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eip title author discussions-to status type category created
820 Pseudo-introspection Registry Contract Jordi Baylina <jordi@baylina.cat>, Jacques Dafflon <jacques.dafflon@gmail.com> https://github.com/ethereum/EIPs/issues/820 Draft Standards Track ERC 2018-01-05

Simple Summary

This standard defines a universal registry smart contract where any address (contract or regular account) can register which interface it supports and which smart contract is responsible for its implementation.

This standard keeps backwards compatibility with ERC165.

Abstract

This standard attempts to define a registry where smart contracts and regular accounts can publish which functionalities they implement—either directly or through a proxy contract .

The rest of the world can query this registry to ask if a specific address implements a given interface and which smart contract handles its implementation.

This registry MAY be deployed on any chain and will share the exact same address.

Interfaces with zeroes (0) as the last 28 bytes are considered ERC165 interfaces, and this registry SHALL forward the call to the contract to see if it implements the interface.

This contract also acts as an ERC165 cache to reduce gas consumption.

Motivation

There has been different approaches to define pseudo-introspection in Ethereum. The first is ERC165 which has the limitation that it cannot be used by regular accounts. The second attempt is ERC672 which uses reverseENS. Using reverseENS has two issues. First, it is unnecessarily complex, and second, ENS is still a centralized contract controlled by a multisig. This multisig, theoretically would be able to modify the system.

This standard is much simpler than ERC672 and it is fully decentralized.

This standard also provides a unique address for all chains. Thus solving the problem of resolving the correct registry address for different chains.

Specification

ERC820 Registry Smart Contract

This is an exact copy of the code of the ERC820 registry smart contract.

/* ERC820: Pseudo-introspection Registry Contract
 * by Jordi Baylina and Jacques Dafflon
 *
 * To the extent possible under law, Jordi Baylina and Jacques Dafflon who
 * associated CC0 with the ERC820: Pseudo-introspection Registry Contract have
 * waived all copyright and related or neighboring rights to the
 * ERC820: Pseudo-introspection Registry Contract.
 *
 * You should have received a copy of the CC0 legalcode along with this work.
 * If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 *
 *    ███████╗██████╗  ██████╗ █████╗ ██████╗  ██████╗
 *    ██╔════╝██╔══██╗██╔════╝██╔══██╗╚════██╗██╔═████╗
 *    █████╗  ██████╔╝██║     ╚█████╔╝ █████╔╝██║██╔██║
 *    ██╔══╝  ██╔══██╗██║     ██╔══██╗██╔═══╝ ████╔╝██║
 *    ███████╗██║  ██║╚██████╗╚█████╔╝███████╗╚██████╔╝
 *    ╚══════╝╚═╝  ╚═╝ ╚═════╝ ╚════╝ ╚══════╝ ╚═════╝
 *
 *    ██████╗ ███████╗ ██████╗ ██╗███████╗████████╗██████╗ ██╗   ██╗
 *    ██╔══██╗██╔════╝██╔════╝ ██║██╔════╝╚══██╔══╝██╔══██╗╚██╗ ██╔╝
 *    ██████╔╝█████╗  ██║  ███╗██║███████╗   ██║   ██████╔╝ ╚████╔╝
 *    ██╔══██╗██╔══╝  ██║   ██║██║╚════██║   ██║   ██╔══██╗  ╚██╔╝
 *    ██║  ██║███████╗╚██████╔╝██║███████║   ██║   ██║  ██║   ██║
 *    ╚═╝  ╚═╝╚══════╝ ╚═════╝ ╚═╝╚══════╝   ╚═╝   ╚═╝  ╚═╝   ╚═╝
 *
 */
pragma solidity 0.4.24;
// IV is value needed to have a vanity address starting with `0x820`.
// IV: 1200

/// @dev The interface a contract MUST implement if it is the implementer of
/// some (other) interface for any address other than itself.
interface ERC820ImplementerInterface {
    /// @notice Indicates whether the contract implements the interface `interfaceHash` for the address `addr` or not.
    /// @param addr Address for which the contract will implement the interface
    /// @param interfaceHash keccak256 hash of the name of the interface
    /// @return ERC820_ACCEPT_MAGIC only if the contract implements `interfaceHash` for the address `addr`.
    function canImplementInterfaceForAddress(address addr, bytes32 interfaceHash) public view returns(bytes32);
}


/// @title ERC820 Pseudo-introspection Registry Contract
/// @author Jordi Baylina and Jacques Dafflon
/// @notice This contract is the official implementation of the ERC820 Registry.
/// @notice For more details, see https://eips.ethereum.org/EIPS/eip-820
contract ERC820Registry {
    /// @notice ERC165 Invalid ID.
    bytes4 constant INVALID_ID = 0xffffffff;
    /// @notice Method ID for the ERC165 supportsInterface method (= `bytes4(keccak256('supportsInterface(bytes4)'))`).
    bytes4 constant ERC165ID = 0x01ffc9a7;
    /// @notice Magic value which is returned if a contract implements an interface on behalf of some other address.
    bytes32 constant ERC820_ACCEPT_MAGIC = keccak256(abi.encodePacked("ERC820_ACCEPT_MAGIC"));

    mapping (address => mapping(bytes32 => address)) interfaces;
    mapping (address => address) managers;
    mapping (address => mapping(bytes4 => bool)) erc165Cached;

