The Vigenere Cipher is a polylphabetic substitution cipher rotating A..Z and a..z characters:

Examples using static factory methods:

Vigenere.Cipher('abc').crypt('ABAB');   // returns: 'ACCB'

var cipher = Vigenere.Cipher('abc');
cipher.crypt('AB');                     // returns: 'AC'
cipher.crypt('AB');                     // returns: 'CB'

Vigenere.Decipher('abc').crypt('ACCB'); // returns 'ABAB'

Examples using constructors:

new Vigenere([0, 1, 2]).crypt('ABAB');  // returns: 'ACCB'
new Vigenere([0,-1,-2]).crypt('ACCB');  // returns: 'ABAB'
new Vigenere([0,25,24]).crypt('ACCB');  // returns: 'ABAB'

See Wikipedia for details.