The Caesar cipher is one of the oldest ciphers: every letter is replaced by the letter a fixed number of places further along the alphabet. With a shift of 3, A becomes D, B becomes E and Z wraps around to C. Julius Caesar is said to have used a shift of 3 for military messages. To decode, shift the same number of places back (or forward by 26 minus the shift). Shift 13 is called ROT13 and is its own inverse because 13 + 13 = 26, so applying it twice gives the original text. Only letters change; digits, spaces and punctuation stay as they are and upper/lower case is kept. Because there are only 25 possible shifts, the cipher is trivial to break: the table above shows all of them, so you just look for the line that reads as normal text.
| Shift | HELLO WORLD | Attack at dawn |
|---|---|---|
| 1 | IFMMP XPSME | Buubdl bu ebxo |
| 2 | JGNNQ YQTNF | Cvvcem cv fcyp |
| 3 | KHOOR ZRUOG | Dwwdfn dw gdzq |
| 5 | MJQQT BTWQI | Fyyfhp fy ifbs |
| 13 (ROT13) | URYYB JBEYQ | Nggnpx ng qnja |
Look at the table of all 25 shifts above and pick the line that reads as normal text. With only 25 possibilities this takes seconds. For longer messages you can also count letters: E is the most common letter in English, so the most frequent letter in the ciphertext is probably E shifted.
ROT13 is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, running ROT13 twice returns the original text, so the same operation encodes and decodes. It is used online to hide spoilers and puzzle answers, not for security.
No. It only has 25 useful keys and keeps letter frequencies intact, so it is easily broken by hand. It is a good way to learn how ciphers work, and a fun puzzle format.
They are left unchanged, and capital letters stay capital. Accented letters and non-Latin alphabets are also left unchanged.
For fun and learning only. Do not use a Caesar cipher to protect anything private.
See also: Morse code translator · text to binary · Base64 encoder · anagram checker · hash generator · all tools · Fancy Text Generator