ROT13 / Caesar cipher

The Caesar cipher is one of the oldest known ciphers: each letter is shifted a fixed number of places along the alphabet. Julius Caesar reportedly used a shift of 3. ROT13 is the special case with a shift of 13, which is its own inverse, so applying it twice gives back the original. This tool encodes and decodes both and can show all 25 possible shifts at once.

How to use it

1. Enter the text.
2. Choose ROT13 or set a Caesar shift.
3. Read the result, or open the brute-force table to decode a message with an unknown shift.

Examples

• Caesar shift 3: HELLO becomes KHOOR.
• ROT13: Hello, World! becomes Uryyb, Jbeyq!, and applying ROT13 again restores it.

Only letters change; digits, spaces and punctuation stay as they are, and case is preserved.

Breaking a Caesar cipher

With only 25 possible keys, a Caesar cipher is broken by trying them all, which is what the brute-force table does. Scan the 25 lines and the readable one is the answer. For longer texts, letter frequency analysis also works: the most common letter in English ciphertext usually corresponds to E.

Where ROT13 is still used

• Hiding spoilers and puzzle answers in forums.
• CTF challenges and programming exercises.
• Light obfuscation of email addresses or text from casual reading.

Security warning

Caesar and ROT13 offer no real security. Anyone can decode them in seconds. Use AES or another modern cipher for anything confidential.