ROT18 cipher — ROT13 for letters plus ROT5 for digits

How to use the ROT18 cipher

1. Type or paste your text into Text.
2. The ROT18 result appears straight away.
3. Copy it. To decode, paste the encoded text into the same box.

What it does and when to use it

ROT13 is a well-known way to hide spoilers and answers, but it leaves numbers readable. That is a problem when the secret is a number: a score, a date, a code or the answer to a maths puzzle. ROT18 fixes that by also rotating the digits.

Typical uses are puzzle hunts, quiz answers in a newsletter, geocache hints that contain coordinates, and classroom exercises on substitution ciphers. In capture-the-flag games it sometimes appears as one of several layers.

Like all ROT ciphers it offers no real security. Compare the alternatives: ROT13 rotates letters only, ROT47 rotates every printable ASCII symbol too, and the Atbash cipher mirrors the alphabet instead of rotating it.

How it works

Two simple rules are applied character by character:

Letters:  A B C D E F G H I J K L M  ↔  N O P Q R S T U V W X Y Z
Digits:   0 1 2 3 4  ↔  5 6 7 8 9

Each letter is replaced by the one 13 places further along the alphabet, wrapping from Z back to A, and each digit by the one 5 further along, wrapping from 9 to 0. Every pair works both ways, which is why the same step encodes and decodes. Case is preserved and nothing else changes. The tool runs in your browser.

Worked examples

Hello 123                →  Uryyb 678
Uryyb 678                →  Hello 123
Order 66 ships at 0900   →  Beqre 11 fuvcf ng 5455
Café 2024!               →  Pnsé 7579!

In the third line the time 0900 becomes 5455: 0 → 5, 9 → 4. In the last line the é is not rotated, because it is not one of the basic 26 letters, and the exclamation mark is kept.

Limits and tips

• Word lengths and punctuation stay visible, so short words can be guessed.
• The number of digits does not change, so a four-digit time or PIN still looks like one. Add other clues to the puzzle if that matters.
• For a cipher with a secret key, use the Vigenère cipher, which is still not secure but needs the keyword to read.