Advanced Acéré Cipher Tool

Acéré (French for “sharp”) is the name this page uses for a toolkit of four classic monoalphabetic substitution modes: Atbash, Caesar shift, keyword alphabet and a random mixed alphabet. It is not a standard named cipher and it is not a new algorithm. In every mode each letter is always replaced by the same other letter, which is why the result can be analysed with letter frequencies.

Not secure. Every mode is a simple substitution cipher, and substitution ciphers are broken by frequency analysis. Atbash has no key at all and a Caesar shift has only 25 possible keys. A random mixed alphabet has 26! possible keys (about 4 × 10²⁶, roughly 88 bits), but that large number does not make it safe, because the letter pattern of the message survives encryption and a modest amount of text is enough to solve it. A keyword alphabet has far fewer possible keys than a random one.

No. Use it for puzzles, teaching and games. For real secrets use a modern, reviewed encryption method such as AES-GCM through a standard library, with a long random key or passphrase. You can generate one with our strong random password generator.

Standard (Atbash) reverses the alphabet, so A becomes Z and B becomes Y. Shift (Caesar) moves every letter the same number of places, from 1 to 25. Keyed starts the cipher alphabet with the distinct letters of your key word and then adds the unused letters in order. Random uses a shuffled alphabet generated with the Web Crypto API in which no letter maps to itself.

The alphabet is the key. If you reload the page or close the tab it is gone, and without it the message cannot be decrypted. Press Copy alphabet and keep it somewhere safe. To decrypt later, choose Random, paste the alphabet into the alphabet box and paste the encrypted text into the message box. The alphabet box also accepts an alphabet you typed yourself, as long as it has all 26 letters exactly once.

Only the letters A to Z are converted, in upper and lower case. Digits, punctuation and spaces pass through unchanged. Letters with accents such as é or ñ are also left unchanged, so they show through in the result and the page warns you. Replace them with plain letters before encrypting if you want them hidden.

It counts how often each letter of your message appears and shows the counts as bars, most common first. A substitution cipher only relabels the letters, so the shape of the chart stays the same after encryption. In English the most common letters are usually E, T, A, O, I and N, so the most common letter in a ciphertext is often a stand-in for one of them. Paste a ciphertext into the message box to see its letter counts.

Everything runs in your browser, the page’s content security policy blocks outgoing network requests, and nothing is saved: there is no history, and the only thing kept in local storage is your language choice. Earlier versions of this page kept a history of your messages in your browser, and this version deletes it the first time it loads.