Caesar Cipher
Shift every letter a fixed number of places down the alphabet.
walkthroughWatch it work, step by step
Key = 3 · shift → forward
Each letter moves 3 places forward through the alphabet, wrapping past Z back to A.
Overview
The Caesar cipher is a substitution cipher: each letter of the message is replaced by the letter a fixed number of positions away in the alphabet. That fixed number is the whole key.
With a shift of 3, A becomes D, B becomes E, and so on; letters near the end wrap around, so X, Y, Z become A, B, C. Decryption is simply the same shift in the opposite direction.
History
Named for Julius Caesar, who according to Suetonius used a left shift of three to protect messages of military significance around 58–50 BC.
It is the archetypal cipher — simple enough to do in your head, and the ancestor of every substitution cipher that followed. The name ROT13 refers to a Caesar shift of 13, still used today to hide spoilers and punchlines in plain sight.
Weaknesses
There are only 25 possible non-trivial keys, so an attacker can simply try all of them — a brute-force search that takes seconds by hand.
Even without trying every key, the cipher preserves letter frequencies: the most common ciphertext letter usually maps back to E. Frequency analysis breaks it instantly on any reasonable amount of text.
It is a monoalphabetic cipher — the same plaintext letter always encrypts to the same ciphertext letter — which is exactly the structure that makes it easy to break.
Implementation notes
This implementation enciphers the 26 letters A–Z, preserves case, and passes every other character (spaces, digits, punctuation) through unchanged.