    /// @notice Indicates a contract is the `implementer` of `interfaceHash` for `addr`.
    event InterfaceImplementerSet(address indexed addr, bytes32 indexed interfaceHash, address indexed implementer);
    /// @notice Indicates `newManager` is the address of the new manager for `addr`.
    event ManagerChanged(address indexed addr, address indexed newManager);

    /// @notice Query if an address implements an interface and through which contract.
    /// @param _addr Address being queried for the implementer of an interface.
    /// (If `_addr == 0` then `msg.sender` is assumed.)
    /// @param _interfaceHash keccak256 hash of the name of the interface as a string.
    /// E.g., `web3.utils.keccak256('ERC777Token')`.
    /// @return The address of the contract which implements the interface `_interfaceHash` for `_addr`
    /// or `0x0` if `_addr` did not register an implementer for this interface.
    function getInterfaceImplementer(address _addr, bytes32 _interfaceHash) external view returns (address) {
        address addr = _addr == 0 ? msg.sender : _addr;
        if (isERC165Interface(_interfaceHash)) {
            bytes4 erc165InterfaceHash = bytes4(_interfaceHash);
            return implementsERC165Interface(addr, erc165InterfaceHash) ? addr : 0;
        }
        return interfaces[addr][_interfaceHash];
    }

    /// @notice Sets the contract which implements a specific interface for an address.
    /// Only the manager defined for that address can set it.
    /// (Each address is the manager for itself until it sets a new manager.)
    /// @param _addr Address to define the interface for. (If `_addr == 0` then `msg.sender` is assumed.)
    /// @param _interfaceHash keccak256 hash of the name of the interface as a string.
    /// For example, `web3.utils.keccak256('ERC777TokensRecipient')` for the `ERC777TokensRecipient` interface.
    function setInterfaceImplementer(address _addr, bytes32 _interfaceHash, address _implementer) external {
        address addr = _addr == 0 ? msg.sender : _addr;
        require(getManager(addr) == msg.sender, "Not the manager");

        require(!isERC165Interface(_interfaceHash), "Must not be a ERC165 hash");
        if (_implementer != 0 && _implementer != msg.sender) {
            require(
                ERC820ImplementerInterface(_implementer)
                    .canImplementInterfaceForAddress(addr, _interfaceHash) == ERC820_ACCEPT_MAGIC,
                "Does not implement the interface"
            );
        }
        interfaces[addr][_interfaceHash] = _implementer;
        emit InterfaceImplementerSet(addr, _interfaceHash, _implementer);
    }

    /// @notice Sets the `_newManager` as manager for the `_addr` address.
    /// The new manager will be able to call `setInterfaceImplementer` for `_addr`.
    /// @param _addr Address for which to set the new manager. (If `_addr == 0` then `msg.sender` is assumed.)
    /// @param _newManager Address of the new manager for `addr`. (Pass `0x0` to reset the manager to `_addr` itself.)
    function setManager(address _addr, address _newManager) external {
        address addr = _addr == 0 ? msg.sender : _addr;
        require(getManager(addr) == msg.sender, "Not the manager");
        managers[addr] = _newManager == addr ? 0 : _newManager;
        emit ManagerChanged(addr, _newManager);
    }

    /// @notice Get the manager of an address.
    /// @param _addr Address for which to return the manager.
    /// @return Address of the manager for a given address.
    function getManager(address _addr) public view returns(address) {
        // By default the manager of an address is the same address
        if (managers[_addr] == 0) {
            return _addr;
        } else {
            return managers[_addr];
        }
    }

    /// @notice Compute the keccak256 hash of an interface given its name.
    /// @param _interfaceName Name of the interface.
    /// @return The keccak256 hash of an interface name.
    function interfaceHash(string _interfaceName) public pure returns(bytes32) {
        return keccak256(abi.encodePacked(_interfaceName));
    }

    /* --- ERC165 Related Functions --- */

    /// @notice Checks whether a contract implements an ERC165 interface or not.
    /// The result is cached. If the cache is out of date, it must be updated by calling `updateERC165Cache`.
    /// @param _contract Address of the contract to check.
    /// @param _interfaceId ERC165 interface to check.
    /// @return `true` if `_contract` implements `_interfaceId`, false otherwise.
    /// @dev This function may modify the state when updating the cache. However, this function must have the `view`
    /// modifier since `getInterfaceImplementer` also calls it. If called from within a transaction, the ERC165 cache
    /// is updated.
    function implementsERC165Interface(address _contract, bytes4 _interfaceId) public view returns (bool) {
        if (!erc165Cached[_contract][_interfaceId]) {
            updateERC165Cache(_contract, _interfaceId);
        }
        return interfaces[_contract][_interfaceId] != 0;
    }

    /// @notice Updates the cache with whether the contract implements an ERC165 interface or not.
    /// @param _contract Address of the contract for which to update the cache.
    /// @param _interfaceId ERC165 interface for which to update the cache.
    function updateERC165Cache(address _contract, bytes4 _interfaceId) public {
        interfaces[_contract][_interfaceId] = implementsERC165InterfaceNoCache(_contract, _interfaceId) ? _contract : 0;
        erc165Cached[_contract][_interfaceId] = true;
    }

    /// @notice Checks whether a contract implements an ERC165 interface or not without using nor updating the cache.
    /// @param _contract Address of the contract to check.
    /// @param _interfaceId ERC165 interface to check.
    /// @return `true` if `_contract` implements `_interfaceId`, false otherwise.
    function implementsERC165InterfaceNoCache(address _contract, bytes4 _interfaceId) public view returns (bool) {
        uint256 success;
        uint256 result;

        (success, result) = noThrowCall(_contract, ERC165ID);
        if (success == 0 || result == 0) {
            return false;
        }

        (success, result) = noThrowCall(_contract, INVALID_ID);
        if (success == 0 || result != 0) {
            return false;
        }

        (success, result) = noThrowCall(_contract, _interfaceId);
        if (success == 1 && result == 1) {
            return true;
        }
        return false;
    }

    /// @notice Checks whether the hash is a ERC165 interface (ending with 28 zeroes) or not.
    /// @param _interfaceHash The hash to check.
    /// @return `true` if the hash is a ERC165 interface (ending with 28 zeroes), `false` otherwise.
    function isERC165Interface(bytes32 _interfaceHash) internal pure returns (bool) {
        return _interfaceHash & 0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF == 0;
    }

    function noThrowCall(address _contract, bytes4 _interfaceId)
        internal view returns (uint256 success, uint256 result)
    {
        bytes4 erc165ID = ERC165ID;

        assembly {
                let x := mload(0x40)               // Find empty storage location using "free memory pointer"
                mstore(x, erc165ID)                // Place signature at begining of empty storage
                mstore(add(x, 0x04), _interfaceId) // Place first argument directly next to signature

                success := staticcall(
                    30000,                         // 30k gas
                    _contract,                     // To addr
                    x,                             // Inputs are stored at location x
                    0x08,                          // Inputs are 8 bytes long
                    x,                             // Store output over input (saves space)
                    0x20                           // Outputs are 32 bytes long
                )

                result := mload(x)                 // Load the result
        }
    }
}

Deployment Transaction

Below is the raw transaction which MUST be used to deploy the smart contract on any chain.

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

The string of 820's at the end of the transaction is the s of the signature. From this pattern, one can clearly deduce that it is a deterministic signature generated by a human.

Deployment Method

This contract is going to be deployed using the keyless deployment method—also known as Nick's method—which relies on a single-use address. (See Nick's article for more details). This method works as follows:

  1. Generate a transaction which deploys the contract from a new random account.
  • This transaction MUST NOT use EIP155 in order to work on any chain.
  • This transaction MUST have a relatively high gas price to be deployed on any chain. In this case, it is going to be 100 Gwei.

  1. Set the v, r, s of the transaction signature to the following values:

    v: 27
r: 0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798
s: 0x0820820820820820820820820820820820820820820820820820820820820820

This nice s value---made of a repetion of 820---is a predicatable "random number" generated deterministically by a human.

  1. We recover the sender of this transaction, i.e. the deployment account.

    Thus we obtain an account that can broadcast that transaction, but we also have the warranty that nobody knows the private key of that account.

  2. Send Ether to this deployment account.

  3. Broadcast the transaction.

This operation can be done on any chain, guaranteeing that the contract address is going to always be the same and nobody will be able to mess up that address with a different contract.

Special Registry Deployment Account

0xC3AdeE9B2E23837DF6259A984Af7a437dE4E2ab6

This account is generated by reverse engineering it from its signature for the transaction. This way no one knows the private key, but it is known that it is the valid signer of the deployment transaction.

Deployed contract

0x820d0Bc4d0AD9E0E7dc19BD8cF9C566FC86054ce

The contract has the address above for every chain it is deployed to.

