Classic Cipher Toolkit

Encode and decode the classical substitution ciphers: Caesar with any shift, ROT13, Atbash and Vigenere. A brute-force view lists all twenty-six Caesar shifts at once, which is the standard way to break one in a CTF or a classroom exercise.

How to use it

Choose a cipher and enter your text.
For Caesar, set the shift; for Vigenere, supply a keyword.
Use the brute-force panel when you have ciphertext but no key and want to eyeball all shifts at once.

Case and punctuation are preserved, so the shape of the message survives and the output stays readable.

How each cipher transforms the alphabet

The Caesar cipher rotates every letter a fixed number of places, so with a shift of three A becomes D. There are only twenty-five useful keys, which is why brute force breaks it instantly. ROT13 is simply Caesar with a shift of thirteen, and because thirteen is half of twenty-six it is its own inverse: applying it twice returns the original. It was never a security measure, only a convention for hiding spoilers and punchlines from casual view.

Atbash reverses the alphabet, mapping A to Z and B to Y. It has no key at all, so knowing the method is knowing everything. Vigenere is the interesting one: it applies a different Caesar shift to each position, cycling through the letters of a keyword. For centuries this defeated simple frequency analysis, because the same plaintext letter encrypts differently depending on where it falls, flattening the letter distribution.

Why none of these are secure

Every cipher here falls to classical cryptanalysis. Monoalphabetic substitutions preserve letter frequencies, so counting characters and comparing against the expected distribution of the language recovers the mapping from a surprisingly short sample. Vigenere resisted longer but yields to the Kasiski examination or an index-of-coincidence test, which recover the keyword length; once that is known the ciphertext splits into separate Caesar problems, each solved by frequency analysis.

Use these for capture-the-flag challenges, puzzles and teaching how substitution and key reuse behave. Never use them to protect anything real: modern confidentiality means an authenticated cipher such as AES-GCM or ChaCha20-Poly1305 with a properly managed key. All transformation runs inside your browser, so any text you paste stays local.

Frequently asked questions

How do I break a Caesar cipher without the key?

Generate all twenty-six shifts and read down the list for the one that produces sensible words. That is exactly what the brute-force view does, and it is why the cipher offers no meaningful protection.

Why does the same letter encrypt differently under Vigenere?

Because the shift depends on the position of the letter relative to the repeating keyword. That is what makes it polyalphabetic and why plain frequency analysis fails until the key length is recovered.

Are numbers and symbols encrypted too?

No. These ciphers act on the twenty-six letters of the Latin alphabet; digits, punctuation and spacing pass through unchanged, which is part of why the ciphertext still leaks the structure of the message.

🗝️ Classic Cipher Toolkit