A method and an apparatus ensuring protection of digital data are provided. In addition to re-encrypting the data using an unchangeable key, the data is double re-encrypted using a changeable key. The changeable key is used first and the unchangeable key is then used, or in another case, the unchangeable key is used first, and the changeable key is then used. In the aspect of embodiments, there is a case adopting a software, a case adopting a hardware, or a case adopting the software and the hardware in combination. The hardware using the unchangeable key developed for digital video is available. In adopting the software, encryption/decryption is performed in a region below the kernel which cannot be handled by the user to ensure the security for the program and for the key used. More concretely, encryption/decryption is performed with RTOS using a HAL and a device driver, i.e., a filter driver, a disk driver and a network driver, in an I/O manager. Either one of two filter drivers, with a file system driver between them, may be used. Further, both filter drivers may be used.