The Vigenère cipher is a polyalphabetic substituton cipher (an addition cipher) that uses a "seed" keyword to encrypt a text. To encrypt a text longer than the key, the key is repeated. This page will encode or decode a text for you, given the clear text and a keyword.
Caution: This page assumes a zero-offset alphabet. 'A' has the value of 0, and 'Z' has the value of 25. If you wish to work with a one-offset alphabet, you must use the rotation cipher to correct (ROT+1 after encryption, or ROT+25 after decryption).