Cryptography – Key management – Having particular key generator
Reexamination Certificate
1997-12-15
2001-04-24
Swann, Tod R. (Department: 2767)
Cryptography
Key management
Having particular key generator
C380S277000, C380S281000, C380S284000, C380S285000, C713S150000, C713S152000, C713S171000
Reexamination Certificate
active
06222923
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of securing a system protected by a predefined hierarchy of cryptographic keys, and in particular, for securing a pay TV system, against unauthorized users.
RELATED TECHNOLOGY
A key hierarchy is used in many fields of application to derive a common key for a large number of users from the customers' individual cryptographic keys. A pay TV system is a typical application. With the help of a key hierarchy, the permission to receive a pay TV program can be granted selectively to certain customers only. One possible key hierarchy has the form of a tree structure. On the lowest hierarchical level, each potential customer initially has a chip card or other security module on which a unique key assigned to that customer is stored. The pay TV operator stores all these individual cryptographic keys in a central storage device.
The key hierarchy is built up gradually in that the keys of the lowest level are initially combined into a plurality of subsets of a predefined size on the second level. Each subset is assigned a cryptographic group key, which is transmitted with the help of the cryptographic keys of the lowest level forming the respective subset. Subsequently, the subsets of the second level are combined into a number of subsets with each subset of the third level being larger than each subset of the second level. Each subset of the third level is assigned a cryptographic key, which is transmitted with the help of the cryptographic group keys of the second level, forming the respective subset. This process can be continued until a common key is generated for those customers who are authorized to receive the pay-per-view TV program. Various types of attack on such a system protected by a key hierarchy are conceivable, all of which are based on a dishonest customer knowing his individual key, one or more group keys, or the common key stored on his or another chip card, and forwarding it without authorization to a third party. Three possible types of attack on such a system can be distinguished:
1. The dishonest customer copies the common key and forwards it without authorization, e.g., on a pirate chip card, to other individuals. This type of attack can be averted by the system operator generating the cryptographic keys regenerating the common key in appropriately selected time intervals.
2. A dishonest customer copies his individual cryptographic key and forwards it without authorization to other individuals. In this case the customer can be barred from the use of the system in a relatively simple manner if the individual key that has been copied, e.g., onto a pirate card, is recognized, since there is a unique relationship between the individual cryptographic key and the respective individual.
3. A dishonest customer copies a group key and forwards it to third parties. In this case, the dishonest customer cannot be uniquely identified using the group key that has been copied. The system operator must either bar all customers of the group identified by the group key from the use of the system or tolerate the misuse by the group key that has been copied.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to provide a method to more effectively protect a system protected by a key hierarchy against unauthorized users.
At the lowest level of the key hierarchy, an individual cryptographic key is assigned to each potential system user; this key may be handed out as a chip card or other security module. The individual cryptographic keys of each user are stored in a storage device of the system. At predefined time intervals, at least one higher hierarchical level of cryptographic keys is subsequently formed through the following steps: the cryptographic keys of the immediately lower hierarchical level are combined into a plurality of subsets of a predefined size in any fashion with a cryptographic key assigned to each subset; this cryptographic key is transmitted with the help of the cryptographic keys forming the respective subset and subsequently stored in the storage device. Then at least one individual cryptographic key assigned to a suspected user is determined by forming the intersection of at least two predefined subsets formed at different times and pertaining to the same hierarchical level.
Instead of rebuilding the higher hierarchical levels at predefined discrete points in time, different key hierarchies can be generated simultaneously for different system operators. Each key hierarchy has at least one higher hierarchical level of cryptographic keys. A higher hierarchical level is formed by combining the cryptographic keys of the immediately lower hierarchical level in any desired fashion into several subsets of predefined size, with a cryptographic key, generated from the cryptographic keys forming the respective subset and then stored, assigned to each subset. Thereafter at least one cryptographic key, assigned to a suspected user, is determined by forming the intersection of at least two predefined subsets pertaining to the same hierarchical level of two different key hierarchies.
Increasingly larger subsets can be successively formed, according to the number of hierarchical levels, to implement this method. A key hierarchy of a tree-type structure would be one possible example. A particularly efficient method, however, is the use of geometric structures to combine cryptographic keys into subsets of a predefined size. Geometric structures have the advantage that the characteristics of forming the intersections of different subsets can be very well described.
A key hierarchy generated for a plurality of customers can preferably be implemented with the help of a d-dimensional finite affine space AG(d,q) using body GF(q) (see A. Beutelspacher, “Einführung in die endliche Geometrie I & II” [Introduction to Finite Geometry I & II], BI Wissenschaftsverlag 1982, and A. Beutelspacher and U. Rosenbaum, “Projektive Geometrie” [Projective Geometry], Vieweg Verlag, 1992).
The intersection can be even more easily formed if the geometric structure is a d-dimensional finite projective space PG(d,q) with body GF(q).
REFERENCES:
patent: 4613901 (1986-09-01), Gilhousen et al.
patent: 5220604 (1993-06-01), Gasser et al.
Callahan Paul E.
Deutsche Telekom AG
Kenyon & Kenyon
Swann Tod R.
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