ZKProofs.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.30;

// =============================================================================
// ZK PROOFS & ZK COPROCESSORS IN SMART CONTRACTS
// =============================================================================
// Zero-Knowledge Proofs (ZKPs) allow one party to prove a statement is true
// without revealing any information beyond the truth of the statement.
//
// For smart contracts, ZKPs enable:
//   1. PRIVACY — prove you know a secret without revealing it
//   2. SCALABILITY — prove off-chain computation was done correctly (ZK rollups)
//   3. ZK COPROCESSORS — prove facts about historical blockchain state off-chain
//   4. IDENTITY — prove you meet criteria (age, credit score) without revealing data
//   5. COMPLIANCE — prove regulatory compliance without exposing business data
//
// Types of ZK proof systems (2025-2026):
//   - Groth16  : smallest proof, fastest verify, requires trusted setup per circuit
//   - PLONK    : universal trusted setup (e.g., Aztec's ceremony), slightly larger proof
//   - STARKs   : no trusted setup, quantum-resistant, larger proof, faster prove
//   - Halo2    : no trusted setup, recursive proofs, used by Zcash & Scroll
//
// Key frameworks:
//   - circom + snarkjs   : circuit DSL + JS prover/verifier generation
//   - gnark (Go)         : fast Groth16/PLONK prover
//   - SP1 (Succinct)     : ZK proof of any Rust program (RISC-V)
//   - RISC Zero          : similar to SP1, with Bonsai proving service
//   - Noir (Aztec)       : high-level ZK DSL, Barretenberg backend
//   - Halo2 (ZCash/PSE)  : research-grade, used by Scroll/Taiko
//
// ZK Coprocessors (compute over historical chain data):
//   - Axiom   : prove facts about historical Ethereum state (via storage proofs)
//   - Lagrange : coprocessor network with aggregation
//   - Brevis   : ZK coprocessor with Solidity SDK
//   - Herodotus : storage proofs across chains
// =============================================================================

// =============================================================================
// SECTION 1 — GROTH16 VERIFIER INTERFACE
// =============================================================================
// Groth16 is the most widely deployed ZK proof system on Ethereum.
// The verifier contract is generated by circom/snarkjs and deployed once per circuit.
// Your application contract calls it to verify a proof.

// Standard Groth16 verifier interface (generated by snarkjs)
interface IGroth16Verifier {
    function verifyProof(
        uint256[2]  calldata pA,    // proof.pi_a
        uint256[2][2] calldata pB,  // proof.pi_b
        uint256[2]  calldata pC,    // proof.pi_c
        uint256[]   calldata pubSignals  // public inputs/outputs
    ) external view returns (bool);
}

// =============================================================================
// SECTION 2 — PLONK/UltraPLONK VERIFIER INTERFACE
// =============================================================================
// PLONK verifiers use a single trusted setup (powers-of-tau ceremony).
// The interface differs slightly from Groth16.

interface IPlonkVerifier {
    function verify(
        bytes calldata proof,
        uint256[] calldata publicInputs
    ) external view returns (bool);
}

// =============================================================================
// SECTION 3 — GENERIC ZK VERIFIER WRAPPER
// =============================================================================
// A unified interface your contracts use, abstracting the proof system underneath.
// This lets you swap Groth16 → PLONK → STARK without changing your business logic.

interface IZKVerifier {
    function verify(bytes calldata proof, bytes calldata publicInputs) external view returns (bool);
}

// Adapter: wraps a Groth16 verifier under the generic IZKVerifier interface
contract Groth16VerifierAdapter is IZKVerifier {
    IGroth16Verifier public immutable verifier;
    uint256 public immutable publicInputCount;

    constructor(address _verifier, uint256 _publicInputCount) {
        verifier = IGroth16Verifier(_verifier);
        publicInputCount = _publicInputCount;
    }

    // proof encoding: abi.encode(pA[2], pB[2][2], pC[2])
    // publicInputs encoding: abi.encode(uint256[N])
    function verify(bytes calldata proof, bytes calldata publicInputs)
        external
        view
        override
        returns (bool)
    {
        (
            uint256[2]    memory pA,
            uint256[2][2] memory pB,
            uint256[2]    memory pC
        ) = abi.decode(proof, (uint256[2], uint256[2][2], uint256[2]));

        uint256[] memory signals = abi.decode(publicInputs, (uint256[]));
        require(signals.length == publicInputCount, "Wrong public input count");

        return verifier.verifyProof(pA, pB, pC, signals);
    }
}

// =============================================================================
// SECTION 4 — ZK PROOF OF MEMBERSHIP (WHITELIST WITHOUT REVEALING WHO)
// =============================================================================
// Classic use case: prove you're in a set of N addresses without revealing which one.
// Circuit: "I know a secret s such that MerkleProof(s, root) is valid"
// On-chain: check the proof, use the nullifier to prevent double-use.

