XORBase64 takes two base64-encoded strings and XORs the decoded byte slices
together, returning the final byte slice. It is an error to pass in two
strings that do not have the same length to their base64-decoded byte slice.
XORBytes takes two byte slices and XORs them together, returning the final
byte slice. It is an error to pass in two byte slices that do not have the
same length.