Interface ID

An Interface ID is a unique 4-byte (32-bit) identifier, typically represented as 0x followed by 8 hexadecimal characters (e.g., 0x01ffc9a7 for ERC-165 itself). It is calculated as the XOR (exclusive OR) of all function selectors (the first 4 bytes of the function signature's Keccak-256 hash) defined in the interface. This deterministic calculation ensures global uniqueness for a given set of functions.

The ERC-165 standard formalizes how contracts publish and query interface support. A compliant contract implements a supportsInterface(bytes4 interfaceId) function that returns true if it implements the queried interface. This allows other contracts and clients to discover a contract's capabilities programmatically before interacting with it, preventing runtime errors from calling unimplemented functions.

Interface IDs are critical for upgradeable proxy patterns. The proxy contract can use supportsInterface to route calls to different implementation logic based on the interface being called. This allows for granular upgrades where new interfaces can be added to a deployed contract system without breaking existing integrations that query for specific functionality.

Major token standards like ERC-721 (NFTs) and ERC-1155 (Multi-Token) rely on ERC-165 for feature detection. Their core interfaces have defined IDs (e.g., 0x80ac58cd for ERC-721). Wallets and marketplaces query these IDs to confirm a contract is a valid NFT contract before displaying assets. Optional extension interfaces (like metadata or enumeration) also have their own IDs for detecting added features.

To calculate an Interface ID:

1. Take the function signature (e.g., balanceOf(address)).
2. Compute its Keccak-256 hash and take the first 4 bytes (the function selector).
3. For an interface with multiple functions, compute the XOR (^) of all its function selectors.

Example: The ERC-721 interface ID 0x80ac58cd is the XOR of selectors for balanceOf(address), ownerOf(uint256), etc. Tools like cast from Foundry can perform this calculation automatically.

Using a 4-byte identifier is highly gas-efficient for on-chain lookups compared to storing full function signatures. It provides a security benefit by creating a formal, verifiable declaration of a contract's external API. This prevents signature collisions and ensures that a contract claiming to support an interface implements all required functions, not just a subset.

ERC-165 is the foundational standard that defines how to calculate and query an Interface ID. A contract implements supportsInterface(bytes4 interfaceId) to return true if it supports a given interface. The ID is calculated as the XOR of all function selectors in the interface.

• Purpose: Enables safe, on-chain discovery of a contract's capabilities.
• Mechanism: interfaceId = bytes4(keccak256('function signature()')) ^ bytes4(keccak256('anotherFunction()'))

The ERC-20 token standard interface ID is 0x36372b07. This ID is derived from the XOR of the selectors for its core functions:

• totalSupply()
• balanceOf(address)
• transfer(address,uint256)
• transferFrom(address,address,uint256)
• approve(address,uint256)
• allowance(address,address)

This allows wallets and decentralized exchanges to reliably identify standard fungible token contracts.

The ERC-721 Non-Fungible Token standard uses the interface ID 0x80ac58cd. It is calculated from the selectors of functions like:

• balanceOf(address)
• ownerOf(uint256)
• safeTransferFrom(address,address,uint256,bytes)
• transferFrom(address,address,uint256)
• approve(address,uint256)
• setApprovalForAll(address,bool)
• getApproved(uint256)
• isApprovedForAll(address,address)

This ID is critical for NFT marketplaces to verify compliance.

The ERC-1155 Multi-Token standard, which can represent both fungible and non-fungible assets in a single contract, has the interface ID 0xd9b67a26. Key functions contributing to this ID include:

• balanceOf(address,uint256)
• balanceOfBatch(address[],uint256[])
• setApprovalForAll(address,bool)
• isApprovedForAll(address,address)
• safeTransferFrom(address,address,uint256,uint256,bytes)
• safeBatchTransferFrom(address,address,uint256[],uint256[],bytes)

This enables efficient batch operations and mixed asset types.

ERC-2981 defines a standard way to retrieve royalty payment information for NFTs. Its interface ID is 0x2a55205a, derived from the single function selector for royaltyInfo(uint256 _tokenId, uint256 _salePrice). This function returns the recipient address and royalty amount.

