotated frequency | E is still the tallest bar, just relabelled |
IIThe key
The key is one integer, and only 25 of them do anything. A shift of 26 returns the plaintext, a shift of 27 is identical to a shift of 1, so 25 is the whole universe of distinct keys.
log2(25) is roughly 4.64 bits. For comparison, a modern symmetric key is 128 bits: the Caesar keyspace is smaller than a coin flip repeated five times.
The square matters more than the number. Because the encryption of any letter depends only on that letter, the cipher has no memory. Encrypting AAAA with shift 3 always produces DDDD; there is no context, no chaining, no state. A cipher with no state can be attacked one letter at a time.
Rule of thumb learned here: if the key is smaller than the message, the message is the key.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →