Even if you're using a separate key for voting, the passport key had to sign it. How do you prove legitimacy of the voting key without exposing the passport key? It's not like in blockchain where your anonymity normally comes from people just not knowing which irl person owns a pubkey. (Though I know Monero etc use homomorphic enc for anon payments)

I'm also assuming here that the govt is signing all the passport keys, cause idk how else that would work.