That’s an interesting idea to set up a blockchain to register voters.
Does it mean the registrations would have to be performed for each election?
The current registrations would expire and be purged a certain time after recent elections?
How would you compartmentalize a voter’s personal info from how they voted?
A blockchain of voter registrations could solve a lot of fraud problems that currently run in the existing voter databases.
I would like to do that. Every election a valid voter could turn invalid (death, felony, moved, etc). To reregister, the voter would create a new private key / public key pair, send the public key and voting credentials to the registrar. The registrar would canonicalize the credentials (very important to prevent multiple registrations), then hash them, then check the blockchain for prior registrations. If none, then the hash will be added as part of a transaction to the hashed public key. The voter could see their hashed public key and know they have the right to vote.
The current registrations would expire and be purged a certain time after recent elections?
The bitcoin / cryptocurrency mantra is that bitcoins can never be purged or expire, it is part of their egalitarian philosophy. A privately operated version of the blockchain can't really purge either. But expiration is possible. If you were issued the credit to vote in one particular election by a transaction and you missed the election, and then you tried to spend that credit in the next election it could be not counted by policy. That would not be hard to enforce without invalidating and valid votes.
How would you compartmentalize a voter’s personal info from how they voted?
The voter's personal information is canonicalized and turned into a hash. That hash is stored on the blockchain as part of the transaction that credits the voter with one vote in the upcoming election. The hash cannot be reversed to find out the private info. However private info could be hashed in a brute force algorithm to try to match up with the hashed in the blockchain to find out how people voted. To prevent that I would have the registrar encrypt the hash before adding it to the blockchain with the registration transaction.
Keep in mind I am an applied cryptographer, not a cryptanalyst. I just implement the code, someone else designs it or at least reviews it to look for weaknesses and bugs. I also do blockchain and the same caveat applies. I can see some potential weaknesses, and can by no means make an exhaustive list.
For one thing we would want to prevent people from registering more than once with different addresses (e.g. mother's basement, college dorm, shared apartment). The blockchain has just a hash or encrypted hash so it is harder to figure that out than with a database. If the registration uses name and date of birth that might eliminate the multiple address problem, but the registrar has to access a date of birth database that can't be hacked or corrupted. I don't have enough familiarity with such a database to know what to do. Also there can be same name individuals with the same DoB.
Tackling the multiple registration problem across different jurisdictions is difficult. To check that some student doesn't register and vote in both Virginia and Ohio means that the registrar in Virginia has to check Ohio and vice versa potentially in hundreds of localities unless each state runs a single blockchain. With encrypted hashes the checking state would have to submit their unencrypted hash to the registrar in the other state. The other state would encrypt and look for matches in their blockchain. The presence of a match could be a false positive that could prevent a legitimate voter from registering. And like I said, there are undoubtedly other problems with my incomplete design.