contract ZKWhitelist {
    IZKVerifier public immutable verifier;

    // Merkle root of whitelisted addresses (built off-chain by the admin)
    bytes32 public merkleRoot;
    // Nullifiers prevent the same membership proof from being used twice
    mapping(bytes32 => bool) public usedNullifiers;

    address public owner;

    event ProofVerified(bytes32 nullifier, address indexed user);
    event MerkleRootUpdated(bytes32 newRoot);

    error InvalidProof();
    error NullifierAlreadyUsed();

    constructor(address _verifier) {
        verifier = IZKVerifier(_verifier);
        owner = msg.sender;
    }

    function updateMerkleRoot(bytes32 newRoot) external {
        require(msg.sender == owner, "Not owner");
        merkleRoot = newRoot;
        emit MerkleRootUpdated(newRoot);
    }

    // User provides a ZK proof that their address is in the whitelist.
    // The proof contains:
    //   Public inputs:  [merkleRoot, nullifier]
    //   Private inputs: [secretKey, merklePathSiblings...]
    // The circuit proves: hash(secretKey) = leaf ∈ tree(merkleRoot)
    function proveAndAct(
        bytes calldata proof,
        bytes32 nullifier,
        bytes32 claimedRoot
    ) external {
        // Verify the claimed root matches current root
        require(claimedRoot == merkleRoot, "Root mismatch");
        // Prevent double-use of same membership proof
        if (usedNullifiers[nullifier]) revert NullifierAlreadyUsed();

        // Public inputs: [merkleRoot (as uint256), nullifier (as uint256)]
        bytes memory publicInputs = abi.encode(
            uint256(claimedRoot),
            uint256(nullifier)
        );

        if (!verifier.verify(proof, publicInputs)) revert InvalidProof();

        usedNullifiers[nullifier] = true;
        emit ProofVerified(nullifier, msg.sender);

        // Proceed with the action (transfer, access, etc.)
        _grantAccess(msg.sender);
    }

    function _grantAccess(address user) internal virtual {
        // Override in subclass to define what "access" means
    }
}

// =============================================================================
// SECTION 5 — ZK AGE VERIFICATION (PRIVACY-PRESERVING KYC)
// =============================================================================
// Prove you're over 18 without revealing your birthdate or identity.
// Circuit: "I know a signed credential that certifies I am over 18"
// Public inputs: [issuerPublicKey, currentTimestamp, nullifier]

contract ZKAgeVerification {
    IZKVerifier public immutable verifier;

    // Trusted credential issuers (e.g., government ID provider)
    mapping(address => bool) public trustedIssuers;
    // Per-user nullifiers (user can only verify once per issuer claim)
    mapping(bytes32 => bool) public verifiedNullifiers;
    // Users who have passed age verification
    mapping(address => bool) public ageVerified;

    address public owner;

    event AgeVerified(address indexed user, bytes32 nullifier);

    error InvalidProof();
    error NullifierUsed();
    error UntrustedIssuer();

    constructor(address _verifier) {
        verifier = IZKVerifier(_verifier);
        owner = msg.sender;
    }

    function trustIssuer(address issuer) external {
        require(msg.sender == owner, "Not owner");
        trustedIssuers[issuer] = true;
    }

    // Prove age ≥ 18 using a ZK proof of a signed credential.
    // The circuit verifies:
    //   1. credential.birthdate signed by trustedIssuer
    //   2. currentDate - credential.birthdate >= 18 years
    //   3. nullifier = hash(userSecret, credentialId) — user-specific, non-replayable
    // None of the private data (birthdate, credential, userSecret) is revealed.
    function verifyAge(
        bytes calldata proof,
        address issuer,
        bytes32 nullifier
    ) external {
        if (!trustedIssuers[issuer]) revert UntrustedIssuer();
        if (verifiedNullifiers[nullifier]) revert NullifierUsed();

