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ZK-SNARK Circuits for Sensor Privacy

This document outlines the design and implementation of the ZK-SNARK circuits used by the Utility-Drip system to preserve sensor data privacy.

Overview

The goal is to allow a hardware device (meter) to prove it has consumed a specific amount of energy/water without revealing the raw, granular sensor readings. The contract verifies this proof and deducts the appropriate balance.

Circuit Specification (Circom)

The circuit is implemented in Circom and uses the Groth16 proof system.

Private Inputs (Witness)

  • usage_raw: The raw, high-precision sensor reading.
  • salt: A random salt to ensure commitment privacy.
  • last_usage: The previous raw reading stored locally on the device.

Public Inputs

  • units_consumed: The calculated units to be billed (e.g., (usage_raw - last_usage) * rate).
  • is_peak_hour: Whether the current time is a peak hour.
  • nullifier: A unique value to prevent proof replay.
  • commitment: A hash of the current state.

Constraints

  1. Integrity: units_consumed must be correctly calculated from the change in raw usage.
  2. Range Proof: units_consumed must be within a valid range (e.g., < 1,000,000).
  3. Commitment: commitment == Poseidon(usage_raw, salt).
  4. Nullifier: nullifier == Poseidon(last_usage, salt).

Proving & Verification Flow

  1. Hardware Device:
    • Reads sensor data.
    • Generates a Groth16 proof using the local witness.
    • Submits the proof and public inputs to the contract via submit_zk_usage_report.
  2. Smart Contract:
    • Uses native Soroban BN254 host functions (pairing_check, g1_add, g1_mul) to verify the proof.
    • Verifies the nullifier hasn't been used before.
    • Deducts the balance based on the verified units_consumed.

Optimization for Soroban

To stay within the ledger's instruction limits, the verifier is optimized by:

  • Using pre-computed components in the Verification Key.
  • Utilizing optimized host functions for all elliptic curve operations.
  • Avoiding expensive big-integer arithmetic in WASM guest code.

Key Files

  • contracts/utility_contracts/src/lib.rs: Contains the verify_groth16_proof logic.
  • meter-simulator/src/meter-device.js: Simulates the proving process for testing.

Deployment

  1. Generate the Verification Key (verification_key.json) using snarkjs.
  2. Format the key for Soroban (Big-Endian bytes).
  3. Call set_zk_verification_key on the contract to register the key for your meter.