Raw metadata of ./contracts/ERC820Registry.sol 
{
  "compiler": {
    "version": "0.4.24+commit.e67f0147"
  },
  "language": "Solidity",
  "output": {
    "abi": [
      {
        "constant": false,
        "inputs": [
          {
            "name": "_addr",
            "type": "address"
          },
          {
            "name": "_interfaceHash",
            "type": "bytes32"
          },
          {
            "name": "_implementer",
            "type": "address"
          }
        ],
        "name": "setInterfaceImplementer",
        "outputs": [],
        "payable": false,
        "stateMutability": "nonpayable",
        "type": "function"
      },
      {
        "constant": true,
        "inputs": [
          {
            "name": "_addr",
            "type": "address"
          }
        ],
        "name": "getManager",
        "outputs": [
          {
            "name": "",
            "type": "address"
          }
        ],
        "payable": false,
        "stateMutability": "view",
        "type": "function"
      },
      {
        "constant": false,
        "inputs": [
          {
            "name": "_addr",
            "type": "address"
          },
          {
            "name": "_newManager",
            "type": "address"
          }
        ],
        "name": "setManager",
        "outputs": [],
        "payable": false,
        "stateMutability": "nonpayable",
        "type": "function"
      },
      {
        "constant": true,
        "inputs": [
          {
            "name": "_interfaceName",
            "type": "string"
          }
        ],
        "name": "interfaceHash",
        "outputs": [
          {
            "name": "",
            "type": "bytes32"
          }
        ],
        "payable": false,
        "stateMutability": "pure",
        "type": "function"
      },
      {
        "constant": false,
        "inputs": [
          {
            "name": "_contract",
            "type": "address"
          },
          {
            "name": "_interfaceId",
            "type": "bytes4"
          }
        ],
        "name": "updateERC165Cache",
        "outputs": [],
        "payable": false,
        "stateMutability": "nonpayable",
        "type": "function"
      },
      {
        "constant": true,
        "inputs": [
          {
            "name": "_addr",
            "type": "address"
          },
          {
            "name": "_interfaceHash",
            "type": "bytes32"
          }
        ],
        "name": "getInterfaceImplementer",
        "outputs": [
          {
            "name": "",
            "type": "address"
          }
        ],
        "payable": false,
        "stateMutability": "view",
        "type": "function"
      },
      {
        "constant": true,
        "inputs": [
          {
            "name": "_contract",
            "type": "address"
          },
          {
            "name": "_interfaceId",
            "type": "bytes4"
          }
        ],
        "name": "implementsERC165InterfaceNoCache",
        "outputs": [
          {
            "name": "",
            "type": "bool"
          }
        ],
        "payable": false,
        "stateMutability": "view",
        "type": "function"
      },
      {
        "constant": true,
        "inputs": [
          {
            "name": "_contract",
            "type": "address"
          },
          {
            "name": "_interfaceId",
            "type": "bytes4"
          }
        ],
        "name": "implementsERC165Interface",
        "outputs": [
          {
            "name": "",
            "type": "bool"
          }
        ],
        "payable": false,
        "stateMutability": "view",
        "type": "function"
      },
      {
        "anonymous": false,
        "inputs": [
          {
            "indexed": true,
            "name": "addr",
            "type": "address"
          },
          {
            "indexed": true,
            "name": "interfaceHash",
            "type": "bytes32"
          },
          {
            "indexed": true,
            "name": "implementer",
            "type": "address"
          }
        ],
        "name": "InterfaceImplementerSet",
        "type": "event"
      },
      {
        "anonymous": false,
        "inputs": [
          {
            "indexed": true,
            "name": "addr",
            "type": "address"
          },
          {
            "indexed": true,
            "name": "newManager",
            "type": "address"
          }
        ],
        "name": "ManagerChanged",
        "type": "event"
      }
    ],
    "devdoc": {
      "author": "Jordi Baylina and Jacques Dafflon",
      "methods": {
        "getInterfaceImplementer(address,bytes32)": {
          "params": {
            "_addr": "Address being queried for the implementer of an interface. (If `_addr == 0` then `msg.sender` is assumed.)",
            "_interfaceHash": "keccak256 hash of the name of the interface as a string. E.g., `web3.utils.keccak256('ERC777Token')`."
          },
          "return": "The address of the contract which implements the interface `_interfaceHash` for `_addr` or `0x0` if `_addr` did not register an implementer for this interface."
        },
        "getManager(address)": {
          "params": {
            "_addr": "Address for which to return the manager."
          },
          "return": "Address of the manager for a given address."
        },
        "implementsERC165Interface(address,bytes4)": {
          "details": "This function may modify the state when updating the cache. However, this function must have the `view` modifier since `getInterfaceImplementer` also calls it. If called from within a transaction, the ERC165 cache is updated.",
          "params": {
            "_contract": "Address of the contract to check.",
            "_interfaceId": "ERC165 interface to check."
          },
          "return": "`true` if `_contract` implements `_interfaceId`, false otherwise."
        },
        "implementsERC165InterfaceNoCache(address,bytes4)": {
          "params": {
            "_contract": "Address of the contract to check.",
            "_interfaceId": "ERC165 interface to check."
          },
          "return": "`true` if `_contract` implements `_interfaceId`, false otherwise."
        },
        "interfaceHash(string)": {
          "params": {
            "_interfaceName": "Name of the interface."
          },
          "return": "The keccak256 hash of an interface name."
        },
        "setInterfaceImplementer(address,bytes32,address)": {
          "params": {
            "_addr": "Address to define the interface for. (If `_addr == 0` then `msg.sender` is assumed.)",
            "_interfaceHash": "keccak256 hash of the name of the interface as a string. For example, `web3.utils.keccak256('ERC777TokensRecipient')` for the `ERC777TokensRecipient` interface."
          }
        },
        "setManager(address,address)": {
          "params": {
            "_addr": "Address for which to set the new manager. (If `_addr == 0` then `msg.sender` is assumed.)",
            "_newManager": "Address of the new manager for `addr`. (Pass `0x0` to reset the manager to `_addr` itself.)"
          }
        },
        "updateERC165Cache(address,bytes4)": {
          "params": {
            "_contract": "Address of the contract for which to update the cache.",
            "_interfaceId": "ERC165 interface for which to update the cache."
          }
        }
      },
      "title": "ERC820 Pseudo-introspection Registry Contract"
    },
    "userdoc": {
      "methods": {
        "getInterfaceImplementer(address,bytes32)": {
          "notice": "Query if an address implements an interface and through which contract."
        },
        "getManager(address)": {
          "notice": "Get the manager of an address."
        },
        "implementsERC165Interface(address,bytes4)": {
          "notice": "Checks whether a contract implements an ERC165 interface or not. The result is cached. If the cache is out of date, it must be updated by calling `updateERC165Cache`."
        },
        "implementsERC165InterfaceNoCache(address,bytes4)": {
          "notice": "Checks whether a contract implements an ERC165 interface or not without using nor updating the cache."
        },
        "interfaceHash(string)": {
          "notice": "Compute the keccak256 hash of an interface given its name."
        },
        "setInterfaceImplementer(address,bytes32,address)": {
          "notice": "Sets the contract which implements a specific interface for an address. Only the manager defined for that address can set it. (Each address is the manager for itself until it sets a new manager.)"
        },
        "setManager(address,address)": {
          "notice": "Sets the `_newManager` as manager for the `_addr` address. The new manager will be able to call `setInterfaceImplementer` for `_addr`."
        },
        "updateERC165Cache(address,bytes4)": {
          "notice": "Updates the cache with whether the contract implements an ERC165 interface or not."
        }
      }
    }
  },
  "settings": {
    "compilationTarget": {
      "./contracts/ERC820Registry.sol": "ERC820Registry"
    },
    "evmVersion": "byzantium",
    "libraries": {},
    "optimizer": {
      "enabled": true,
      "runs": 200
    },
    "remappings": []
  },
  "sources": {
    "./contracts/ERC820Registry.sol": {
      "content": "/* ERC820: Pseudo-introspection Registry Contract\n * by Jordi Baylina and Jacques Dafflon\n *\n * To the extent possible under law, Jordi Baylina and Jacques Dafflon who\n * associated CC0 with the ERC820: Pseudo-introspection Registry Contract have\n * waived all copyright and related or neighboring rights to the\n * ERC820: Pseudo-introspection Registry Contract.\n *\n * You should have received a copy of the CC0 legalcode along with this work.\n * If not, see .\n *\n *    ███████╗██████╗  ██████╗ █████╗ ██████╗  ██████╗\n *    ██╔════╝██╔══██╗██╔════╝██╔══██╗╚════██╗██╔═████╗\n *    █████╗  ██████╔╝██║     ╚█████╔╝ █████╔╝██║██╔██║\n *    ██╔══╝  ██╔══██╗██║     ██╔══██╗██╔═══╝ ████╔╝██║\n *    ███████╗██║  ██║╚██████╗╚█████╔╝███████╗╚██████╔╝\n *    ╚══════╝╚═╝  ╚═╝ ╚═════╝ ╚════╝ ╚══════╝ ╚═════╝\n *\n *    ██████╗ ███████╗ ██████╗ ██╗███████╗████████╗██████╗ ██╗   ██╗\n *    ██╔══██╗██╔════╝██╔════╝ ██║██╔════╝╚══██╔══╝██╔══██╗╚██╗ ██╔╝\n *    ██████╔╝█████╗  ██║  ███╗██║███████╗   ██║   ██████╔╝ ╚████╔╝\n *    ██╔══██╗██╔══╝  ██║   ██║██║╚════██║   ██║   ██╔══██╗  ╚██╔╝\n *    ██║  ██║███████╗╚██████╔╝██║███████║   ██║   ██║  ██║   ██║\n *    ╚═╝  ╚═╝╚══════╝ ╚═════╝ ╚═╝╚══════╝   ╚═╝   ╚═╝  ╚═╝   ╚═╝\n *\n */\npragma solidity 0.4.24;\n// IV is value needed to have a vanity address starting with `0x820`.\n// IV: 1200\n\n/// @dev The interface a contract MUST implement if it is the implementer of\n/// some (other) interface for any address other than itself.\ninterface ERC820ImplementerInterface {\n    /// @notice Indicates whether the contract implements the interface `interfaceHash` for the address `addr` or not.\n    /// @param addr Address for which the contract will implement the interface\n    /// @param interfaceHash keccak256 hash of the name of the interface\n    /// @return ERC820_ACCEPT_MAGIC only if the contract implements `interfaceHash` for the address `addr`.\n    function canImplementInterfaceForAddress(address addr, bytes32 interfaceHash) public view returns(bytes32);\n}\n\n\n/// @title ERC820 Pseudo-introspection Registry Contract\n/// @author Jordi Baylina and Jacques Dafflon\n/// @notice This contract is the official implementation of the ERC820 Registry.\n/// @notice For more details, see https://eips.ethereum.org/EIPS/eip-820\ncontract ERC820Registry {\n    /// @notice ERC165 Invalid ID.\n    bytes4 constant INVALID_ID = 0xffffffff;\n    /// @notice Method ID for the ERC165 supportsInterface method (= `bytes4(keccak256('supportsInterface(bytes4)'))`).\n    bytes4 constant ERC165ID = 0x01ffc9a7;\n    /// @notice Magic value which is returned if a contract implements an interface on behalf of some other address.\n    bytes32 constant ERC820_ACCEPT_MAGIC = keccak256(abi.encodePacked(\"ERC820_ACCEPT_MAGIC\"));\n\n    mapping (address => mapping(bytes32 => address)) interfaces;\n    mapping (address => address) managers;\n    mapping (address => mapping(bytes4 => bool)) erc165Cached;\n\n    /// @notice Indicates a contract is the `implementer` of `interfaceHash` for `addr`.\n    event InterfaceImplementerSet(address indexed addr, bytes32 indexed interfaceHash, address indexed implementer);\n    /// @notice Indicates `newManager` is the address of the new manager for `addr`.\n    event ManagerChanged(address indexed addr, address indexed newManager);\n\n    /// @notice Query if an address implements an interface and through which contract.\n    /// @param _addr Address being queried for the implementer of an interface.\n    /// (If `_addr == 0` then `msg.sender` is assumed.)\n    /// @param _interfaceHash keccak256 hash of the name of the interface as a string.\n    /// E.g., `web3.utils.keccak256('ERC777Token')`.\n    /// @return The address of the contract which implements the interface `_interfaceHash` for `_addr`\n    /// or `0x0` if `_addr` did not register an implementer for this interface.\n    function getInterfaceImplementer(address _addr, bytes32 _interfaceHash) external view returns (address) {\n        address addr = _addr == 0 ? msg.sender : _addr;\n        if (isERC165Interface(_interfaceHash)) {\n            bytes4 erc165InterfaceHash = bytes4(_interfaceHash);\n            return implementsERC165Interface(addr, erc165InterfaceHash) ? addr : 0;\n        }\n        return interfaces[addr][_interfaceHash];\n    }\n\n    /// @notice Sets the contract which implements a specific interface for an address.\n    /// Only the manager defined for that address can set it.\n    /// (Each address is the manager for itself until it sets a new manager.)\n    /// @param _addr Address to define the interface for. (If `_addr == 0` then `msg.sender` is assumed.)\n    /// @param _interfaceHash keccak256 hash of the name of the interface as a string.\n    /// For example, `web3.utils.keccak256('ERC777TokensRecipient')` for the `ERC777TokensRecipient` interface.\n    function setInterfaceImplementer(address _addr, bytes32 _interfaceHash, address _implementer) external {\n        address addr = _addr == 0 ? msg.sender : _addr;\n        require(getManager(addr) == msg.sender, \"Not the manager\");\n\n        require(!isERC165Interface(_interfaceHash), \"Must not be a ERC165 hash\");\n        if (_implementer != 0 && _implementer != msg.sender) {\n            require(\n                ERC820ImplementerInterface(_implementer)\n                    .canImplementInterfaceForAddress(addr, _interfaceHash) == ERC820_ACCEPT_MAGIC,\n                \"Does not implement the interface\"\n            );\n        }\n        interfaces[addr][_interfaceHash] = _implementer;\n        emit InterfaceImplementerSet(addr, _interfaceHash, _implementer);\n    }\n\n    /// @notice Sets the `_newManager` as manager for the `_addr` address.\n    /// The new manager will be able to call `setInterfaceImplementer` for `_addr`.\n    /// @param _addr Address for which to set the new manager. (If `_addr == 0` then `msg.sender` is assumed.)\n    /// @param _newManager Address of the new manager for `addr`. (Pass `0x0` to reset the manager to `_addr` itself.)\n    function setManager(address _addr, address _newManager) external {\n        address addr = _addr == 0 ? msg.sender : _addr;\n        require(getManager(addr) == msg.sender, \"Not the manager\");\n        managers[addr] = _newManager == addr ? 0 : _newManager;\n        emit ManagerChanged(addr, _newManager);\n    }\n\n    /// @notice Get the manager of an address.\n    /// @param _addr Address for which to return the manager.\n    /// @return Address of the manager for a given address.\n    function getManager(address _addr) public view returns(address) {\n        // By default the manager of an address is the same address\n        if (managers[_addr] == 0) {\n            return _addr;\n        } else {\n            return managers[_addr];\n        }\n    }\n\n    /// @notice Compute the keccak256 hash of an interface given its name.\n    /// @param _interfaceName Name of the interface.\n    /// @return The keccak256 hash of an interface name.\n    function interfaceHash(string _interfaceName) public pure returns(bytes32) {\n        return keccak256(abi.encodePacked(_interfaceName));\n    }\n\n    /* --- ERC165 Related Functions --- */\n\n    /// @notice Checks whether a contract implements an ERC165 interface or not.\n    /// The result is cached. If the cache is out of date, it must be updated by calling `updateERC165Cache`.\n    /// @param _contract Address of the contract to check.\n    /// @param _interfaceId ERC165 interface to check.\n    /// @return `true` if `_contract` implements `_interfaceId`, false otherwise.\n    /// @dev This function may modify the state when updating the cache. However, this function must have the `view`\n    /// modifier since `getInterfaceImplementer` also calls it. If called from within a transaction, the ERC165 cache\n    /// is updated.\n    function implementsERC165Interface(address _contract, bytes4 _interfaceId) public view returns (bool) {\n        if (!erc165Cached[_contract][_interfaceId]) {\n            updateERC165Cache(_contract, _interfaceId);\n        }\n        return interfaces[_contract][_interfaceId] != 0;\n    }\n\n    /// @notice Updates the cache with whether the contract implements an ERC165 interface or not.\n    /// @param _contract Address of the contract for which to update the cache.\n    /// @param _interfaceId ERC165 interface for which to update the cache.\n    function updateERC165Cache(address _contract, bytes4 _interfaceId) public {\n        interfaces[_contract][_interfaceId] = implementsERC165InterfaceNoCache(_contract, _interfaceId) ? _contract : 0;\n        erc165Cached[_contract][_interfaceId] = true;\n    }\n\n    /// @notice Checks whether a contract implements an ERC165 interface or not without using nor updating the cache.\n    /// @param _contract Address of the contract to check.\n    /// @param _interfaceId ERC165 interface to check.\n    /// @return `true` if `_contract` implements `_interfaceId`, false otherwise.\n    function implementsERC165InterfaceNoCache(address _contract, bytes4 _interfaceId) public view returns (bool) {\n        uint256 success;\n        uint256 result;\n\n        (success, result) = noThrowCall(_contract, ERC165ID);\n        if (success == 0 || result == 0) {\n            return false;\n        }\n\n        (success, result) = noThrowCall(_contract, INVALID_ID);\n        if (success == 0 || result != 0) {\n            return false;\n        }\n\n        (success, result) = noThrowCall(_contract, _interfaceId);\n        if (success == 1 && result == 1) {\n            return true;\n        }\n        return false;\n    }\n\n    /// @notice Checks whether the hash is a ERC165 interface (ending with 28 zeroes) or not.\n    /// @param _interfaceHash The hash to check.\n    /// @return `true` if the hash is a ERC165 interface (ending with 28 zeroes), `false` otherwise.\n    function isERC165Interface(bytes32 _interfaceHash) internal pure returns (bool) {\n        return _interfaceHash & 0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF == 0;\n    }\n\n    function noThrowCall(address _contract, bytes4 _interfaceId)\n        internal view returns (uint256 success, uint256 result)\n    {\n        bytes4 erc165ID = ERC165ID;\n\n        assembly {\n                let x := mload(0x40)               // Find empty storage location using \"free memory pointer\"\n                mstore(x, erc165ID)                // Place signature at begining of empty storage\n                mstore(add(x, 0x04), _interfaceId) // Place first argument directly next to signature\n\n                success := staticcall(\n                    30000,                         // 30k gas\n                    _contract,                     // To addr\n                    x,                             // Inputs are stored at location x\n                    0x08,                          // Inputs are 8 bytes long\n                    x,                             // Store output over input (saves space)\n                    0x20                           // Outputs are 32 bytes long\n                )\n\n                result := mload(x)                 // Load the result\n        }\n    }\n}\n",
      "keccak256": "0xbc18b6a4ad0aeb405e2ebae976b0ab653c17b3ba60eeaf0f1cf740a26bdcb7f1"
    }
  },
  "version": 1
}

