Shutter Shielded Onchain Voting

This code demonstrates a proof-of-concept for a shielded onchain voting system using the Shutter API, a threshold encryption service by Shutter Network.

Credits to @williamhwgeorge for the protocol which does not require a trusted oracle during the vote reveal step.

Automation of the reveal step improves UX compared to typical 2-steps commitment/reveal protocols without additional trust assumption beyond Shutter Network.

Overview

The voting process consists of two steps: commitment and reveal.

sequenceDiagram
    %% Participants
    participant Voter as Voter
    participant Bot as Reveal Bot<br>(anyone)
    participant ShutterSDK as Shutter SDK
    participant Shutter as Shutter Network
    participant Contract as Voting Contract

    %% --- Vote Commitment Phase ---
    Note over Voter,Contract: Voting session opens
    Voter->>Shutter: register_identity(decryptionTimestamp)
    Shutter-->>Voter: {identity, eon_key}
    Voter->>Voter: generate random salt
    Voter->>ShutterSDK: encrypt(message: {choice, justification, salt}, identity, eon_key)
    ShutterSDK-->>Voter: {encryptedVote}
    Voter->>Voter: commitHash = keccak256(choice, justification, salt)
    Voter->>Contract: castCommit(voteIDs, commitHash, identity, encryptedVote)
    Contract-->>Contract: emits CommitCast(voter, voteIDs, commitHash, identity, encryptedVote)

    %% --- Vote Reveal Phase (after delay) ---
    Note over Voter,Contract: Voting session ends, Shutter Network releases decryption key
    Bot->>Contract: query CommitCast event({voter})
    Contract-->>Bot: CommitCast event(voter, voteIDs, commitHash, identity, encryptedVote)
    Bot->>ShutterSDK: decrypt(encryptedVote, identity)
    ShutterSDK->>Shutter: get_decryption_key(identity)
    Shutter-->>ShutterSDK: decryption_key
    ShutterSDK-->>Bot: {choice, justification, salt}
    Bot->>Contract: castVote(voteIDs, choice, salt, justification)
    Contract-->>Contract: verify vote hash against commitHash
    Contract-->>Contract: emits VoteCast(voteIDs, voter, choice, justification, salt)

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1. Vote commitment

A message is encrypted offchain by the Shutter Network consisting of the vote choice, the justification and a random salt. The Shutter network is instructed to release the decryption key at a later time once the voting session is over (after 10 seconds for this example).

The vote commitment is cast onchain by the voter and consists of the salted hash of the vote choice and a justification.

The vote remains hidden until the decryption key is released by the Shutter Network.

To enable later decryption, the Shutter identity and encrypted vote are emitted as part of CommitCast event.

2. Vote reveal

After the voting session is over, anyone may retrieve the decrypted message from the Shutter Network using the identity and encrypted vote emitted earlier.

Anyone in possession of the vote, justification and salt from the decrypted message may cast the vote onchain and effectively reveal it.

The contract computes the salted hash of the vote choice and a justification and compares it to the commitment. If they match, the vote is counted.

The vote is emitted as part of VoteCast event.

Demo

Implementation

• Voting contract: ShutterShieldedVoting.sol
• Voting commitment and reveal bot: shutterAutoVote.ts
• Shutter API wrapper: shutter.ts

Integration into a voting application

Typically the vote commitment step is performed by the user via a voting web application.

The vote reveal step is performed by a bot continuously watching for CommitCast events and submitting the vote onchain once the decryption key is released. Anyone watcher may participate in this step.

Getting started

Setup

yarn install
yarn compile

Execution

Shell 1: local node

yarn local-node

Shell 2: voting

yarn deploy
yarn auto-vote

Running again

The contract supports only a single voting session for simplicity so it must be redeployed. Simply stop the local node and re-run the above steps.