EIP191

EIP-191, short for Ethereum Improvement Proposal 191, defines a general scheme for signing arbitrary data to be used by Ethereum wallets and applications. The goal is to provide a simple, domain-separated method so that signatures created for messages cannot be mistaken for approvals of transactions.

The core of the standard is a prefix-based hashing scheme. The most widely used variant prefixes the message with a string such as “\x19Ethereum Signed Message:\n” followed by the decimal length of the message and then the message itself. This prefixed message is hashed with keccak256, and the resulting 32-byte hash is what is actually signed using the secp256k1 ECDSA key. The resulting signature consists of r, s, and v components, with v indicating the recovery identifier to recover the signer’s address.

Common usage and impact: The Ethereum Signed Message format is the de facto standard for personal signing and eth_sign operations in many wallets, including MetaMask and client implementations like Geth and Parity. It enables users to verify or prove control of an address by signing a readable message rather than a transaction.

Relation to other standards: EIP-191 is distinct from EIP-712, which specifies a more complex, typed structured data signing scheme intended for safer and more verifiable interactions. EIP-191 provides a simpler, broadly compatible method for signing arbitrary data.

Security notes: The prefixing and length encoding help prevent misuse of signatures intended for messages as signatures for transactions. Users should understand that a signed message is not a transaction and should be interpreted accordingly.