Standard Interface Detection

Standard Interface Detection

Abstract

Herein, we standardize the following:

1. How interfaces are identified
2. How a contract will publish the interfaces it implements
3. How to detect if a contract implements CIP-165
4. How to detect if a contract implements any given interface

Motivation

For some "standard interfaces" like the Core20 token interface, it is sometimes useful to query whether a contract supports the interface and if yes, which version of the interface, in order to adapt the way in which the contract is to be interacted with. This proposal standardizes the concept of interfaces and standardizes the identification (naming) of interfaces.

Specification

How Interfaces are Identified

For this standard, an interface is a set of function selectors as defined by the Core Blockchain ABI. This is a subset of Solidity's concept of interfaces and the interface keyword definition which also defines return types, mutability and events.

We define the interface identifier as the XOR of all function selectors in the interface. This code example shows how to calculate an interface identifier:

pragma solidity ^0.8.0;

interface Solidity101 {
    function hello() external pure;
    function world(int) external pure;
}

contract Selector {
    function calculateSelector() public pure returns (bytes4) {
        Solidity101 i;
        return i.hello.selector ^ i.world.selector;
    }
}

Note: interfaces do not permit optional functions, therefore, the interface identity will not include them.

How a Contract will Publish the Interfaces it Implements

A contract that is compliant with CIP-165 shall implement the following interface (referred as CIP165.sol):

pragma solidity ^0.8.0;

interface CIP165 {
    /// @notice Query if a contract implements an interface
    /// @param interfaceID The interface identifier, as specified in CIP-165
    /// @dev Interface identification is specified in CIP-165. This function
    ///  uses less than 30,000 gas.
    /// @return `true` if the contract implements `interfaceID` and
    ///  `interfaceID` is not 0xffffffff, `false` otherwise
    function supportsInterface(bytes4 interfaceID) external view returns (bool);
}

The interface identifier for this interface is 0x01ffc9a7. You can calculate this by running bytes4(keccak256('supportsInterface(bytes4)')); or using the Selector contract above.

Therefore the implementing contract will have a supportsInterface function that returns:

• true when interfaceID is 0x01ffc9a7 (CIP-165 interface)
• false when interfaceID is 0xffffffff
• true for any other interfaceID this contract implements
• false for any other interfaceID

This function must return a bool and use at most 30,000 gas.

Implementation note, there are several logical ways to implement this function. Please see the example implementations and the discussion on gas usage.

How to Detect if a Contract Implements CIP-165

1. The source contract makes a STATICCALL to the destination address with input data: 0x01ffc9a701ffc9a700000000000000000000000000000000000000000000000000000000 and gas 30,000. This corresponds to contract.supportsInterface(0x01ffc9a7).
2. If the call fails or return false, the destination contract does not implement CIP-165.
3. If the call returns true, a second call is made with input data 0x01ffc9a7ffffffff00000000000000000000000000000000000000000000000000000000.
4. If the second call fails or returns true, the destination contract does not implement CIP-165.
5. Otherwise it implements CIP-165.

How to Detect if a Contract Implements any Given Interface

1. If you are not sure if the contract implements CIP-165, use the above procedure to confirm.
2. If it does not implement CIP-165, then you will have to see what methods it uses the old-fashioned way.
3. If it implements CIP-165 then just call supportsInterface(interfaceID) to determine if it implements an interface you can use.

Rationale

We tried to keep this specification as simple as possible. This implementation is also compatible with the current Solidity version and Core Blockchain architecture.

Backward Compatibility

The mechanism described above (with 0xffffffff) should work with most of the contracts previous to this standard to determine that they do not implement CIP-165.

Test Cases

Following is a contract that detects which interfaces other contracts implement:

pragma solidity ^0.8.0;

contract CIP165Query {
    bytes4 constant InvalidID = 0xffffffff;
    bytes4 constant CIP165ID = 0x01ffc9a7;

    function doesContractImplementInterface(address _contract, bytes4 _interfaceId) external view returns (bool) {
        uint256 success;
        uint256 result;

        (success, result) = noThrowCall(_contract, CIP165ID);
        if ((success==0)||(result==0)) {
            return false;
        }

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

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

    function noThrowCall(address _contract, bytes4 _interfaceId) internal view returns (uint256 success, uint256 result) {
        bytes4 cip165ID = CIP165ID;

        assembly {
                let x := mload(0x40)               // Find empty storage location using "free memory pointer"
                mstore(x, cip165ID)                // Place signature at beginning 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
                                    0x24,          // Inputs are 36 bytes long
                                    x,             // Store output over input (saves space)
                                    0x20)          // Outputs are 32 bytes long

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

Implementation

This approach uses a view function implementation of supportsInterface. The execution cost is 586 gas for any input. But contract initialization requires storing each interface (SSTORE is 20,000 gas). The CIP165MappingImplementation contract is generic and reusable.

pragma solidity ^0.8.0;

import "./CIP165.sol";

contract CIP165MappingImplementation is CIP165 {
    /// @dev You must not set element 0xffffffff to true
    mapping(bytes4 => bool) internal supportedInterfaces;

    constructor() {
        supportedInterfaces[this.supportsInterface.selector] = true;
    }

    function supportsInterface(bytes4 interfaceID) external view override returns (bool) {
        return supportedInterfaces[interfaceID];
    }
}

interface Simpson {
    function is2D() external returns (bool);
    function skinColor() external returns (string);
}

contract Lisa is CIP165MappingImplementation, Simpson {
    constructor() {
        supportedInterfaces[this.is2D.selector ^ this.skinColor.selector] = true;
    }

    function is2D() external returns (bool){}
    function skinColor() external returns (string){}
}

Following is a pure function implementation of supportsInterface. The worst-case execution cost is 236 gas, but increases linearly with a higher number of supported interfaces.

pragma solidity ^0.8.0;

import "./CIP165.sol";

interface Simpson {
    function is2D() external returns (bool);
    function skinColor() external returns (string);
}

contract Homer is CIP165, Simpson {
    function supportsInterface(bytes4 interfaceID) external view override returns (bool) {
        return
          interfaceID == this.supportsInterface.selector || // CIP-165
          interfaceID == this.is2D.selector
                         ^ this.skinColor.selector; // Simpson
    }

    function is2D() external returns (bool){}
    function skinColor() external returns (string){}
}

With three or more supported interfaces (including CIP-165 itself as a required supported interface), the mapping approach (in every case) costs less gas than the pure approach (at worst case).

Security Considerations

• Ensure proper validation of interface IDs to prevent malicious contracts from claiming to implement interfaces they don't support
• Consider gas costs when implementing interface detection in frequently called functions
• Validate that the 0xffffffff interface ID is never set to true in mapping implementations

Conclusion

The CIP-165 standard provides a robust and efficient method for interface detection on Core Blockchain, enabling smart contracts to dynamically discover and interact with each other based on their implemented interfaces.

Copyright

Copyright and related rights waived via CC0.