feat: As an attestation issuer, I would like to remain compatible with the ERC-721 standard

src/example/NFTPortal.sol

@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.21;
+
+import { IERC721, ERC721 } from "openzeppelin-contracts/contracts/token/ERC721/ERC721.sol";
+import { IERC165 } from "openzeppelin-contracts/contracts/utils/introspection/ERC165.sol";
+import { AbstractPortal } from "../interface/AbstractPortal.sol";
+import { Attestation, AttestationPayload } from "../types/Structs.sol";
+
+/**
+ * @title NFT Portal
+ * @author Consensys
+ * @notice This contract aims to provide ERC 721 compatibility
+ * @dev This Portal implements parts of ERC 721 - balanceOf and ownerOf functions
+ */
+contract NFTPortal is AbstractPortal, ERC721 {
+  mapping(bytes owner => uint256 numberOfAttestations) private numberOfAttestationsPerOwner;
+
+  constructor(
+    address[] memory modules,
+    address router
+  ) AbstractPortal(modules, router) ERC721("NFTPortal", "NFTPortal") {}
+
+  /** @notice Count all attestations assigned to an owner
+   * @param owner An address for whom to query the balance
+   * @return The number of attestations owned by `owner`, possibly zero
+   */
+  function balanceOf(address owner) public view virtual override returns (uint256) {
+    return numberOfAttestationsPerOwner[abi.encode(owner)];
+  }
+
+  /** @notice Find the owner of an attestation
+   * @param tokenId The identifier for an attestation
+   * @return The address of the owner of the attestation
+   */
+  function ownerOf(uint256 tokenId) public view virtual override returns (address) {
+    bytes32 attestationId = bytes32(tokenId);
+    Attestation memory attestation = attestationRegistry.getAttestation(attestationId);
+    return abi.decode(attestation.subject, (address));
+  }
+
+  /**
+   * @notice Method run before a payload is attested
+   * @param attestationPayload the attestation payload supposed to be attested
+   */
+  function _onAttest(AttestationPayload memory attestationPayload) internal override {
+    numberOfAttestationsPerOwner[attestationPayload.subject]++;
+  }
+
+  function withdraw(address payable to, uint256 amount) external override {}
+
+  /**
+   * @notice Verifies that a specific interface is implemented by the Portal, following ERC-165 specification
+   * @param interfaceID the interface identifier checked in this call
+   * @return The list of modules addresses linked to the Portal
+   */
+  function supportsInterface(bytes4 interfaceID) public pure virtual override(AbstractPortal, ERC721) returns (bool) {
+    return
+      interfaceID == type(AbstractPortal).interfaceId ||
+      interfaceID == type(IERC165).interfaceId ||
+      interfaceID == type(IERC721).interfaceId;
+  }
+}

test/example/NFTPortal.t.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.21;
+
+import { Test } from "forge-std/Test.sol";
+import { NFTPortal } from "../../src/example/NFTPortal.sol";
+import { Router } from "../../src/Router.sol";
+import { AbstractPortal } from "../../src/interface/AbstractPortal.sol";
+import { AttestationPayload } from "../../src/types/Structs.sol";
+import { AttestationRegistryMock } from "../mocks/AttestationRegistryMock.sol";
+import { ModuleRegistryMock } from "../mocks/ModuleRegistryMock.sol";
+import { IERC721 } from "openzeppelin-contracts/contracts/token/ERC721/ERC721.sol";
+import { IERC165 } from "openzeppelin-contracts/contracts/utils/introspection/ERC165.sol";
+
+contract NFTPortalTest is Test {
+  address public attester = makeAddr("attester");
+  NFTPortal public nftPortal;
+  address[] public modules = new address[](0);
+  ModuleRegistryMock public moduleRegistryMock = new ModuleRegistryMock();
+  AttestationRegistryMock public attestationRegistryMock = new AttestationRegistryMock();
+  Router public router = new Router();
+
+  event Initialized(uint8 version);
+  event AttestationRegistered();
+  event BulkAttestationsRegistered();
+
+  function setUp() public {
+    router.initialize();
+    router.updateModuleRegistry(address(moduleRegistryMock));
+    router.updateAttestationRegistry(address(attestationRegistryMock));
+
+    nftPortal = new NFTPortal(modules, address(router));
+  }
+
+  function test_balanceOf_ownerOf() public {
+    // Create attestation payload
+    AttestationPayload memory attestationPayload = AttestationPayload(
+      bytes32(uint256(1)),
+      uint64(block.timestamp + 1 days),
+      abi.encode(address(1)), // Convert address(1) to bytes and use as subject
+      new bytes(1)
+    );
+    // Create validation payload
+    bytes[] memory validationPayload = new bytes[](0);
+    vm.expectEmit(true, true, true, true);
+    emit AttestationRegistered();
+    nftPortal.attest(attestationPayload, validationPayload);
+    vm.expectEmit(true, true, true, true);
+    emit AttestationRegistered();
+    nftPortal.attest(attestationPayload, validationPayload);
+
+    uint256 balance = nftPortal.balanceOf(address(1));
+    assertEq(balance, 2);
+
+    address ownerOfFirstAttestation = nftPortal.ownerOf(1);
+    address ownerOfSecondAttestation = nftPortal.ownerOf(2);
+    assertEq(ownerOfFirstAttestation, address(1));
+    assertEq(ownerOfSecondAttestation, address(1));
+  }
+
+  function testSupportsInterface() public {
+    bool isIERC165Supported = nftPortal.supportsInterface(type(IERC165).interfaceId);
+    assertEq(isIERC165Supported, true);
+    bool isIERC721Supported = nftPortal.supportsInterface(type(IERC721).interfaceId);
+    assertEq(isIERC721Supported, true);
+    bool isEASAbstractPortalSupported = nftPortal.supportsInterface(type(AbstractPortal).interfaceId);
+    assertEq(isEASAbstractPortalSupported, true);
+  }
+}