ROT13 / Caesar Cipher

The ROT13 / Caesar Cipher tool is a free browser-based utility that encodes and decodes text using the classic letter-substitution cipher invented by Julius Caesar. It supports the standard ROT13 rotation (shift of 13) as well as any custom rotation from 1 to 25, giving you full control over the Caesar cipher shift amount.

The Caesar cipher is one of the oldest known encryption techniques. It works by shifting each letter in the plaintext by a fixed number of positions in the alphabet. For example, with a shift of 3, the letter A becomes D, B becomes E, and so on. Non-alphabetic characters such as digits, punctuation, and whitespace pass through unchanged.

ROT13 is the most widely used variant of the Caesar cipher. Because the English alphabet has 26 letters, a shift of 13 is exactly half the alphabet, which makes ROT13 its own inverse — applying ROT13 twice returns the original text. This self-reciprocal property made ROT13 the standard for hiding spoilers and puzzle answers on Usenet, forums, and email lists during the early internet era.

Every operation runs entirely in your browser using pure JavaScript string manipulation. No data is transmitted to any server, no cookies are set, and no input is logged. This makes the tool safe for any content you want to encode or decode. You can also use our Base64 Encode/Decode tool for binary-safe encoding, our String Escape/Unescape tool for format-specific escaping, or our Hash Generator for cryptographic hashing.

While the Caesar cipher is not secure by modern standards — it can be broken instantly by brute force since there are only 25 possible keys — it remains an excellent educational tool for understanding substitution ciphers, modular arithmetic, and the fundamentals of cryptography.

1. Select Encode or Decode mode using the toggle at the top.
2. Adjust the Rotation slider to set the shift amount (1–25). The default is 13 (ROT13).
3. Paste or type your text into the Input textarea on the left.
4. The encoded or decoded result appears instantly in the read-only Output panel on the right.
5. Click Copy to copy the output to your clipboard, or use Ctrl+Shift+C.
6. Use the Swap button to move the output back into the input for round-trip verification.
7. Click Clear to reset both fields and start over.