Illegitimate use can never be prevented whether completely manual or completely automated or something in-between. But the credentials can be controlled in a blockchain system. The user generates their own credential, whether illegal alien or citizen. The next step is they register to vote. That is a process where they turn in the public key and their proof that they are a citizen. The government official looks at the proof and adds the public key into the database of voters.
Can the database be hacked and new public keys added? Maybe, but the addition of those keys is in a block chain and such fraud would be easy to detect. Can illegal aliens present fake citizenship credentials? Possible, but only to the extent they can today.
The beauty of the block chain once the registration step is done is that it cannot be hacked.
The registration process is relatively unchanged. In MA that is the point where you have prove you are eligible.
Sorry but I’ve already told you how to beat every aspect of your system. There are examples happening right now that show this idea is simply unsecure and worthless. Every script kiddie and welfare fraud knows how to get around this.