Historical retrospective · Ethereum
EIP-155 added chain IDs to transaction signatures
EIP-155 altered the transaction-signing scheme to include a chain identifier, helping prevent transactions signed for one chain from being replayed on another.
Replay protection became particularly important when Ethereum and Ethereum Classic continued as distinct networks after the DAO fork.
The proposal tied a historical chain split to a durable wallet-security improvement.
Users and exchanges still need to select the intended network; signatures only protect against replay when implementations use the right chain ID.
Primary source
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