Interface name

Any interface name is hashed using keccak256 and sent to getInterfaceImplementer().

If the interface is part of a standard, it is best practice to explicitly state the interface name and link to this published ERC820 such that other people don't have to come here to look up these rules.

Approved ERCs

If the interface is part of an approved ERC, it MUST be named ERC###XXXXX where ### is the number of the ERC and XXXXX should be the name of the interface in CamelCase. The meaning of this interface SHOULD be defined in the specified ERC.

Examples:

  • keccak256("ERC20Token")
  • keccak256("ERC777Token")
  • keccak256("ERC777TokensSender")
  • keccak256("ERC777TokensRecipient")

ERC165 Compatible Interfaces

Any interface where the last 28 bytes are zeroes (0) SHALL be considered an ERC165 interface.

ERC165 Cache

Whether a contract implements an ERC165 interface or not is automatically cached during lookup if the lookup is part of a transaction.

If a contract dynamically changes its interface, that contract SHOULD update the cache manually---there is no automatic cache invalidation or cache update. The cache update MUST be done using the updateERC165Cache function:

function updateERC165Cache(address _contract, bytes4 _interfaceId) public

parameters
_contract: Address of the contract for which to update the cache.
_interfaceHash: ERC165 interface for which to update the cache.

Private User-defined Interfaces

This scheme is extensible. You MAY make up your own interface name and raise awareness to get other people to implement it and then check for those implementations. Have fun but please, you MUST not conflict with the reserved designations above.

Set An Interface For An Address

For any address to set a contract as the interface implementation, it must call the following function of the ERC820 registry:

function setInterfaceImplementer(address _addr, bytes32 _interfaceHash, address _implementer) public

Sets the contract that will handle a specific interface.

