Individuals can see their own vote, and who they voted for, right?

If that's the case, it would be evidence that an individual voted for a specific candidate. This is, essentially, proof for anyone who'd want to purchase votes.

If they can't see who they voted for, they'd need some alternative way to verify that their vote counted for the candidate they wanted to vote for, without actually disclosing who that candidate is. Which would add some complexity to the solution.

If the chain were public, individuals could look up their vote by the hash but would have no way of proving the hash was their vote.

You could also add a second layer of security here pretty easily. People would probably need to be given a printout with their hash on it. To avoid this receipt being proof, that receipt would include a large number of hashes with various voting combinations also shown. The voter knows which hash is his, but nobody else does. And he's free to claim whichever one he likes for himself.

To solve the issue of first voters (when there would be no other valid hashes for their printout) simply seed all candidates with a billion votes to start. Which hash is real and which is 'fake' is irrelevant since all that matters is the sum total of which we know 1 billion are to be removed.

How would that work? If they would get any hash that is not bound to them in any verifiable way, how does that prove to THEM that they are verifying their own vote?

Another complexity is ensuring there aren't superfluous votes. Everyone can verify their own, but what if there were several million "hacked" extra votes.

This is a pretty easy one to solve since the number of votes is not privileged information. Simply publicly record the number of votes as each person goes into vote. This can be verified/observed by whoever wishes to do so. You could even require a large public display at each station. Pair that with a camera record that can also be verified. One person enters, the display should go up by one. And from there you have simple math:

The sums of the polling stations should equal the district.

The sums of the districts should equal the state.

The sums of the states should equal the nation.

I have to say it also makes me wonder what in the world darpa is thinking. Modern technology possible, somewhat trivially, elections that would be nearly impossible to fake in any way, shape, or form. And I'm certain this isn't exactly a secret to darpa.

You'd want a system where each citizen receives an ID/key, and there is no way to who a particular ID/key voted for, but if you know your ID/key, you can look up whether you voted. This should deter anyone from voting in anyone else's name, since they'd be likely to be exposed if they did it at an impactful scale.

The only other way to forge votes would be to invent new people that don't really exist, or maybe piggyback on recently deceased people. These are both problems that should be fairly straightforward to address if you have decently efficient administrations keeping track of things.

The issue is verifying whether the system is broken or hacked, otherwise you wouldn't even need to verify your own vote.

> The only other way to forge votes would be to invent new people that don't really exist

Exactly.

> straightforward to address

The question is how? I don't think it's considered that straightforward in blockchain voting circles.

A simple way would be to use a public key ID that can be used to check if that key voted, and a private signing key. The list of all legitimate IDs is public (though not linked to identities). Anyone can see the total number of legitimate keys, and anyone can check to see how many of those keys voted, and verify that the number of voting keys from the national list is in fact equal to the global vote total. That same total and be cross-checked with the census and other sources of voting age population data.