* similar to Vigenère but with alphabet written backwards
* can be descibed by

given K = k(1) k(2) ... k(d)
then f(i) (a) = (k(i) - a) (mod n)

and its inverse is

f(i) ^(-1)(a) = (k(i) - c) (mod n)

Key = d
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: DCBAZYXWVUTSRQPONMLKJIHGFE