"The encrypted data along with the signature of the encrypted vote is written to the bigquery table (and not directly to the air-gapped server) which is written to another temp folder (only the account that has access permissions can write to this folder) before being downloaded and processed on air-gapped server."

No, you say it is. From the client's side, all I do is POST my encrypted vote and signature ( https://github.com/VOTEism/server/blob/d8d3752af99d7f4c25c0b... ), encrypted with a public key I just got from an HTTP request to the same server, no? ( https://github.com/VOTEism/server/blob/d8d3752af99d7f4c25c0b... ). I can't tell what you're doing with it after that.

And there's what may be an even worse issue (not even requiring tampering with the public key) - the user's signature appears to go outside of the encrypted blob. Now, I can't tell what the signature is actually over - but if it's, say, just a candidate name or ID, then... without ever decrypting the "encrypted vote", I can figure out who voted for what by just trying possible values and seeing if I can validate their signature.