Given ch, a character to encode, and k, the appropriate character in k.  We
encode as follows.

(1) t1 = ch + k
(2) t2 = t1 mod NUM_VALID_CHARACTERS
(3) t3 = t2 + FIRST_VALID_CHARACTER

How about decoding?  We want to invert that.

(4) t2 = t3 - FIRST_VALID_CHARACTER
(5) t1 = t2 // Hard to undo mod
(6) ch = t1 - k.

Problem!  What if this is outside the valid range?  (I.e., because we
used mod.)  I *think* this means that we'll get a negative number, so
we can just add it.