Caesar Cipher
The Caesar cipher slides every letter a fixed number of places along the alphabet. This tool lets you write, read and crack shift messages in seconds.
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What it does
Type a message, pick a shift between 1 and 25 with the slider, and the coded version appears alongside. Two rows of letters under the slider show which plain letter becomes which coded letter, so learners can see the rule rather than just the answer. Switch to Decode to reverse a message you have received. Only A to Z change; capitals stay capital, and spaces, digits and punctuation are left alone.
If you have a Caesar message but no key, the Try all 25 shifts button lists every possibility, ranked by how much each looks like English. The right answer usually comes out on top. That makes it perfect for showing why a cipher with so few keys is weak, and for history work on how Romans may have hidden letters. The Roman writer Suetonius recorded that Julius Caesar used a shift of three.
How to use it
- Type or paste your message into the text box.
- Leave Encode selected to hide a message, or choose Decode to read one.
- Move the slider to set the shift; check the two letter rows to see the mapping.
- Press Copy result to paste the answer into a worksheet or message.
- Stuck without the key? Press Try all 25 shifts and look at the top suggestions. Read a few, because a very short message can sometimes make a wrong shift look sensible.
More tips
- Make a paper cipher wheel alongside the tool so children can feel how the alphabet wraps round from Z to A.
- Set a 'secret shift of the day' on the board and greet the class with a coded message they decode during registration.
Ideas for the classroom and home learning
- KS2 (7–11)History link: after learning about Roman Britain, pupils write a short 'legion order' with shift 3 and swap with a partner to decode.
- KS2 (7–11)Maths link: practise counting on and back within 26, treating the alphabet as a loop like a clock face.
- KS3 (11–14)Use Try all 25 shifts to introduce brute-force attacks: if every key can be tested quickly, the cipher is not safe.
- KS4 (14–16)Connect shifts to modular arithmetic (mod 26) and ask students to write the encoding rule as a formula.
Questions
What shift did Caesar use?
According to Suetonius, three places, so A became D.
Can I decode without knowing the shift?
Yes. Put the message in the box and use the "Try all 25 shifts" button; the most English-like result comes first.
Are numbers changed?
No. Digits stay exactly as typed; just letters move.
Is it safe for real secrets?
No. With only 25 keys it can be cracked almost instantly, so keep it for games and lessons.
Opens Unscramble Words 4U, our free word-puzzle site, in a new tab. Free, with no sign-up.
The Caesar cipher is a teaching and puzzle tool, not real security. Never use it for passwords or personal information.