A method of digital forensics based on blockchain technology and a fine-grained access control scheme, using a finite state machine (FSM) based on smart contracts to manage a digital certificate, where a law enforcement agency ends the warrant request to a court and upon approval of the request collects the forensics data by obtaining the master secret key for accessing the forensic data from a plurality of authorities. The forensics data are encrypted by D-KP-ABE (Distributed Key Police Attribute-based Encryption) with privacy-preserved access policy. The secret sharings are required to form the decryption key for accessing the forensics data. The secret sharings are distributed among the plurality of authorized authorities so that no individual authority has a complete master key by itself. Each state of the FSM requires digital signature(s) of at least one specific authorized authority for transitioning to a next state.