Privacy-Preserving Voting Smart Contract

A Clarity smart contract implementing anonymous voting through a commitment-reveal scheme on the Stacks blockchain.

Overview

This contract enables privacy-preserving polls where voters commit to their choices cryptographically before revealing them, preventing vote manipulation and maintaining anonymity during the voting process.

Key Features

🔒 Privacy Protection

• Commit-Reveal Scheme: Two-phase voting prevents vote observation during commit phase
• Cryptographic Commitments: SHA-256 hashing with salt prevents vote tampering
• Anonymous Voting: Vote choices hidden until reveal phase begins

🗳️ Three-Phase Voting Process

1. Registration Phase: Voters register for specific polls
2. Commit Phase: Voters submit hashed votes (commitment)
3. Reveal Phase: Voters prove their votes with salt verification

🛡️ Security Features

• Input validation for all user data
• Double-voting prevention
• Time-locked phases using block heights
• Registered voters only
• Emergency shutdown capability

Contract Functions

Public Functions

create-poll

Creates a new poll with multiple options.

(create-poll 
  (title (string-ascii 100))
  (description (string-ascii 500))
  (options (list 10 (string-ascii 200)))
  (commit-duration uint)
  (reveal-duration uint))

Parameters:

• title: Poll title (1-100 characters)
• description: Poll description (1-500 characters)
• options: List of 2-10 voting options (each 1-200 characters)
• commit-duration: Number of blocks for commit phase
• reveal-duration: Number of blocks for reveal phase

Returns: Poll ID

---

register-voter

Register to participate in a poll.

(register-voter (poll-id uint))

Requirements:

• Poll must be active
• Must register before commit phase ends
• Cannot register twice

---

commit-vote

Submit a cryptographic commitment of your vote.

(commit-vote (poll-id uint) (commitment-hash (buff 32)))

Commitment Hash Calculation:

hash = SHA256(option-id + salt + voter-principal)

Requirements:

• Must be registered voter
• Only during commit phase
• Cannot commit twice
• Hash must be exactly 32 bytes

---

reveal-vote

Reveal your vote with proof.

(reveal-vote 
  (poll-id uint) 
  (option-id uint) 
  (salt (buff 32)))

Requirements:

• Must have committed in commit phase
• Only during reveal phase
• Hash must match commitment
• Cannot reveal twice

---

close-poll

Close the poll after reveal phase ends (creator only).

(close-poll (poll-id uint))

---

emergency-close-poll

Emergency shutdown (contract owner only).

(emergency-close-poll (poll-id uint))

Read-Only Functions

• get-poll (poll-id uint): Get poll details
• get-poll-option (poll-id uint) (option-id uint): Get option details and vote count
• get-commitment (poll-id uint) (voter principal): Get voter's commitment
• is-registered (poll-id uint) (voter principal): Check registration status
• get-vote-reveal (poll-id uint) (voter principal): Get revealed vote
• get-poll-counter: Get total number of polls
• is-commit-phase (poll-id uint): Check if in commit phase
• is-reveal-phase (poll-id uint): Check if in reveal phase

Usage Example

1. Create a Poll

(contract-call? .voting-contract create-poll
  "Best Programming Language"
  "Vote for your favorite programming language"
  (list "Clarity" "Solidity" "Rust" "JavaScript")
  u144  ;; ~1 day commit phase
  u144) ;; ~1 day reveal phase

2. Register as Voter

(contract-call? .voting-contract register-voter u1)

3. Generate Commitment

Off-chain calculation:

const optionId = 2; // Voting for option 2
const salt = generateRandom32Bytes();
const voter = "ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM";
const commitment = sha256(optionId + salt + voter);

4. Commit Vote

(contract-call? .voting-contract commit-vote 
  u1 
  0x<commitment-hash>)

5. Reveal Vote

(contract-call? .voting-contract reveal-vote 
  u1 
  u2 
  0x<original-salt>)

6. Check Results

(contract-call? .voting-contract get-poll-option u1 u2)
;; Returns: {description: "Solidity", vote-count: u5}

Error Codes

Code	Error	Description
u100	err-owner-only	Only contract owner can perform action
u101	err-not-authorized	Caller not authorized
u102	err-poll-not-found	Poll does not exist
u103	err-poll-closed	Poll is not active
u104	err-already-voted	Already committed/revealed
u105	err-invalid-option	Invalid option ID
u106	err-reveal-phase-not-started	Still in commit phase
u107	err-commitment-not-found	No commitment found
u108	err-invalid-reveal	Hash verification failed
u109	err-poll-active	Poll still active
u110	err-already-registered	Already registered
u111	err-invalid-options-count	Invalid options list
u112	err-invalid-duration	Invalid duration value
u113	err-invalid-input	Input validation failed

Security Considerations

✅ Protections Implemented

• Input Validation: All user inputs validated before processing
• Cryptographic Verification: SHA-256 ensures vote integrity
• Time-Locking: Block-height-based phases prevent timing attacks
• Access Control: Registration and authorization checks
• State Management: Prevents double voting and unauthorized actions

⚠️ Important Notes

• Salt Security: Keep salt secret until reveal phase
• Gas Costs: Revealing votes requires transaction fees
• Block Timing: Phase durations are in blocks (~10 min per block on Stacks)
• Commitment Storage: Store commitment details off-chain for reveal

Deployment

1. Deploy contract to Stacks blockchain
2. Contract deployer becomes owner
3. Create polls with appropriate durations
4. Share poll ID with voters

Testing Recommendations

• Test all three phases sequentially
• Verify commitment hash calculations
• Test error conditions (double voting, wrong phase, etc.)
• Validate time-lock enforcement
• Test emergency shutdown functionality