1. A broadcast encryption method, comprising the steps of:
allocating, to each of a plurality of subscribers, a corresponding set of subscriber keys;
broadcasting encrypted content to the plurality of subscribers using a set of broadcast keys, wherein the encrypted content is decoded by a given subscriber using the subscriber’s corresponding set of subscriber keys;
modifying the set of broadcast keys, which are used for broadcasting encrypted content, by excluding compromised subscriber keys; and
updating a set of subscriber keys corresponding to at least one subscriber when the at least one subscriber’s set of subscriber keys comprises an amount of active keys that falls below a first predetermined threshold.
2. The method of claim 1, wherein each set of subscriber keys is encoded on a smartcard that is issued to the corresponding subscriber.
3. The method of claim 2, further comprising the steps of:
identifying a compromised smartcard; and
identifying each subscriber key contained on the compromised smartcard as a compromised key.
4. The method of claim 3, wherein a compromised smartcard comprises one of a pirate smartcard and a smartcard of an excluded subscriber.
5. The method of claim 4, wherein the step of updating comprises the steps of:
tracking a total amount of compromised cards; and
reissuing a smartcard comprising the updated set of subscriber keys when the total amount of compromised cards meets a second predefined threshold.
6. The method of claim 1, wherein the first predetermined threshold is one key.
7. A broadcast encryption method, comprising the steps of:
allocating a set of subscriber keys to each of a plurality of n subscribers, wherein each set of subscriber keys is generated by randomly selecting r keys from a universal set comprising K keys;
broadcasting encrypted content to the n subscribers using a set of broadcast keys Sp selected from the universal set of keys;
identifying at least one compromised subscriber key;
adjusting Sp by excluding the at least one compromised subscriber key; and
updating a set of subscriber keys corresponding to at least one subscriber when the at least one subscriber’s set of subscriber keys comprises an amount of active keys that falls below a first predetermined threshold.
8. The method of claim 7, wherein the step of allocating is performed using a randomized broadcast encryption scheme wherein K is selected to ensure an (m,\u03b1) cover free family with high probability that the sets of subscriber keys corresponding to a coalition of m subscribers can not cover a fraction \u03b1 of r keys comprising the set of subscriber keys of another subscriber.
9. The method of claim 8, wherein the step of broadcasting is performed using an (\u03b1r, |Sp|)-threshold broadcast protocol.
10. The method of claim 9, wherein \u03b1r=1.
11. The method of claim 7, wherein each set of subscriber keys is encoded on a separate smartcard that is issued to the corresponding subscriber.
12. The method of claim 11, wherein the step of identifying at least one compromised subscriber key comprises the steps of:
identifying a compromised smartcard; and
identifying each subscriber key contained on the compromised card as a compromised key.
13. The method of claim 12, wherein a compromised smartcard comprises one of a pirate smartcard and a smartcard of an excluded subscriber.
14. The method of claim 12, wherein the step of updating comprises the steps of:
tracking a total amount of compromised smartcards; and
reissuing a smartcard comprising the updated set of subscriber keys when the total amount of compromised cards meets a second predefined threshold d.
15. The method of claim 14, wherein d is substantially equal to Kr.
16. The method of claim 14, wherein the step of reissuing comprises the steps of:
generating a new key for each compromised key to update the universal set of keys; and
randomly selecting r keys from the updated universal set of keys to generate the updated set of subscriber keys.
17. The method of claim 14, wherein K, r and d are selected to obtain a bound on the number of subscribers that are reissued smartcards in recarding sessions.
18. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for performing broadcast encryption, the method steps comprising:
allocating, to each of a plurality of subscribers, a corresponding set of subscriber keys;
broadcasting encrypted content to the plurality of subscribers using a set of broadcast keys, wherein the encrypted content is decoded by a given subscriber using the subscriber’s corresponding set of subscriber keys;
modifying the set of broadcast keys, which are used for broadcasting encrypted content, by excluding compromised subscriber keys; and
updating a set of subscriber keys corresponding to at least one subscriber when the at least one subscriber’s set of subscriber keys comprises an amount of active keys that falls below a first predetermined threshold.
19. The program storage device of claim 18, further comprising instructions for performing the step of:
identifying each subscriber key contained on a compromised smartcard as a compromised key.
20. The program storage device of claim 19, wherein the instructions for performing the step of updating comprise instructions for performing the steps of:
tracking a total amount of compromised cards; and
encoding a smartcard with the updated set of subscriber keys when the total amount of compromised cards meets a second predefined threshold.
21. The program storage device of claim 18, wherein the first predetermined threshold comprises one key.
22. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for broadcast encryption, the method comprising the steps of:
allocating a set of subscriber keys to each of a plurality of n subscribers, wherein each set of subscriber keys is generated by randomly selecting r keys from a universal set comprising K keys;
broadcasting encrypted content to the n subscribers using a set of broadcast keys Sp selected from the universal set of keys;
identifying at least one compromised subscriber key;
adjusting Sp by excluding the at least one compromised subscriber key; and
updating a set of subscriber keys corresponding to at least one subscriber when the at least one subscriber’s set of subscriber keys comprises an amount of active keys that falls below a first predetermined threshold.
23. The program storage device of claim 22, wherein the instructions for performing the step of allocating comprise instructions for performing a randomized broadcast encryption scheme wherein K is selected to ensure an (m, \u03b1) cover free family with high probability that the sets of subscriber keys corresponding to a coalition of m subscribers can not cover a fraction \u03b1 of r keys comprising the set of subscriber keys of another subscriber.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A repetitive control device comprising:
an adder to which a compensated signal is inputted;
a feedback signal system for sequentially updating and storing an output signal from the adder, and outputting the signal to the adder;
said feedback signal system comprising,
a filter which has, as a delay element, a memory which stores signal information for one rotation of a disc into divided plural memory areas of the memory,
a gain element which multiplies an output from the filter by a value not larger than 1, and inputs the resultant to the adder, and
said filter being operated using a clock signal that is equal to an operation frequency of a driving signal, or a divided frequency thereof.
2. A repetitive control device as defined in claim 1 wherein
said filter has a register connected in parallel with the memory, and records higher-order bit data of an input signal in the memory while records lower-order bit data in the register, during a filtering process by the filter.
3. A repetitive control device as defined in claim 2 wherein
a word length of the higher-order bit data recorded in the memory is equal to a word length of data of the compensated signal that is added in the adder, or a word length of a DA converter to which the output of the adder is inputted.
4. A repetitive control device as defined in claim 1 wherein
said filter includes a low-pass filter, and uses said memory as a delay element for the low-pass filter.
5. A repetitive control device as defined in claim 1 wherein
said filter is a band-pass filter comprising a low-pass filter and a high-pass filter, and uses said memory as a delay element for the low-pass filter.
6. A repetitive control device including an integral term operation part of a PID servo filter, which performs an arithmetic processing using a clock signal that is equal to an operation frequency of a driving signal, or a divided frequency thereof, said integral term operation part comprising:
an adder to which a servo error signal is inputted; and
a feedback signal system for sequentially updating an output signal from the adder and inputting the signal to the adder; and
said feedback signal system comprising,
a memory in which signal information for one rotation of a disc is divided to be stored in plural memory areas, and
a gain element which multiplies an output from the memory by a value not larger than 1, and inputs the resultant to the adder,
wherein, when performing an integral operation to the inputted servo error signal, the integral operation uses the memory as a delay element for the input signal.
7. An optical disc device performing recording or playback of an optical disc, which is equipped with a repetitive control device as defined in claim 1.
8. An optical disc device performing recording or playback of an optical disc, which is equipped with a repetitive control device as defined in claim 6.