Only the manager defined for that address can set it. (Each address is the manager for itself, see the manager section for more details.)

NOTE: If _addr and _implementer are two different addresses, then:

  • The _implementer MUST implement the ERC820ImplementerInterface (detailed below).
  • Calling canImplementInterfaceForAddress on _implementer with the given _addr and _interfaceHash MUST return the ERC820_ACCEPT_MAGIC value.

NOTE: The _interfaceHash MUST NOT be an ERC165 interface—it MUST NOT end with 28 zeroes (0).

parameters
_addr: Address to define the interface for (if _addr == 0 them msg.sender: is assumed)
_interfaceHash: keccak256 hash of the name of the interface as a string, for example web3.utils.keccak256('ERC777TokensRecipient') for the ERC777TokensRecipient interface.

Get An Implementation Of An Interface For An Address

Anyone MAY query the ERC820 Registry to obtain the address of a contract implementing an interface on behalf of some address using the getInterfaceImplementer function.

function getInterfaceImplementer(address _addr, bytes32 _interfaceHash) public view returns (address)

Query if an address implements an interface and through which contract.

NOTE: If the last 28 bytes of the _interfaceHash are zeroes (0), then the first 4 bytes are considered an ERC165 interface and the registry SHALL forward the call to the contract at _addr to see if it implements the ERC165 interface (the first 4 bytes of _interfaceHash). The registry SHALL also cache ERC165 queries to reduce gas consumption. Anyone MAY call the erc165UpdateCache function to update whether a contract implements an interface or not.

parameters
_addr: Address being queried for the implementer of an interface. (If _addr == 0 them msg.sender is assumed.)
_interfaceHash: keccak256 hash of the name of the interface as a string. E.g. web3.utils.keccak256('ERC777Token')
returns: The address of the contract which implements the interface _interfaceHash for _addr or 0x0 if _addr did not register an implementer for this interface.

Interface Implementation (ERC820ImplementerInterface)

interface ERC820ImplementerInterface {
    /// @notice Indicates whether the contract implements the interface `interfaceHash` for the address `addr`.
    /// @param addr Address for which the contract will implement the interface
    /// @param interfaceHash keccak256 hash of the name of the interface
    /// @return ERC820_ACCEPT_MAGIC only if the contract implements `ìnterfaceHash` for the address `addr`.
    function canImplementInterfaceForAddress(address addr, bytes32 interfaceHash) public view returns(bytes32);
}

Any contract being registered as the implementation of an interface for a given address MUST implement said interface. In addition if it implements an interface on behalf of a different address, the contract MUST implement the ERC820ImplementerInterface shown above.

function canImplementInterfaceForAddress(address addr, bytes32 interfaceHash) view public returns(bytes32);

Indicates whether a contract implements an interface (interfaceHash) for a given address (addr).

If a contract implements the interface (interfaceHash) for a given address (addr), it MUST return ERC820_ACCEPT_MAGIC when called with the addr and the interfaceHash. If it does not implement the interfaceHash for a given address (addr), it MUST NOT return ERC820_ACCEPT_MAGIC.

parameters
addr: Address for which the interface is implemented
interfaceHash: Hash of the interface which is implemented

The special value ERC820_ACCEPT_MAGIC is defined as the keccka256 hash of the string "ERC820_ACCEPT_MAGIC".

bytes32 constant ERC820_ACCEPT_MAGIC = keccak256("ERC820_ACCEPT_MAGIC");

The reason to return ERC820_ACCEPT_MAGIC instead of a boolean is to prevent cases where a contract fails to implement the canImplementInterfaceForAddress but implements a fallback function which does not throw. In this case, since canImplementInterfaceForAddress does not exist, the fallback function is called instead, executed without throwing and returns 1. Thus making it appear as if canImplementInterfaceForAddress returned true.

Manager

The manager of an address (regular account or a contract) is the only entity allowed to register implementations of interfaces for the address. By default, any address is its own manager.

The manager can transfer its role to another address by calling setManager with the address for which to transfer the manager and the address of the new manager.

setManager function

function setManager(address _addr, address _newManager) public

Sets the _newManager as manager for the _addr address.

The new manager will be able to call setInterfaceImplementer for _addr.

If _addr is 0x0, msg.sender is assumed for _addr.
If _newManager is 0x0, the manager is reset to _addr itself as the manager.

parameters
_addr: Address for which to set the new manager. (Pass 0x0 to use msg.sender as the address.)
_newManager: The address of the new manager for _addr. (Pass 0x0 to reset the manager to _addr.)

getManager function

function getManager(address _addr) public view returns(address)

Get the manager of an address.

parameters
_addr: Address for which to return the manager.
returns: Address of the manager for a given address.

Backward Compatibility

This standard is backward compatible with ERC165, as both methods MAY be implemented without conflicting with each other.

Test Cases

Please check the jbaylina/eip820 repository for the full test suite.

Implementation

The implementation is available in the repo: jbaylina/eip820.

Copyright

Copyright and related rights waived via CC0.