Unfortunately that’s not really all you need. It needs integrity too. Need to be able to verify that the output came from the input and hasn’t been modified or tampered with.

Also need to ensure that, despite being anonymous, people can only vote once and can’t vote on behalf of someone else.

Also that whoever is receiving and counting the votes can’t miscount or lie about the count or figure out which votes came from where by decrypting individual votes as they’re received.

The scheme they were using is “Helios” which involves people encrypting their votes such that a group of authorities can combine all the encrypted votes together homomorphically to count them and then decrypt the results without ever knowing any one vote. They then use zero-knowledge proofs to prove that they did it correctly and nobody could have known what any vote was or tampered with any results at any point.

Someone just derped and lost their private key so they couldn’t decrypt the results after they’d been combined…