Disclosed herein are methods, systems, and apparatus for controlling authorization of access to user data. One of the methods includes generating a first decentralized identifier based on information about a second decentralized identifier and information about a third decentralized identifier; registering a first set of records and the first decentralized identifier at a second blockchain, the second blockchain includes one or more blockchain contracts configured to, upon execution, perform one or more operations associated with the first decentralized identifier, and registering the first set of records includes storing a hash value for each record in the first set of records in the second blockchain; and controlling authorization of access to the first set of records based on the first decentralized identifier, including in response to receiving a request to grant a second user access to a particular record in the first set of records, determining whether the request includes the first decentralized identifier.