The present invention relates to a storage, in which a recording medium such as a magnetic disk and a control device for accessing this recording medium are integrated, and an access control method for such a storage. In particular, the invention relates to a technique of preventing time tampering of a built-in clock and preventing illegal accesses.
Among storages, a magnetic disk device is put to practical use as a built-in storage for information apparatuses such as a computer and digital home appliances because a large capacity of device can be realized relatively easily. In order to digitally store content data, which is likely to include a copyright, such as video data, music, and still images in a storage, it is necessary to apply a copyright protection technique for the storage or a host terminal apparatus using the storage such that unlimited copying and use are not performed. In particular, when it is necessary to set an expiration date on the content data, it is necessary to refer to reliable time in order to compare the time with expiration date information set on the content data. Except where it is possible to always make connection to a reliable clock through a safe path without intervention of artificiality, it is likely that, in a constitution including a battery-driven clock such as a portable apparatus, deviation of the clock accumulates according to elapse of a long time or the clock cannot tick away the minutes normally because of exhaustion of the battery. Therefore, it is essential to provide a mechanism for referring to more reliable time and setting time immediately after replacing the battery or when deviation of time increases. If, for example, a user is allowed to use such a mechanism for setting time of an internal clock freely, there is a problem in that time is likely to be tampered by a malicious user. It is also likely that a user acts in an artificial way to prevent the built-in clock from functioning normally by setting an exhausted battery intentionally or removing a battery.
As measures to solve the problems, for example, patent document 1 (JP-A-2004-86494) discloses an example in which a user is not allowed to operate synchronizing means for a built-in clock freely and safe communication is established between a reliable standard time server and the synchronizing means. Patent document 2 (JP-A-2002-366031) describes a clock that can be operated only in a determined procedure in synchronization with time acquired on a network. Moreover, patent document 3 (JP-A-2003-258792) describes a system in which client apparatuses connected to a server authenticate time among the clients to make it difficult to commit fraudulence in time authentication. As a system for judging whether synchronization of a built-in clock is necessary, patent document 4 (JP-A-2003-167788) describes a system that, when the built-in clock is not in a reliable state, makes this built-in clock “invalid” to thereafter prevent the built-in clock from being referred to by processing that requires time reference.
It is also possible to perform expiration date management in a host system apparatus that handles a storage in which the content data is recorded. In this case, expiration date information is read out from the storage to the host system apparatus and compared with the present time. Then, propriety of use of a content is determined according to judgment of the host system apparatus. In any case, the content data and information essential for use of the data such as an encryption key can be read out from the storage regardless of the present time. In particular, in a portable storage, it is likely that the expiration date management in the host system apparatus is not performed and the content data flows out to the outside. There is a problem in that system design is complicated in terms of copyright protection or protection intensity decreases. Thus, it is conceivable to adopt a method of setting a clock for measuring time inside a storage and of counting time for an expiration date for a recorded content using this clock. For example, patent document 5 (JP-A-2002-268951) describes a storage incorporating a counter and an oscillator for driving the counter and a battery for supplying power to the counter and the oscillator.