Caesar Cipher — Classical Cipher Tool

A static, zero-dependency web tool implementing five classical ciphers with brute-force decryption and letter frequency analysis.

Live demo: https://sen.ltd/portfolio/caesar-cipher/

Features

• Caesar — shift each letter by N positions (slider 0–25)
• ROT13 — Caesar shift 13; applying twice restores original
• Vigenère — polyalphabetic cipher using a keyword
• Atbash — alphabet reversal (A↔Z)
• Rail Fence — zigzag transposition with N rails
• Encode and decode for all ciphers
• Brute force Caesar — view all 26 shifts at once; click a row to apply
• Letter frequency analysis — chi-squared against English frequencies to suggest the best shift
• Japanese / English UI
• Dark / light theme

Setup

No build step required. Open index.html directly in a browser, or run:

npm run serve   # python3 -m http.server 8080

Then visit http://localhost:8080.

Tests

npm test        # node --test tests/*.test.js

Project structure

caesar-cipher/
├── index.html          # App shell
├── style.css           # Styles (light & dark theme)
├── src/
│   ├── main.js         # DOM, event wiring
│   ├── ciphers.js      # Pure cipher implementations
│   └── i18n.js         # JA/EN translations
├── tests/
│   └── ciphers.test.js # 30+ unit tests
├── assets/             # Screenshots and media
└── package.json

License

MIT — Copyright (c) 2026 SEN LLC (SEN 合同会社)

Links

• 🌐 Demo: https://sen.ltd/portfolio/caesar-cipher/
• 📝 dev.to: https://dev.to/sendotltd/classical-ciphers-with-chi-squared-frequency-analysis-for-auto-decryption-3kc1