ROT13 / Caesar Cipher
Apply ROT13 or any Caesar cipher shift to your text.
Non-letter characters are unchanged
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Type or paste your text, then use the slider to set the shift amount. The output updates live.
- ROT13: Click the ROT13 button to set the shift to 13. ROT13 is its own inverse: apply it twice to any text and you get the original back. This makes it useful for hiding spoilers and puzzle answers.
- Caesar cipher: Drag the slider to any shift from 1 to 25. To decode, use the complementary shift: if encoded with shift 3, decode with shift 23 (26 - 3). Or simply apply the same shift 26 times to cycle back to the original.
- Rules: Only letters A-Z and a-z are shifted. Numbers, spaces, and punctuation pass through unchanged. The case of each letter is preserved.
About Caesar Ciphers
The Caesar cipher is one of the oldest and simplest encryption techniques, named after Julius Caesar who used it in his private correspondence with a shift of 3. It works by replacing each letter with the letter that appears a fixed number of positions later in the alphabet. With shift 3, A becomes D, B becomes E, Z becomes C (wrapping around). ROT13 (rotate by 13) became popular in early internet culture because applying it twice returns the original text, making it trivially reversible. It is used today mainly to obscure spoilers in online forums rather than for actual security.
Frequently Asked Questions
What is ROT13?
ROT13 (rotate by 13) is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, rotating by 13 twice returns the original text. This makes ROT13 self-reciprocal: the same operation both encodes and decodes. It was widely used on Usenet newsgroups in the 1980s and 1990s to hide spoilers, offensive content, and puzzle answers. It offers no real security but is still used for lightweight obfuscation.
How do I decode a Caesar cipher if I don't know the shift?
Try all 25 possible shifts (1 through 25) and look for the one that produces readable text. This is called a brute-force attack and is easy because there are only 25 possibilities. English text can also be decoded by frequency analysis: the most common letter in the ciphertext likely represents E (the most frequent English letter), giving you the shift directly.
Is the Caesar cipher secure?
No. The Caesar cipher provides essentially zero security for modern purposes. With only 25 possible shifts, any encrypted message can be cracked by hand in minutes without any computer. It is also vulnerable to frequency analysis. It is a great educational tool for learning about encryption concepts, but should never be used to protect sensitive information. Use AES-256 or RSA encryption for real security needs.
What is the difference between a Caesar cipher and a Vigenere cipher?
A Caesar cipher uses a single fixed shift for every letter. A Vigenere cipher uses a keyword to determine the shift at each position, cycling through the letters of the keyword. This makes Vigenere significantly harder to crack by frequency analysis, since the same letter can be encoded differently depending on its position. However, Vigenere is still a classical cipher and is not secure by modern standards.