        // Public inputs: [issuerAddress, currentTimestamp, nullifier, minAge=18]
        bytes memory publicInputs = abi.encode(
            uint256(uint160(issuer)),
            block.timestamp,
            uint256(nullifier),
            uint256(18)
        );

        if (!verifier.verify(proof, publicInputs)) revert InvalidProof();

        verifiedNullifiers[nullifier] = true;
        ageVerified[msg.sender] = true;

        emit AgeVerified(msg.sender, nullifier);
    }
}

// =============================================================================
// SECTION 6 — ZK COPROCESSOR: AXIOM-STYLE HISTORICAL STATE PROOFS
// =============================================================================
// Axiom lets you prove facts about any historical Ethereum block/state/tx.
// Example: "this address held ≥ 1000 tokens at block N" — without re-syncing
// the entire chain. The proof is generated off-chain using Axiom SDK,
// then verified on-chain via their AxiomV2Query contract.

// Simplified Axiom callback interface
interface IAxiomV2Client {
    function axiomV2Callback(
        uint64 sourceChainId,
        address callerAddress,
        bytes32 querySchema,
        uint256 queryId,
        bytes32[] calldata axiomResults, // the proven values
        bytes calldata extraData
    ) external;
}

// Query schema encoding for Axiom circuits
// Schema = keccak256(circuitId, circuitVersion)

contract AxiomHistoricalProver is IAxiomV2Client {
    address public immutable axiomV2QueryAddress;
    bytes32 public immutable querySchema; // identifies our specific circuit

    // Proven historical balances: user => blockNumber => balance
    mapping(address => mapping(uint256 => uint256)) public provenBalance;

    event HistoricalBalanceProven(
        address indexed user,
        uint256 blockNumber,
        uint256 balance
    );

    error NotAxiom();
    error WrongSchema();

    constructor(address _axiomQuery, bytes32 _schema) {
        axiomV2QueryAddress = _axiomQuery;
        querySchema = _schema;
    }

    // Called by Axiom after the ZK proof is verified on their contract.
    // axiomResults contains the proven output values from the circuit.
    function axiomV2Callback(
        uint64 sourceChainId,
        address callerAddress,
        bytes32 _querySchema,
        uint256 queryId,
        bytes32[] calldata axiomResults,
        bytes calldata extraData
    ) external override {
        // Only Axiom's contract can call this
        if (msg.sender != axiomV2QueryAddress) revert NotAxiom();
        if (_querySchema != querySchema)        revert WrongSchema();

        // Decode results from the circuit's public outputs
        // Our circuit outputs: [userAddress, blockNumber, tokenBalance]
        address user        = address(uint160(uint256(axiomResults[0])));
        uint256 blockNumber = uint256(axiomResults[1]);
        uint256 balance     = uint256(axiomResults[2]);

        provenBalance[user][blockNumber] = balance;
        emit HistoricalBalanceProven(user, blockNumber, balance);
    }

    // Application logic: gate access based on proven historical balance
    function claimRewardIfEarlyHolder(uint256 blockNumber) external {
        uint256 balance = provenBalance[msg.sender][blockNumber];
        require(balance >= 1000e18, "Was not an early holder with enough tokens");
        // Distribute reward...
    }
}

// =============================================================================
// SECTION 7 — RISC ZERO / SP1 STYLE: PROVE ARBITRARY COMPUTATION
// =============================================================================
// SP1 and RISC Zero can generate ZK proofs of arbitrary Rust/C programs.
// Example: prove that a complex sorting or ML inference was done correctly.
// The verifier on-chain is a universal verifier — one contract for all programs.

// SP1 verifier interface (simplified)
interface ISP1Verifier {
    function verifyProof(
        bytes32 programVKey,  // identifies the specific Rust program
        bytes calldata publicValues,
        bytes calldata proofBytes
    ) external view;
}

// RISC Zero verifier interface (simplified)
interface IRiscZeroVerifier {
    function verify(
        bytes calldata seal,    // the proof
        bytes32 imageId,       // identifies the guest program
        bytes32 journalDigest  // hash of public outputs
    ) external view;
}

// Example: use SP1 to prove off-chain sorting of 10,000 elements
// then use the proven sorted output on-chain
contract SP1SortingProver {
    ISP1Verifier public immutable sp1Verifier;
    bytes32 public immutable SORT_PROGRAM_VKEY; // hash of the Rust sort binary

    // Proven sorted arrays stored on-chain
    mapping(bytes32 => uint256[]) public provenSortedArrays;

    event SortProven(bytes32 indexed inputHash, uint256[] sortedArray);

    constructor(address _verifier, bytes32 _vkey) {
        sp1Verifier = ISP1Verifier(_verifier);
        SORT_PROGRAM_VKEY = _vkey;
    }

