Method-Specific Identifier

A Method-Specific Identifier (MSI), commonly known as a function selector, is the first four bytes of the Keccak-256 hash of a smart contract function's signature, used to uniquely identify which function to execute in an Ethereum Virtual Machine (EVM) transaction. This compact identifier is the critical piece of data in the data field of a transaction that tells the receiving contract precisely which logic to run, distinguishing between functions like transfer(address,uint256) and approve(address,uint256).

The process of generating an MSI is deterministic: the function's canonical signature—its name and the parenthesized, comma-separated list of its parameter types—is hashed using Keccak-256, and the first four bytes (eight hexadecimal characters) of the resulting hash are taken. For example, the widely used transfer(address,uint256) function hashes to 0xa9059cbb..., making its MSI 0xa9059cbb. This mechanism allows the EVM to efficiently route calls without needing to parse complex human-readable strings during execution.

Understanding MSIs is essential for developers interacting with or analyzing smart contracts. They appear directly in raw transaction data on-chain, in event logs, and are used internally for function dispatch within a contract's bytecode. When building transactions programmatically using libraries like web3.js or ethers.js, the MSI is automatically computed and appended to the call data. Analysts and block explorers use these identifiers to decode and interpret transaction purposes, making them a fundamental element for blockchain transparency and interoperability.

A Method-Specific Identifier (MSI) is the component of a Decentralized Identifier (DID) that follows the did:<method-name>: prefix and is uniquely defined by that specific DID method. For example, in the DID did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a, the MSI is the Ethereum address 0xb9c5714089478a327f09197987f16f9e5d936e8a. The DID method's specification dictates the syntax, generation rules, and resolution logic for this identifier, making it the key that the resolver uses to query the underlying ledger, blockchain, or system where the associated DID Document is anchored.

During the DID resolution process, the resolver first parses the DID string to extract the method name and the MSI. The resolver then invokes the corresponding DID method driver—a software module that understands how to interact with that specific decentralized system. The driver uses the MSI as a direct lookup key or input to a specific algorithm. For instance, a did:key method might decode the MSI directly into a public key, while a did:ion method would use the MSI to query a Sidetree-based blockchain for anchor files containing the DID Document's state.

The design of the MSI is critical for deterministic resolution. A well-formed MSI must allow any compliant resolver to locate the exact same DID Document data from the target system, ensuring global consistency. This is why MSIs are often cryptographically derived values like public key hashes or ledger-specific addresses. The separation of the method prefix from the MSI enables the DID architecture to be extensible and interoperable, allowing new verification methods and ledger technologies to be integrated without altering the core resolution framework, as long as they define a clear scheme for their method-specific identifiers.

The identifier is generated by taking the first four bytes (the most significant 32 bits) of the Keccak-256 hash of the function's canonical signature. This signature is the function name followed by the parenthesized, comma-separated list of its parameter types, without spaces or parameter names (e.g., transfer(address,uint256)). The resulting 8-character hexadecimal prefix, like 0xa9059cbb, acts as a compact, collision-resistant pointer within the contract's bytecode.

When a user or another contract initiates a transaction, the call data must begin with this 4-byte selector. The EVM uses it to jump to the correct function logic. This mechanism is fundamental to contract ABI (Application Binary Interface) interoperability, allowing external systems to encode calls predictably. Without the correct selector, a transaction will either revert or, in rare cases, invoke a fallback function.

Understanding selectors is crucial for developers performing low-level calls using call(), delegatecall(), or staticcall(), as they must construct the calldata manually. They are also key in signature replay security and event topic indexing, where the selector of a function forms the first topic of its event log. Tools like Ethers.js and Web3.js libraries abstract this calculation, but the underlying hash is always present.

A Method-Specific Identifier, commonly known as a function selector or method ID, is the first four bytes of the Keccak-256 hash of a function's canonical signature, used to uniquely identify which function to execute within an Ethereum Virtual Machine (EVM) smart contract. This compact, four-byte prefix is prepended to the encoded call data in every transaction or message call, allowing the EVM to route execution to the correct contract logic efficiently. For example, the selector for a function transfer(address,uint256) is 0xa9059cbb.

The canonical signature is derived by taking the function name and the parenthesized, comma-separated list of its parameter types, without spaces or parameter names. This standardization is crucial for interoperability, ensuring that wallets, indexers, and other contracts compute the same identifier. Selectors are deterministic and collision-resistant, though the four-byte space is finite, leading to potential selector clashes where different function signatures hash to the same identifier, a critical security consideration during contract development.

Beyond basic function dispatch, method IDs are fundamental to Ethereum's Application Binary Interface (ABI) specification, which defines how data is encoded for the EVM. They enable higher-level functionalities such as contract introspection and the implementation of generic proxy patterns like the ERC-1967 upgradeable proxy standard, where a fallback function delegates calls based on the selector. Understanding selector generation and usage is essential for debugging transactions and building secure, composable decentralized applications.