• Utility: Allows marketplaces to automatically pay out royalties to creators on secondary sales.
• Adoption: Widely implemented by major NFT collections and marketplaces.

An Interface ID is not arbitrary; it is computed deterministically. For any Solidity interface, the ID is the bitwise XOR (^) of the function selectors (the first 4 bytes of keccak256("functionSignature")) of all functions in the interface.

Example Calculation (Simplified ERC-20):

solidityCopy
// Selectors (simplified for example)
bytes4 a = bytes4(keccak256('totalSupply()')); // 0x18160ddd
bytes4 b = bytes4(keccak256('balanceOf(address)')); // 0x70a08231
// ... XOR all selectors
interfaceId = a ^ b ^ c ^ d ^ e ^ f; // Result: 0x36372b07

Tools and libraries like OpenZeppelin's IERC165 helper perform this calculation.

The primary application of an Interface ID is to declare a smart contract's compliance with a standard, most notably ERC-165. A contract's supportsInterface function returns true when queried with a specific IID, enabling other contracts and user interfaces to programmatically verify its capabilities.

• Key Use: Enables wallets and dApps to detect if a contract is an NFT (ERC-721), fungible token (ERC-20), or other standard.
• Mechanism: The IID is a 4-byte hash (typically the XOR of the function selectors) of the interface's functions.

Interface IDs are critical for managing upgradeable proxy patterns. When a proxy's logic contract is upgraded, the new implementation must guarantee it supports all the interface IDs of the previous version to maintain compatibility.

• Prevents Breaking Changes: Tools can verify that an upgrade doesn't remove required functionality by checking IID support.
• Security Check: Auditors use IID checks to ensure upgrade safety and prevent accidental interface violations.

Advanced DEX aggregators and routers use Interface IDs to identify the capabilities of liquidity pools dynamically. This allows a router to determine if a pool contract supports specific functions like swap, addLiquidity, or flash loans without prior knowledge of its exact type.

• Dynamic Integration: Enables support for new pool types (e.g., Uniswap v2, v3, Balancer) as long as they publish a standard IID.
• Efficiency: Avoids failed transactions by checking for required function support before interacting.

User-facing applications like MetaMask and blockchain explorers use Interface ID queries to provide accurate contextual interfaces. When you connect to a contract, your wallet can detect if it's a token, a marketplace, or a lending vault and display relevant buttons (e.g., 'Approve', 'List for Sale', 'Deposit').

• Improved UX: Automatically tailors the UI based on discovered contract features.
• Error Prevention: Prevents users from calling non-existent functions on a contract.

Cross-chain messaging and bridging protocols (e.g., LayerZero, Axelar) use Interface IDs to standardize message formats and payloads across different blockchain environments. A target chain's contract can verify an incoming message is destined for a contract that supports a specific interface, ensuring proper execution.

• Standardized Communication: Defines a canonical way to identify contract types across heterogeneous chains.
• Security: Adds a layer of validation to cross-chain calls, reducing the risk of executing calls on incompatible contracts.

In modular DAO frameworks (e.g., Aragon OSx), Interface IDs act as a registry key for plugins and governance modules. The DAO core can query if an attached module supports the required interfaces for voting, token distribution, or treasury management.

• Modular Design: Allows DAOs to safely compose functionality from verified, standards-compliant modules.
• Composability: Developers can build new modules that are automatically discoverable and integrable by any DAO using the standard IID system.

An Interface ID is the unique identifier for a smart contract interface, calculated as the bitwise XOR of all the function selectors (the first 4 bytes of the keccak256 hash of the function signature) defined within that interface. For example, the Interface ID for ERC-721 is 0x80ac58cd. This deterministic calculation ensures that any contract claiming to support an interface must implement all its functions.

The primary purpose of an Interface ID is to enable the ERC-165 standard, which allows contracts to publish and detect the interfaces they implement. A contract declares support by returning true when its supportsInterface(bytes4 interfaceId) function is called with the correct ID. This prevents errors by allowing other contracts or clients to programmatically verify a contract's capabilities before interacting with it.

