Add key with the pixels

How to add 1 4 key with an image size 256*256 in matlab. e.g

ii=i(:,:,3);
rng(78932);    %agree upon a seed
rand_key = uint8(randi([0 255], 1, 4));
enc=bitxor(ii,rand_key);

apparantly addition is not possible Is there a way to do addition