ROT13 encoder and decoder

How it works

ROT13 rotates every letter by exactly 13 positions in the 26-letter alphabet. A becomes N, B becomes O, and M becomes Z. The second half maps back to the first: N becomes A, O becomes B, and Z becomes M. Numbers and punctuation are not part of the substitution.

Because 13 is half of 26, the transformation is self-inverse. There is no different decryption rule. Apply ROT13 once to hide the text and apply the same operation again to restore it. The Encode and Decode buttons are both provided for clarity, but they intentionally produce the same transformation.

This tool converts letters to uppercase. It strips spaces and punctuation by default to match compact classical-cipher notation. Enable Preserve spaces and punctuation if you want sentence spacing, line breaks, digits, and punctuation to stay in their original positions.

ROT13 has no keyword and no adjustable shift. That makes it convenient for reversible obfuscation, but it also means every person uses the exact same mapping. A reader who recognizes the pattern can recover the original text immediately.

Worked example

Transform HELLO with ROT13. Each letter moves 13 places, exactly halfway around the alphabet.

1. Enter the text: Type HELLO in the Input text field.
2. Move each letter by 13: Map H to U, E to R, L to Y, and O to B.
3. Read the transformed text: Combine the mapped letters to get URYYB.
4. Reverse it: Apply ROT13 to URYYB again to recover HELLO.

The output is URYYB. Apply ROT13 again: U becomes H, R becomes E, each Y becomes L, and B becomes O. The second pass therefore returns HELLO without selecting a different key or direction.

History and origin

ROT13 became closely associated with early online communities, especially Usenet in the 1980s and 1990s. Writers used it to obscure joke punch lines, puzzle answers, plot spoilers, and material that some readers might prefer not to see automatically. It was never presented as strong encryption; the point was to require a deliberate decoding step.

The method is a direct descendant of the Caesar cipher. Its special shift of 13 is useful because the English alphabet has 26 letters. That symmetry made ROT13 easy to implement in text programs, mail readers, and command-line utilities: one function handles both directions.

How to break it

ROT13 provides no cryptographic security. There is no secret key, and the algorithm is publicly recognizable. Applying ROT13 to suspected text instantly reveals the original. Even without recognizing the name, a Caesar brute-force search will include shift 13 among only 25 candidates.

The substitution also preserves language patterns. Repeated letters remain repeated, word lengths remain visible when spaces are kept, and common letter frequencies simply move to fixed partners. A message with several instances of the same short word gives an analyst immediate clues.

Use ROT13 as a courtesy screen, a puzzle mechanic, or a reversible text effect. Do not use it for passwords, private records, account details, or confidential conversations. Obscuring text from a casual glance is not the same as encrypting it against an attacker.

Related ciphers

• The Caesar cipher generalizes ROT13 by allowing any shift from 1 through 25 and includes a brute-force decoder.
• Atbash is also self-inverse, but it mirrors the alphabet rather than rotating it by 13 places.
• The string encoder combines ROT13 with URL, HTML entity, Unicode, and Base64 transformations.
• The cipher tools directory provides a crawlable overview of the classical substitution family.

Frequently asked questions

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