Abstract: Disclosed herein are methods, devices, and apparatuses, including computer programs stored on computer-readable media, for protecting sensitive data of a transaction activity based on a smart contract in a blockchain. One of the methods includes: receiving activity data for creating an account for the transaction activity from a user, the activity data including at least an identifier of the smart contract in the blockchain and an expiration time of the transaction activity; sending the activity data to an authentication server for requesting a digital certificate of the activity data, wherein the digital certificate stores the identifier of the smart contract, and an expiration time of the digital certificate is set to be the expiration time of the transaction activity; and causing the account for the transaction activity to be created after to receiving the digital certificate from the authentication server.