Right-click any selected text on any web page to instantly see all 25 Caesar-cipher shifts. Offline, zero tracking, open source. Companion to caesarcipher.org.
- One right-click → all 26 shift candidates (0 = original, 1–25 = decoded)
- 100% offline — no network calls, no tracking, no storage
- Only one Chrome permission:
contextMenus - Click-to-copy on any candidate row
- Handles selections up to 2000 characters; Unicode passes through unchanged
- Chrome Web Store: (link added once published)
- From source:
- Clone this repo
- Open
chrome://extensions - Enable Developer mode
- Click Load unpacked, select the repo folder
This extension is deliberately single-purpose. For Vigenère, Atbash, Autokey, Rail Fence, Playfair, and 20+ other classical ciphers — plus encoding, frequency analysis, and step-by-step walkthroughs — use the full web toolkit at caesarcipher.org.
The algorithm is a standard Caesar shift applied only to ASCII A–Z / a–z characters; everything else (spaces, punctuation, digits, CJK, emoji) passes through untouched.
For each candidate shift n ∈ [0, 25], the ciphertext is reversed by n positions modulo 26 — i.e., the "Shift n" row shows what the plaintext would be if the ciphertext had been encrypted with Caesar +n. So ciphertext encrypted with a +3 shift is decoded on the "Shift 3" row. The result page shows all 26 rows at once so you can eyeball the plaintext — for short ciphertexts that's the fastest solution. Implementation is ~20 lines in lib/caesar.js.
The extension icons are generated from the main-site SVG logo:
brew install librsvg # one-time
./scripts/build-icons.sh # writes icons/icon-{16,48,128}.png
npm test # runs node:test unit tests for the algorithm
To iterate on the UI, load the extension unpacked, edit files, and click the refresh icon on its card in chrome://extensions.
./scripts/build-zip.sh # writes dist/caesar-cipher-decoder-<version>.zip
The zip excludes dev-only files (tests, scripts, store-assets, docs, README, LICENSE, PRIVACY.md, package.json, .git).
See PRIVACY.md. Short version: the extension collects, transmits, and stores nothing.
MIT — see LICENSE.