A method, user device, and service providing server are provided for information management using a specific tree based on a blockchain. Aspects of the described approach include registering in an open ledger of the blockchain private data that belongs to a set of private data that identifies a particular subject. Furthermore, pseudo-public data including prefix values that correspond to specific private data are generated, registered as sub-data corresponding to a sub-structure of a specific leaf node to which the specific private data is allocated. Also, a pseudo-public function is created based on the pseudo-public data allocated to the specific leaf node.