A storage device includes a nonvolatile memory device, and a controller that manages a data encryption key (DEK). The DEK is used to encrypt data to be written in a storage space of the nonvolatile memory device by a first user and to decrypt data read from the storage space. The controller grants a second user authority to access the storage space by encrypting the DEK based on a Diffie-Hellman (DH) algorithm, grants a second user authority to access the encrypted DEK, and decrypts the encrypted DEK based on the DH algorithm.