Free Caesar Cipher – Encrypt & Decrypt Text Online

What the Caesar Cipher Is and How the Shift Is Calculated

The Caesar cipher shifts every letter a fixed number of positions along the alphabet. It is named for Julius Caesar, who according to Suetonius used a shift of three for military correspondence, which makes it roughly two thousand years old and still the first cipher almost everyone learns. People come to it for three reasons: a school or university exercise on substitution ciphers, a puzzle or escape-room clue that needs decoding, and the simple pleasure of passing a coded note. Pick a shift, choose encrypt or decrypt, and read the result. The arithmetic is modular. Each letter is converted to a position from 0 to 25, the shift is added, the result is taken modulo 26 so it wraps from Z back to A, and the number becomes a letter again. Encryption adds the shift; decryption subtracts it, which is why the same key recovers your original exactly. Case is preserved by handling upper and lower case in separate ranges, so a capital stays capital. Digits, spaces and punctuation pass through untouched, which is worth noting because it leaks information — word lengths and sentence structure survive intact and give a codebreaker an enormous head start. Any whole number works as a key: 29 behaves identically to 3, and a negative shift simply moves the other way, both handled by the modulo. A worked example. Take Attack at dawn! with a shift of 3. A is position 0, plus 3 is 3, which is D. The double t at positions 19 becomes 22, which is w. Working through, the output is Dwwdfn dw gdzq! — note the space and the exclamation mark survived, and the two-word-then-two-word shape is unchanged. Decrypt that with a shift of 3 and you get Attack at dawn! back precisely. Try shift 5 on Meet me at 7 PM and you get Rjjy rj fy 7 UR, where the digit 7 is untouched and the capital P and M stay capital because each case range is shifted separately. Where it turns up. A computer science lecturer sets a first cryptography assignment on it. An escape-room designer engraves a shifted word on a prop and needs the exact output before it goes to the workshop. A parent builds a treasure hunt where each clue is shifted by the number found in the previous one. A puzzle-hunt competitor tries all 25 shifts on an unknown string until one produces English. Two friends pass notes with an agreed key, which works fine because nobody is seriously attacking them. A museum education officer prints a shifted Latin phrase on an activity sheet beside a Roman exhibit so children decode it themselves. A board game designer hides a bonus objective in a shifted line on a card back, so only players who spot the pattern claim it. Be blunt about the security: there are only 25 useful keys, so anyone can break this by trying all of them in under a minute, and frequency analysis breaks it even faster on a longer message. It offers no protection whatsoever for a password, a personal detail or anything you genuinely need kept private, and people do sometimes make that mistake. If you are decoding an unknown Caesar message, the practical method is to try every shift and look for the one where common words like the and and appear. Note also that shift 13 is exactly ROT13, which is its own inverse. Non-Latin scripts such as Devanagari or Arabic pass through completely unchanged and therefore gain no concealment whatsoever from the shift. Everything runs in your browser, so nothing you type is uploaded, stored or logged.

Caesar Cipher Formula & Method

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