Incorrect Interface ID calculation or declaration is a major security risk. If a contract incorrectly reports support for an interface it does not fully implement, it can cause catastrophic interaction failures for integrating protocols and users. Auditors must verify that:

• The declared Interface ID matches the XOR of all live functions.
• The supportsInterface function is present and correct.
• Interface support is not removed in future upgrades without proper signaling.

Common implementation errors include:

• Forgotten Functions: Omitting a function from the XOR calculation after adding it to the interface.
• Selector Collisions: While statistically near-impossible for defined interfaces, using type(IInterface).interfaceId in Solidity (v0.8.x+) is the safest method to avoid manual calculation errors.
• Upgrade Inconsistency: Adding or removing functions in a new interface version without creating a new, corresponding Interface ID, breaking existing integrations.

Developers should use established tools to avoid manual errors:

• Solidity Compiler: Use type(IMyInterface).interfaceId for compile-time calculation.
• Testing Suites: Include rigorous tests that call supportsInterface for all claimed IDs.
• Blockchain Explorers: Platforms like Etherscan display verified Interface IDs for audited contracts, providing a public reference.

A Function Selector is the foundational element of an Interface ID. It is the first 4 bytes of the keccak256 hash of a function's signature (e.g., balanceOf(address)). The Interface ID is the aggregate of these selectors. Understanding this relationship is crucial for low-level debugging and writing secure proxy patterns or fallback handlers that decode calldata.

The Application Binary Interface (ABI) is the standard way to interact with contracts in the Ethereum ecosystem, both from outside the blockchain and for contract-to-contract interaction. It defines the function signatures (names, argument types, return types) and event structures of a smart contract. The Interface ID is derived from this ABI.

• Encoding/Decoding: The ABI specifies how to encode function calls and decode returned data into a low-level byte format the EVM understands.
• Essential for Clients: Wallets, explorers, and dApps use the ABI to construct valid transactions and parse contract state.

ERC-165 is the Ethereum standard that formalizes how smart contracts publish and detect the interfaces they implement, using the Interface ID. A contract compliant with ERC-165 includes a supportsInterface(bytes4 interfaceId) function that returns true if it implements that specific interface.

• How it Works: Before interacting with an unknown contract, a client can call supportsInterface to check for required functionality (e.g., ERC-721 for NFTs).
• Prevents Errors: This standard prevents calls to non-existent functions, which would revert the transaction.

A Function Selector is the first four bytes of the Keccak-256 hash of a function's signature (e.g., balanceOf(address)). It's the compact identifier used in transaction data to specify which contract function to execute. The Interface ID is conceptually similar but broader: it is the XOR (exclusive OR) of all function selectors in an interface.

• Key Difference: A function selector identifies a single method; an Interface ID identifies an entire set of methods (an interface or standard).
• Transaction Calldata: The selector is the critical piece of data at the start of a transaction's data field.

Contract Bytecode is the compiled, low-level machine code (opcodes) that is deployed and executed by the Ethereum Virtual Machine (EVM). The Interface ID and ABI are metadata about the contract's capabilities, while the bytecode is the contract's executable logic.

• Deployment: The bytecode is what is stored on-chain.
• Runtime vs. Interface: The EVM runs bytecode. External systems use the ABI and Interface ID to know how to call it. A contract can have the same bytecode but expose different interfaces.

An Ethereum Request for Comments (ERC) is a standard technical document describing an application-level convention or interface for the Ethereum platform. Prominent examples include ERC-20 (fungible tokens) and ERC-721 (non-fungible tokens). Each standard defines a specific Interface ID.

• Standardization: ERCs ensure interoperability between independent dApps and contracts.
• Interface ID Role: The fixed Interface ID for an ERC (e.g., 0x80ac58cd for ERC-721) allows for universal detection of compliant contracts via ERC-165.

In development frameworks like ethers.js and Hardhat, an Interface Object is a programming abstraction that represents a contract's ABI. Developers use these objects to easily encode function calls, decode logs, and interact with contracts. The underlying Interface ID can be derived from this object.

• Developer Tooling: These libraries compute selectors and Interface IDs from the provided ABI.
• Example (ethers.js): const iface = new ethers.Interface(abi); const interfaceId = iface.getSighash("interfaceId");