Method for distributing keys among a number of secure devices, method for communicating with a number of secure devices, security system, and set of secure devices

#The invention relates to a method for distributing keys among a number of secure devices. The invention further relates to a method for communicating with a number of secure devices, to a security system in which this method is used, and to a set of secure devices obtained by the distributing method.

#It is known to protect content against unauthorised copying by using conditional access like technology. The term content in the present application is used as an indication of any type of information, such as audio or video signals, computer software etc. To protect the content, the content is scrambled using a control word. The term “control word” refers to the key which is used in the scrambling algorithm to scramble the content. The control word is generally transferred to the descrambling location in an encrypted message. In a consumer electronic system, such as for example a CD or DVD player or a PC, a secure device, such as a smart card, is used to decrypt the encrypted message to obtain the control word and the decrypted control word is used by the electronic system to descramble the content. As a large number of secure devices is open to attack by hackers, it is not unlikely on the long term that the security of a secure device will be breached so that the content is available for unauthorized commercial purposes. In a commonly used method in conditional access systems, breaches of security are managed by distributing new keys which are used to encrypt the control word. However in particular in off-line circumstances, i.e. in case of distribution of scrambled content on CD's and DVD's, for example, such a distribution method can not be used.

#The invention aims to provide a method for distributing keys among a number of secure devices, which is in particular suitable for distributing keys in stored media applications.

#The invention will be further explained by reference to the drawing.

#FIG. 1 schematically shows a content provider and a number of users of the content.

#FIG. 2 shows a system for descrambling a scrambled content with a secure device.

#FIG. 1 shows a content provider system 1 operating according to an embodiment of the method for scrambling a content according to the invention. The scrambled content is distributed among a number of users by means of a distribution network 2. This distribution network 2 can be, for example, the Internet, a broadcast network or a number of shops selling CD's, DVD's or other storage media. Each user has a system 3 for descrambling the scrambled content co-operating with a secure device 4, such as a smart card. The system 3 can be part of a CD or DVD, player, a PC or can be implemented by means of a suitable software program running on a microprocessor which is part of such equipment.

#In order to prevent unauthorized copying of the content provided by the system 1, a provider will scramble the content using a suitable scrambling algorithm, wherein a key is used to scramble this content. The key used to scramble the content will be indicated as control word in this description. The control word is delivered to the users as an encrypted control message or cryptogram. It is noted that this control message may contain further entitlement information such as number of uses of the content, period during which the content may be used or the like. This part of the control message is not part of the present invention and will not be described further. The control message is encrypted using a key which is unique to the secure device 4 of a restricted number of users only. The manner in which the keys are distributed among a number of secure devices 4 will explained by reference to the following example. 1 a b c d e A 01 11 21 31 41 02 12 22 32 42 03 13 23 33 43 04 14 24 34 44 05 15 25 35 45 B 01 11 21 31 41 42 02 12 22 32 33 43 03 13 23 24 34 44 04 14 15 25 35 45 05 C 01 11 21 31 41 32 42 02 12 22 13 23 33 43 03 44 04 14 24 34 25 35 45 05 15 D 01 11 21 31 41 22 32 42 02 12 43 03 13 23 33 14 24 34 44 04 35 45 05 15 25 E 01 11 21 31 41 12 22 32 42 02 23 33 43 03 13 34 44 04 14 24 45 05 15 25 35

#As indicated in these tables, the secure devices are divided into sets A,B,C,D and E and each set has a plurality of subsets a,b,c,d and e. Subset A,a comprises secure #01-#05, subset A,b comprises secure devices #11-15 subset A,c comprises secure devices #21-#25, subset A,d comprises secure devices #31-#35 and subset A,e comprises secure devices #41-#45. The secure devices of each subset receive the same unique key, for example the secure devices #01-#05 of subset A,a receive the unique key A,a. This means that for example secure device #01 has the following set of unique keys A,a; B,a; C,a; D,a and E,a. As shown in the above tables, each secure device is a member of a number of sets A-E such that any two or more secure devices which are a member of a subset, are not a member of the same subset in another set. In this manner each secure device 4 will receive a unique combination of subset keys.