feat(contracts): add standard way to call external verifiers and aggr…

…egate their logic

packages/contracts/contracts/src/AdvancedChecker.sol

@@ -2,19 +2,22 @@
 pragma solidity ^0.8.20;
 
 import {IAdvancedChecker, Check, CheckStatus} from "./interfaces/IAdvancedChecker.sol";
+import {Checker} from "./Checker.sol";
 
 /// @title AdvancedChecker.
 /// @notice Multi-phase validation checker with pre, main, and post checks.
 /// @dev Base contract for implementing complex validation logic with configurable check phases.
-abstract contract AdvancedChecker is IAdvancedChecker {
+abstract contract AdvancedChecker is IAdvancedChecker, Checker {
+    constructor(address[] memory _verifiers) Checker(_verifiers) {}
+
     /// @notice Entry point for validation checks.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @param checkType Type of check (PRE, MAIN, POST).
     /// @return checked Validation result.
     function check(
         address subject,
-        bytes memory evidence,
+        bytes[] calldata evidence,
         Check checkType
     ) external view override returns (bool checked) {
         return _check(subject, evidence, checkType);
@@ -26,7 +29,7 @@ abstract contract AdvancedChecker is IAdvancedChecker {
     /// @param evidence Validation data.
     /// @param checkType Check type to perform.
     /// @return checked Validation result.
-    function _check(address subject, bytes memory evidence, Check checkType) internal view returns (bool checked) {
+    function _check(address subject, bytes[] calldata evidence, Check checkType) internal view returns (bool checked) {
         if (checkType == Check.PRE) {
             return _checkPre(subject, evidence);
         }
@@ -43,19 +46,19 @@ abstract contract AdvancedChecker is IAdvancedChecker {
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @return checked Validation result.
-    function _checkPre(address subject, bytes memory evidence) internal view virtual returns (bool checked) {}
+    function _checkPre(address subject, bytes[] calldata evidence) internal view virtual returns (bool checked) {}
 
     /// @notice Main validation implementation.
     /// @dev Override to implement main check logic.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @return checked Validation result.
-    function _checkMain(address subject, bytes memory evidence) internal view virtual returns (bool checked) {}
+    function _checkMain(address subject, bytes[] calldata evidence) internal view virtual returns (bool checked) {}
 
     /// @notice Post-condition validation implementation.
     /// @dev Override to implement post-check logic.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @return checked Validation result.
-    function _checkPost(address subject, bytes memory evidence) internal view virtual returns (bool checked) {}
+    function _checkPost(address subject, bytes[] calldata evidence) internal view virtual returns (bool checked) {}
 }

packages/contracts/contracts/src/AdvancedPolicy.sol

@@ -43,7 +43,7 @@ abstract contract AdvancedPolicy is IAdvancedPolicy, Policy {
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @param checkType Type of check (PRE, MAIN, POST).
-    function enforce(address subject, bytes calldata evidence, Check checkType) external override onlyTarget {
+    function enforce(address subject, bytes[] calldata evidence, Check checkType) external override onlyTarget {
         _enforce(subject, evidence, checkType);
     }
 
@@ -59,7 +59,7 @@ abstract contract AdvancedPolicy is IAdvancedPolicy, Policy {
     /// @custom:throws PreCheckNotEnforced If PRE check is required but not done.
     /// @custom:throws MainCheckNotEnforced If MAIN check is required but not done.
     /// @custom:throws MainCheckAlreadyEnforced If multiple MAIN checks not allowed.
-    function _enforce(address subject, bytes calldata evidence, Check checkType) internal {
+    function _enforce(address subject, bytes[] calldata evidence, Check checkType) internal {
         if (!ADVANCED_CHECKER.check(subject, evidence, checkType)) {
             revert UnsuccessfulCheck();
         }

packages/contracts/contracts/src/BaseChecker.sol

@@ -2,18 +2,21 @@
 pragma solidity ^0.8.20;
 
 import {IBaseChecker} from "./interfaces/IBaseChecker.sol";
+import {Checker} from "./Checker.sol";
 
 /// @title BaseChecker
 /// @notice Abstract base contract for implementing validation checks.
 /// @dev Provides a standardized interface for implementing custom validation logic
 /// through the internal _check method.
-abstract contract BaseChecker is IBaseChecker {
+abstract contract BaseChecker is Checker, IBaseChecker {
+    constructor(address[] memory _verifiers) Checker(_verifiers) {}
+
     /// @notice Validates evidence for a given subject address.
     /// @dev External view function that delegates to internal _check implementation.
     /// @param subject Address to validate.
     /// @param evidence Custom validation data.
     /// @return checked Boolean indicating if the check passed.
-    function check(address subject, bytes memory evidence) external view override returns (bool checked) {
+    function check(address subject, bytes[] calldata evidence) external view override returns (bool checked) {
         return _check(subject, evidence);
     }
 
@@ -22,5 +25,5 @@ abstract contract BaseChecker is IBaseChecker {
     /// @param subject Address to validate.
     /// @param evidence Custom validation data.
     /// @return checked Boolean indicating if the check passed.
-    function _check(address subject, bytes memory evidence) internal view virtual returns (bool checked) {}
+    function _check(address subject, bytes[] calldata evidence) internal view virtual returns (bool checked) {}
 }

packages/contracts/contracts/src/BasePolicy.sol

@@ -35,7 +35,7 @@ abstract contract BasePolicy is Policy, IBasePolicy {
     /// @custom:throws AlreadyEnforced if check was previously enforced.
     /// @custom:throws UnsuccessfulCheck if the check fails.
     /// @custom:emits Enforced when check succeeds.
-    function enforce(address subject, bytes calldata evidence) external override onlyTarget {
+    function enforce(address subject, bytes[] calldata evidence) external override onlyTarget {
         _enforce(subject, evidence);
     }
 