    // Submit a ZK proof that a given array was correctly sorted.
    // publicValues = abi.encode(inputHash, sortedArray)
    function submitSortProof(
        bytes calldata publicValues,
        bytes calldata proof
    ) external {
        // SP1 verifies the proof against our sort program's vkey
        sp1Verifier.verifyProof(SORT_PROGRAM_VKEY, publicValues, proof);

        // Decode the proven public values
        (bytes32 inputHash, uint256[] memory sortedArray) =
            abi.decode(publicValues, (bytes32, uint256[]));

        provenSortedArrays[inputHash] = sortedArray;
        emit SortProven(inputHash, sortedArray);
    }
}

// =============================================================================
// SECTION 8 — NULLIFIER PATTERN (PREVENT DOUBLE-SPEND/DOUBLE-USE)
// =============================================================================
// Nullifiers are the key primitive for ZK privacy systems.
// A nullifier is a deterministic value derived from a secret, used to
// mark a "note" or "credential" as spent without revealing which one.
//
// Used in: Tornado Cash, Zcash, Aztec, ZK airdrop systems

contract NullifierRegistry {
    // Global nullifier set — once spent, cannot be spent again
    mapping(bytes32 => bool) public nullifiers;

    // Each ZK application registers a "topic" to namespace its nullifiers
    mapping(address => bool) public registeredApps;
    address public owner;

    event NullifierSpent(bytes32 indexed nullifier, address indexed app);

    error NullifierAlreadySpent();
    error AppNotRegistered();

    constructor() {
        owner = msg.sender;
    }

    function registerApp(address app) external {
        require(msg.sender == owner, "Not owner");
        registeredApps[app] = true;
    }

    // ZK application marks a nullifier as spent after verifying proof
    function spendNullifier(bytes32 nullifier) external {
        if (!registeredApps[msg.sender]) revert AppNotRegistered();
        if (nullifiers[nullifier]) revert NullifierAlreadySpent();
        nullifiers[nullifier] = true;
        emit NullifierSpent(nullifier, msg.sender);
    }

    function isSpent(bytes32 nullifier) external view returns (bool) {
        return nullifiers[nullifier];
    }
}

// =============================================================================
// SECTION 9 — ZK AIRDROP (PROVE ELIGIBILITY WITHOUT REVEALING ADDRESS)
// =============================================================================
// Airdrop where: merkle tree of eligible addresses is public,
// but each claim doesn't reveal WHICH address is claiming.
// The ZK proof shows: "I know an address in the tree and the secret for it."
// The nullifier = hash(secret) prevents double-claiming.

contract ZKAirdrop {
    IZKVerifier public immutable verifier;
    bytes32 public immutable merkleRoot;
    NullifierRegistry public immutable nullifierRegistry;

    address public immutable rewardToken;
    uint256 public constant REWARD_AMOUNT = 1000e18;

    mapping(address => bool) public claimed; // by receiving address

    event Claimed(address indexed to, bytes32 nullifier);

    error InvalidProof();
    error AlreadyClaimed();

    constructor(
        address _verifier,
        bytes32 _merkleRoot,
        address _nullifierRegistry,
        address _rewardToken
    ) {
        verifier = IZKVerifier(_verifier);
        merkleRoot = _merkleRoot;
        nullifierRegistry = NullifierRegistry(_nullifierRegistry);
        rewardToken = _rewardToken;
    }

    // Claim airdrop with ZK proof.
    // Proof shows: I own a secret s such that leaf(s) is in tree(merkleRoot).
    // Nullifier = hash(s, "airdrop_v1") — unique per claim, hides identity.
    // The caller can be ANY address — no link between claimer and eligible address.
    function claim(
        address recipient,
        bytes32 nullifier,
        bytes calldata proof
    ) external {
        // Public inputs: [merkleRoot, nullifier]
        bytes memory publicInputs = abi.encode(uint256(merkleRoot), uint256(nullifier));

        if (!verifier.verify(proof, publicInputs)) revert InvalidProof();

        // Spend nullifier — prevents double claiming
        nullifierRegistry.spendNullifier(nullifier);

        // Send reward to recipient (could be a fresh wallet for privacy)
        IERC20(rewardToken).transfer(recipient, REWARD_AMOUNT);

        emit Claimed(recipient, nullifier);
    }
}

interface IERC20 {
    function transfer(address to, uint256 amount) external returns (bool);
}

