// source: https://cryptopals.com/sets/1/challenges/2
/*
Fixed XOR
Write a function that takes two equal-length buffers and produces their XOR combination.
If your function works properly, then when you feed it the string:
1c0111001f010100061a024b53535009181c
... after hex decoding, and when XOR'd against:
686974207468652062756c6c277320657965
... should produce:
746865206b696420646f6e277420706c6179
*/