VITON13 SCHOOL · Codes and ciphers

Caesar cipher decoder

Type a message and it is encoded or decoded at once. Turn the wheel, try a Vigenère keyword, or let the page find the key of a text you cannot read.

By Vitalii Tarasov, Founder of VITON13 SCHOOL

Alphabet

26 letters. Accented letters, Cyrillic, digits and punctuation pass through unchanged.

Treat accented letters as plain ones (é → e)

Shift

Plain text

Cipher text

Phhw ph eb wkh rog oleudub dw qrrq. Eulqj wkh pds!

38 letters changed · 12 other characters kept

Drag the inner ring or use the arrow keys. Outer ring: plain letters, inner ring: cipher letters.

Letter by letter

Language of the hidden text

Most likely key

Shift 0 · English

Clear match

Meet me by the old library at noon. Bring the map!

The input field is read as cipher text. Each key is scored by how closely the decoded letters match the language.

For learning and puzzles only: these ciphers do not protect secrets.

How to use the cipher tool

Pick a cipher and a keyCaesar moves every letter by the same number, Vigenère by the letters of a keyword, Atbash mirrors the alphabet. Choose Latin or Russian letters.
Type or paste the textThe result appears as you type. Capital letters, digits, spaces and punctuation keep their places. Swap sends the result back as the new input.
Break a message without the keyPaste the cipher text and open “Break it”: every shift is scored against the language’s letter frequencies, and for Vigenère the key length and the key are estimated.

How the automatic break works

According to Suetonius, Julius Caesar wrote confidential letters by putting the fourth letter of the alphabet, D, in place of A, and so on — a shift of three. Any shift from 1 to 25 works the same way, and ROT13, a shift of 13, is its own reverse because 13 + 13 = 26.

Every language uses its letters unevenly: in English, E, T and A are common and Q, X and Z are rare. A shift moves the whole pattern along the alphabet without changing its shape, so the tool decodes the text with every key and measures how far its letter counts are from the language’s table with the chi-squared statistic. The smallest value marks the most likely key; with only a few words the guess can be wrong, which is why every shift is listed too.

A Vigenère keyword changes the shift from letter to letter and flattens the frequencies. The tool first estimates the key length with the index of coincidence: split into one column per key letter, each column is an ordinary Caesar cipher, and its letters repeat about as often as in plain language. Repeated letter groups and their distances (the Kasiski examination) are shown as a second opinion, then each column is broken like a Caesar cipher. This needs a reasonably long text: at least 60 letters, and a few hundred for long keys.

These ciphers are for learning, games and puzzles, not for protecting secrets. A Caesar cipher has only 25 keys, and a Vigenère cipher falls to the statistics on this page. Real data needs modern, well-reviewed encryption, such as the encryption built into your devices and messengers.

Questions about the Caesar cipher

How do I decode a Caesar cipher without the key?

Paste the text and open “Break it”. The tool tries every shift, scores each result against English, Spanish or French letter frequencies and highlights the most likely one. “All shifts” lists every result, so a short message can also be read by eye.

What is the Vigenère cipher?

A Caesar cipher whose shift follows a keyword: with the key LEMON the first letter moves by 11 (L), the second by 4 (E), and so on, then the keyword repeats. Decoding moves each letter back by the same amounts. Only letters advance the key; spaces and punctuation are skipped.

Does it work with the Russian alphabet?

Yes. Choose 33 letters with Ё, the modern alphabet, or 32 letters without Ё, where Ё is read as Е. The two give different results for the same shift, so both sides need the same choice. Latin letters stay unchanged while a Russian alphabet is chosen, and Cyrillic stays unchanged with Latin.

Is the Caesar cipher secure?

No. With 25 keys anyone can try them all in seconds, letter frequencies reveal the key even faster, and Vigenère is broken by the statistics on this page. Use these ciphers to learn how encryption works, not to hide passwords or personal data.

Sources

Related guides

Caesar cipher: how it works, how to decode it and why it is easy to breakHow the Caesar cipher works: encoding and decoding with a worked example, ROT13, why it breaks in minutes, Vigenère as the next step, and its history.

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