Caesar cipher translator

Type on the left, read the shifted text on the right. Swap the boxes to decode, and set Shift to any number from 1 to 25.

If the shift is a mystery, leave the tool on any setting and look at the panel below. Every shift is listed and ranked by how much the result reads like English, so the answer is usually the top row.

Pick a number and move every letter that far along the alphabet, wrapping from Z back to A. At a shift of 3, A becomes D and dawn becomes gdzq. Decoding runs the same distance backwards.

Only letters move. Digits, spaces and punctuation stay exactly where they were, which keeps the shape of the message and makes it easy to spot word lengths. That's also the weakness: a three-letter word at the start of a sentence is probably "the".

Suetonius describes Julius Caesar shifting letters by 3 in his private correspondence, which is where the name comes from. He adds that Augustus preferred a shift of 1, and wrote AA for X, the last letter of his alphabet, instead of wrapping round.

The idea survived into the 20th century as a toy. Radio serials in the 1930s and 1940s posted decoder badges to listeners, who used them to read a secret message at the end of an episode. Captain Midnight's Code-O-Graph matched letters to numbers on a dial. Its alphabet was scrambled, so it was a cousin of the Caesar wheel rather than the thing itself.

ROT13 is this cipher with the shift welded to 13, the setting that makes it undo itself. Atbash mirrors the alphabet instead of rotating it.

If the message has digits and symbols in it, a cipher is the wrong guess. Try the cipher identifier, which checks the encodings too.

Ages 9 and up, about 30 minutes. In pairs, each student writes a short message and encodes it with a secret shift from 1 to 25, then swaps with their partner, who has to find the shift. The quickest way is to try every shift on the first word, which is exactly how a computer breaks it.

Take it further: ask why a Caesar shift is weak (only 25 keys) and what would make it stronger. That leads straight to the Vigenère cipher, and the escape room puzzle maker prints a Caesar sheet with the shift as a clue.