Vigenère cipher

Each letter is shifted by the corresponding letter of a repeating keyword, so the same plaintext letter encrypts differently depending on where it falls. "attackatdawn" with the key "lemon" gives "lxfopvefrnhr".

This is what makes Vigenère genuinely more interesting than a Caesar shift: because the shift changes from letter to letter, simple frequency analysis fails — E does not map to any one symbol. It was called le chiffre indéchiffrable for roughly three centuries, until Kasiski published a method in 1863 that finds the key length by looking for repeated sequences in the ciphertext, after which the cipher collapses into several Caesar shifts and falls immediately. It is still not secure. One implementation detail this gets right and many tools get wrong: the key advances only on letters, so punctuation does not shift the key out of step and decoding works on text with spaces.