Where does "xoxo" come from

when you see a love letter signed "xoxo, john" what is the origin of "xoxo?"

The Xs and Os stand for "love and kisses." They may appear in other sequences, such as x OOO x OOO. I forget whether the Xs represent love and the Os kisses or the other way round, as well as the basis for this symbolism. Anybody out there know?