Caesar Cipher

No — it is one of the weakest classical ciphers. With only 25 possible keys, an attacker can try all of them in microseconds. This tool is for education, puzzles, and fun, not for protecting secrets. The brute force mode demonstrates this: it scores all 26 candidates and highlights the most likely plaintext. If you need real confidentiality, use AES-256 (a modern symmetric cipher) or X25519 + ChaCha20 (for key exchange + encryption). The Caesar cipher is historically interesting because it was used by Julius Caesar around 50 BC for military messages, and it illustrates the basic idea of substitution ciphers — every letter is replaced by another letter at a fixed offset.

ROT13 is Caesar cipher with shift 13. The special property of shift 13 is that applying it twice returns the original text: ROT13(ROT13(x)) = x. This is because the English alphabet has 26 letters and 13 is exactly half of 26, so each letter maps to the letter 13 positions away, and mapping 13 positions away from that letter brings you back to the start. ROT13 is widely used online to obscure spoilers, puzzle answers, NSFW content, and mildly offensive jokes — anyone who wants to read the text can apply ROT13 in their head (or with a tool like this) and the original content is hidden from casual scanning. It is not encryption because there is no key — the shift is always 13.

The brute force mode runs all 26 shifts on your cipher text and scores each candidate plaintext by counting how many of the words match a small built-in list of ~100 common English words ("the", "and", "to", "of", "is", "are", etc.). The shift with the highest score gets a star marker. If your cipher text is short (under 30 characters) or contains mostly proper nouns / unusual words, the star may be wrong — common words are a heuristic, not a guarantee. Read each candidate by eye, and use the star as a hint, not a definitive answer. You can also use the copy button next to each candidate to grab that particular shift's output.

Use "A-Z case-preserved" (default) for most English text. It shifts A→A+shift, B→B+shift, etc. while keeping the case of the original letter, and passes numbers, punctuation, and whitespace through unchanged. Use "a-z only" or "A-Z only" if you want strict single-case behavior (useful for some puzzle formats). Use "Printable ASCII (32-126)" if your text contains non-letter characters that you also want shifted — for example, if you have an obfuscated file path or a string of mixed letters and digits where you want everything shifted. Note that with the ASCII option, spaces and punctuation are also shifted, which can make the output harder to read.

Yes. The cipher is pure client-side JavaScript inlined in the HTML. There are no network calls, no API uploads, no analytics, no CDN. Your plaintext, cipher text, and chosen shift stay in your browser. The copy button uses the standard browser clipboard API. Even the brute force scoring (which uses a small built-in word list) is fully local. You can encode or decode any text without any data being transmitted.