// =============================================================================
// SECTION 10 — PLONKY2/RECURSIVE PROOFS PATTERN
// =============================================================================
// Recursive proofs: one proof aggregates N proofs into 1.
// Used by: Polygon zkEVM, Scroll, zkSync Era, Aztec.
// On-chain verifier only needs to verify the single aggregated proof.

interface IRecursiveVerifier {
    // Verify an aggregated proof that covers N individual proofs
    function verifyAggregated(
        bytes calldata aggregatedProof,
        bytes32[] calldata individualPublicInputHashes
    ) external view returns (bool);
}

contract BatchZKProcessor {
    IRecursiveVerifier public immutable verifier;
    uint256 public processedBatches;

    event BatchProcessed(uint256 batchId, uint256 count);

    constructor(address _verifier) {
        verifier = IRecursiveVerifier(_verifier);
    }

    // Process N user actions in one tx by verifying the aggregated proof.
    // Gas cost: O(1) instead of O(N) if each action required its own proof.
    function processBatch(
        bytes calldata aggregatedProof,
        bytes32[] calldata actionHashes,  // public inputs for each action
        bytes[] calldata encodedActions   // decoded actions to execute
    ) external {
        require(actionHashes.length == encodedActions.length, "Length mismatch");

        // Single verification covers all N actions
        require(
            verifier.verifyAggregated(aggregatedProof, actionHashes),
            "Batch proof invalid"
        );

        // Execute each proven action
        for (uint256 i = 0; i < encodedActions.length; i++) {
            _executeAction(encodedActions[i]);
        }

        uint256 batchId = ++processedBatches;
        emit BatchProcessed(batchId, encodedActions.length);
    }

    function _executeAction(bytes memory actionData) internal virtual {
        // Override in subclass
    }
}

// =============================================================================
// SECTION 11 — ZK PROOF IN PRACTICE: WORKFLOW SUMMARY
// =============================================================================
//
// 1. CIRCUIT DESIGN (off-chain, e.g., circom or Rust for SP1)
//    - Define private inputs (secrets) and public inputs (verifiable outputs)
//    - Encode the computation as arithmetic constraints (R1CS, PLONKish)
//    - Test with unit inputs before generating keys
//
// 2. TRUSTED SETUP (for Groth16/PLONK — not needed for STARKs)
//    - Powers-of-Tau ceremony (public, done once)
//    - Circuit-specific setup (Groth16: per circuit; PLONK: universal)
//    - Store proving key (prover uses) and verification key (verifier uses)
//
// 3. VERIFIER DEPLOYMENT (on-chain)
//    - snarkjs generates a Solidity verifier from the verification key
//    - Deploy the verifier contract once per circuit
//    - Your application contract calls verifier.verifyProof(...)
//
// 4. PROOF GENERATION (off-chain, user's device or proving service)
//    - User provides private inputs + public inputs to the prover
//    - Prover runs the circuit and outputs: proof + public signals
//    - Submit proof + public signals to the on-chain application contract
//
// 5. ON-CHAIN VERIFICATION
//    - Application contract calls IZKVerifier.verify(proof, publicInputs)
//    - If verified: execute business logic (transfer, access grant, etc.)
//    - Spend nullifier to prevent replay

// =============================================================================
// PROFESSIONAL CHECKLIST: ZK PROOF INTEGRATION AUDIT POINTS
// =============================================================================
//
// [ ] Verifier address: immutable or protected from replacement (proxy attack)
// [ ] Public inputs: all application-specific values are in publicInputs (not hidden)
// [ ] Nullifiers: every privacy-preserving action has a nullifier to prevent replay
// [ ] Root/schema versioning: old proofs for old roots should not be accepted
// [ ] Trusted setup: verify the ceremony was public and the toxic waste destroyed
// [ ] Circuit scope: the circuit ONLY proves what you think it proves — audit it
// [ ] Proof malleability: Groth16 proofs are malleable; use snarkjs with proper encoding
// [ ] Front-running: proof + nullifier submission can be front-run; add msg.sender to public inputs
// [ ] Input encoding: match abi.encode layout with circuit field element order exactly
// [ ] Gas cost: Groth16 on-chain verify ~250k gas; PLONK ~500k; STARK ~1M+ — plan accordingly
// [ ] Fallback: always have an admin key or governance bypass if ZK system has bugs
// [ ] Coprocessor trust: Axiom/SP1 proofs rely on the prover's integrity — check audits