1. Field of the Invention
The present invention relates to a broadcast receiving apparatus receiving broadcast signals and to a method of controlling the broadcast receiving apparatus.
2. Description of the Related Art
Content that is scrambled is transmitted in terrestrial digital broadcasting. Content is scrambled by using a Conditional Access System (CAS). B-CAS systems using integrated circuit (IC) cards are employed as the CAS systems at present.
A method for protecting content (particularly, the copyright of content) in broadcast receiving apparatuses is called Rights Management and Protection (RMP). Encryption keys are used to encrypt content in the RMP method. In the current B-CAS systems, for example, three types of encryption keys including a scramble key, a work key, and a master key are hierarchically used (refer to Association of Radio Industries and Businesses (ARIB) Standard (STD) B-25 Version 5.1 in Japan).
In addition, a new content protection method (hereinafter referred to as a “new RMP method”) is standardized in “Dai-san-bu: Jushin-ji no Seigyo Houshiki (Kontentsu Hogo Houshiki) (Section 3: Control method upon reception (Content protection method)” in ARIB STD B-25 Version 5.1 in Japan. Three types of encryption keys including a scramble key, a work key, and a device key are hierarchically used in the new RMP method.
In the new RMP method, an Entitlement Management Message (EMM) for setting the work key and an Entitlement Control Message (ECM) for transmitting the scramble key are transmitted from each broadcast station operating the new RMP method to one broadcast receiving apparatus on a certain cycle.
The EMM is data carrying information specific to each broadcast receiving apparatus and includes identification information (device ID) used for identifying each broadcast receiving apparatus and a work key for decrypting the ECM. An EMM for updating the device key is also transmitted in the new RMP method, and each EMM of this type also include a device ID. The EMM for setting the work key is hereinafter called a work-key setting EMM and the EMM for updating the device key is hereinafter called a device-key updating EMM.
The ECM is data carrying information common to all the broadcast receiving apparatuses and includes a scramble key for decrypting content and information about programs.
The ECM is encrypted with the work key for every RMP business unit whereas the EMM is encrypted with the device key for every device ID. Each broadcast receiving apparatus receives the EMMs that are periodically transmitted and that should be acquired by the own broadcast receiving apparatus to set the work key and so on. While content is watched by a viewer, the broadcast receiving apparatus of the viewer receives the ECM and processes the received ECM to acquire the scramble key and descramble the content by using the scramble key.
The work-key setting EMMs of types corresponding to the types (numbers) of the device IDs are provided and the device-key updating EMMs of types corresponding to the types of the device IDs are provided in the new RMP method. Accordingly, from the viewpoint of a certain broadcast receiving apparatus, the EMMs that are received include a work-key setting EMM for the own apparatus (an own-apparatus work-key setting EMM), a device-key updating EMM for the own apparatus (an own-apparatus device-key updating EMM), and EMMs for multiple other apparatuses.
Broadcasting of new types, such as IP broadcasting, is increasingly used in recent years and the number of broadcasting business companies will be increased along with the increase of the new types of broadcasting. Since different work keys are used for different broadcasting business companies in each CAS system, the number of types of the work key will be increased in the future.
A case will now be considered in which a broadcast receiving apparatus directly records a broadcast stream including, for example, content and the EMMs on a recording medium and then plays back the content. If the work key set for the CAS system does not correspond to the content when the broadcast receiving apparatus plays back the content, it is necessary for the broadcast receiving apparatus to acquire the work key corresponding to the content and to set the acquired work key in the CAS system.
In order for the broadcast receiving apparatus to acquire the work key, it is necessary for the broadcast receiving apparatus to read out the broadcast stream and to acquire the EMMs corresponding to the own apparatus from among the EMMs that are sequentially read out. Since the EMMs corresponding to the own apparatus are periodically transmitted, as described above, it takes a time (a few seconds to several tens of seconds) corresponding to up to one period to acquire the EMMs. The time necessary for acquiring the EMMs depends on the number of device IDs. Since the broadcast receiving apparatus cannot play back the content during the acquisition of the EMMs, the screen of the display device is blacked out. The broadcast receiving apparatus may be configured so that the broadcast receiving apparatus reads out a broadcast stream before the playback of the content is started to acquire the EMMs corresponding to the own apparatus. However, in this case, it is necessary for a user to wait a time corresponding to up to one period before the playback of the content is started.
Although a technology for acquiring a scramble key to reduce the time necessary for channel switching is disclosed in a Japanese Patent Laid-Open No. 2007-129575, it is difficult to resolve the above issues with this technology.