The joke is that, in German, "hast" (hate) and "hasst" (have) are pronounced exactly the same, and "du hast/hasst mich" doesn't give enough context to determine if it means "you have me" or "you hate me."

I thought the joke was the code…
You are right about the German, except you mixed up 'hast' and 'hasst.'
One 's' is have; two is hate.