@@ -45,7 +45,7 @@ abstract contract BasePolicy is Policy, IBasePolicy {
     /// @param evidence Additional data required for verification.
     /// @custom:throws AlreadyEnforced if already enforced for this subject.
     /// @custom:throws UnsuccessfulCheck if BASE_CHECKER.check returns false.
-    function _enforce(address subject, bytes calldata evidence) internal {
+    function _enforce(address subject, bytes[] memory evidence) internal {
         bool checked = BASE_CHECKER.check(subject, evidence);
 
         if (enforced[msg.sender][subject]) revert AlreadyEnforced();

packages/contracts/contracts/src/Checker.sol

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {IChecker} from "./interfaces/IChecker.sol";
+
+/// @title Checker
+/// @notice Abstract base contract for implementing attribute verification logic.
+/// @dev Provides infrastructure to orchestrate third-party verifiers for single checks.
+abstract contract Checker is IChecker {
+    /// @notice Array of third-party contract addresses used for verification.
+    /// @dev Can include existing and already deployed Checkers, NFTs, MACI polls, and/or any other contract
+    /// that provides evidence verification. These contracts should already be deployed and operational.
+    address[] internal verifiers;
+
+    /// @notice Initializes the Checker with an optional list of third-party verification contracts.
+    /// @param _verifiers Array of addresses for existing verification contracts.
+    /// @dev Each address should point to a deployed contract that will be consulted during verification.
+    /// This array can remain empty if there's no reliance on external verifiers.
+    constructor(address[] memory _verifiers) {
+        verifiers = _verifiers;
+    }
+
+    /// @notice Retrieves the list of third-party verifiers' addresses.
+    /// @return Array of addresses for the necessary verification contracts.
+    function getVerifiers() internal view returns (address[] memory) {
+        return verifiers;
+    }
+
+    /// @notice Retrieves the verifier address at a specific index.
+    /// @param index The index of the verifier in the array.
+    /// @return The address of the verifier at the specified index.
+    /// @custom:throws VerifierNotFound if no address have been specified at given index.
+    function getVerifierAtIndex(uint256 index) internal view returns (address) {
+        if (index >= verifiers.length) revert VerifierNotFound();
+
+        return verifiers[index];
+    }
+}

packages/contracts/contracts/src/interfaces/IAdvancedChecker.sol

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.20;
 
+import {IChecker} from "./IChecker.sol";
+
 /// @title Check.
 /// @notice Defines validation phases in the AdvancedChecker system.
 /// @custom:values PRE - Pre-condition validation.
@@ -26,11 +28,11 @@ struct CheckStatus {
 /// @title IAdvancedChecker.
 /// @notice Defines multi-phase validation system interface.
 /// @dev Implement this for custom validation logic with pre/main/post checks.
-interface IAdvancedChecker {
+interface IAdvancedChecker is IChecker {
     /// @notice Validates subject against specified check type.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @param checkType Check phase to execute.
     /// @return checked True if validation passes.
-    function check(address subject, bytes calldata evidence, Check checkType) external view returns (bool checked);
+    function check(address subject, bytes[] calldata evidence, Check checkType) external view returns (bool checked);
 }

packages/contracts/contracts/src/interfaces/IAdvancedPolicy.sol

@@ -27,12 +27,12 @@ interface IAdvancedPolicy is IPolicy {
     /// @param target Protected contract address.
     /// @param evidence Validation data.
     /// @param checkType Type of check performed.
-    event Enforced(address indexed subject, address indexed target, bytes evidence, Check checkType);
+    event Enforced(address indexed subject, address indexed target, bytes[] evidence, Check checkType);
 
     /// @notice Enforces validation check on subject.
     /// @dev Delegates to appropriate check method based on checkType.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @param checkType Check phase to execute.
-    function enforce(address subject, bytes calldata evidence, Check checkType) external;
+    function enforce(address subject, bytes[] calldata evidence, Check checkType) external;
 }

packages/contracts/contracts/src/interfaces/IBaseChecker.sol

@@ -1,12 +1,14 @@
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.20;
 
+import {IChecker} from "./IChecker.sol";
+
 /// @title IBaseChecker.
 /// @notice Defines base validation functionality.
-interface IBaseChecker {
+interface IBaseChecker is IChecker {
     /// @notice Validates subject against evidence.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
     /// @return checked True if validation passes.
-    function check(address subject, bytes calldata evidence) external view returns (bool checked);
+    function check(address subject, bytes[] calldata evidence) external view returns (bool checked);
 }

packages/contracts/contracts/src/interfaces/IBasePolicy.sol

@@ -10,10 +10,10 @@ interface IBasePolicy is IPolicy {
     /// @param subject Address that passed validation.
     /// @param target Protected contract address.
     /// @param evidence Validation data.
-    event Enforced(address indexed subject, address indexed target, bytes evidence);
+    event Enforced(address indexed subject, address indexed target, bytes[] evidence);
 
     /// @notice Enforces validation check on subject.
     /// @param subject Address to validate.
     /// @param evidence Validation data.
-    function enforce(address subject, bytes calldata evidence) external;
+    function enforce(address subject, bytes[] calldata evidence) external;
 }

packages/contracts/contracts/src/interfaces/IChecker.sol

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+/// @title IChecker
+/// @notice Core checker interface for attribute verification functionalities.
+interface IChecker {
+    /// @notice Core error conditions.
+    error VerifierNotFound();
+}