ROT13 and Caesar Cipher

How the Caesar cipher works

Each letter is moved a fixed number of places along the alphabet. With a shift of 3, A becomes D, B becomes E, and the end wraps round so X, Y and Z become A, B and C. Decoding moves the letters back by the same amount.

Only the 26 letters A–Z change. Capitals stay capitals, and digits, spaces, punctuation and accented letters pass through unchanged. A shift of −3 is the same as 23, and decoding with shift 3 equals encoding with shift −3.

ROT13 explained

ROT13 is a Caesar cipher with a shift of 13. Because 13 is half of 26, applying it twice brings back the original, so the same operation encodes and decodes. It was used on Usenet to hide spoilers and punchlines from accidental reading: Why did the chicken cross the road? becomes Jul qvq gur puvpxra pebff gur ebnq?.

ROT47 does the same with all 94 printable ASCII characters from ! to ~, so digits and symbols change too: Pass: 1234! becomes !2DDi abcP`.

Cracking a Caesar cipher: every shift at once

There are only 25 possible shifts, so you can simply try them all. Crack Caesar: show all 25 shifts lists every decoding and marks the one that looks most like English, scored by how common its letters are in English and how many common words (the, and, of…) it contains.

With a sentence or more the mark is reliable. With only two or three words the ranking can be wrong, so read the whole list. If no line makes sense, the text probably isn't a Caesar cipher; try Vigenère, Atbash or ROT47.

Vigenère and Atbash ciphers

Vigenère uses a keyword to shift each letter by a different amount: A shifts by 0, B by 1, and so on, repeating the keyword along the text. Encoding ATTACK AT DAWN with the keyword LEMON gives LXFOPV EF RNHR. The keyword only advances on letters, so spaces and punctuation don't use up key letters.

Atbash reverses the alphabet (A↔Z, B↔Y), so Hello becomes Svool; like ROT13, it is its own inverse. Rail fence writes the text in a zigzag over a number of rails and reads it off row by row; spaces count as characters.

Why these ciphers aren't secure

A Caesar cipher has 25 keys, which anyone can try in seconds, as this page does. Vigenère resisted attack for centuries but has been breakable since the 1860s by finding the key length from repeated patterns, then solving each shift with letter frequencies. Use these ciphers for puzzles, geocaches, escape rooms and teaching. To protect real data, use modern encryption such as AES with a random key from the encryption key generator.