Vigenère Cipher

The Vigenère Cipher is a version of a Caesar Cipher, where each letter is shifted based upon a keyword. In the applet, S is the 19th letter, so the first G in GeoGebra is shifted by 19 to Z. T is the 20th letter, so the E in GeoGebra is shifted by 20 to Y. The letters include the "&" to be used as a space. Use a reasonable key length.