Ask HN: Data integrity in a decentralized trustless system | Hacker News Reader

Ask HN: Data integrity in a decentralized trustless system

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Hi,

I am working on a voting system for a local community, people will vote with their smartphone, tablet or computer. They previously receive a link to the vote form with an access key in the url.

Once someone has voted he cannot see is vote (like in real world polling station), which partly avoid the "cash for vote" problem. "Partly" because someone can still force someone else to vote like he wants. For this, my idea was to allow to vote multiple times but with idem-potency: only the first or the last vote is recorded in the database. "you can give me 5$ to vote like you want, once back home I vote for the one I want".

But since voters cannot check if their vote has been properly recorded they must trust the system.

Block chain that involves several parties (an NGO, an university, etc...) could be an idea but nothing prevent the API that receives the votes and insert records in the blockchain to insert fake data. Code audit (+ CI/CD audit + DNS records audit) can help.

My idea: each party (NGO 1, NGO 2, university 1, university 2, etc...) deploy the API and its database. The source code is obviously open.

Then, a proxy receives the HTTP request and forward them to each API. The goal is to reach eventual consistency.

Outages may occur, if API deployed at university 1 is unreachable some data will be missing in its database. A retry policy at the proxy level can help but only for short time network failure, not for several minutes or hours outage.

So differences between databases at the end of the election will probably happen and should be corrected. If consensus is met on chunks of data (example: 2/3 of the databases have the same data for each 1 hour period) then we can get reach eventual consistency.

The proxy becomes the weak link. Each party should must have access to its configuration for audit purpose and must also have access to the DNS records.

At the ends the voters don't have to understand all these details, they trust the system because they trust the parties who participates.

What do you think ?

Thanks !

I think it's a very important and interesting problem you're tackling and you probably need to iterate a bit further on several points of the architecture and protocol.

As to your main question re the proxy: Why is there a proxy in the first place? The client could just make the same requests the proxy would directly to the multiple parties, obviating the for it, no?

Thanks for your answer.

Without the proxy the client would request a specific instance of the API (university 1, university 2, NGO 1, NGO 2), which then would be responsible for forwarding the data to the other. What if he changes the code and forward a false data:

user A votes for candidate X (HTTP POST request received by API deployed at university 1). API deployed at university A is compromised (by the university itself or not) and the information persisted in the DB is "user A votes for candidate Y". This information is then forwarded to other API.

If a proxy like NGINX is responsible for request forwarding the problem is solved (assuming that all parties trust nginx and its "mirror" module). https://nginx.org/en/docs/http/ngx_http_mirror_module.html

no, no forwarding. have the client contact all sites directly.

> What if he changes the code and forward a false data:

Presumably you will find a cryptographer to help you construct a scheme preventing this via cryptographic signatures. You'd do well to investigate prior art.

You're right, we can get rid of the proxy. If the multiple API calls are made from the browser it is even more traceable, user can see these requests in the source code and from the devtools network console.

Each API has its database which contains the accesskey for each user, if this data is stolen the attacker can send HTTP POST requests to API with valid access key. To reject these forged request a code can be sent to the user in the invitation message while each database has a bcrypt hash of the code along the access key. When an API receives a POST requests it can check if the code present in the request body matches with the hash he has in its database. So having the access key is not enough to send request, someone who steal a db cannot do anything with the data and cannot propagate false data to other APIs.

I recommend you read up some more on prior art and work before digging the design deeper.

Multiple parties maintaining their own APIs for transparency doesnt fully solves the trust issue — it just shifts it. Maybe letting clients broadcast directly to all parties (with signatures) could reduce that dependency.

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