Reliable storage medium access control method and device
A method of and device for granting access to content on a storage medium, including obtaining cryptographic data from a property, such as a wobble, of the storage medium, reading helper data from the storage medium, and granting the access based on an application of a delta-contracting function to the cryptographic data and the helper data. The delta-contracting function allows the choice of an appropriate value of the helper data, such that any value of the cryptographic data which sufficiently resembles the original primary input value leads to the same output value. Substantially different values of the cryptographic data lead to different values of the output.
1. A method of calculating reliable bits from cryptographic data; the method comprising acts of:
obtaining the cryptographic data from a property of a non-transitory medium,
reading helper data from a storage medium, and
calculating the reliable bits based on an application of a delta-contracting function to the cryptographic data and the helper data, wherein the helper data defines ranges of values for the cryptographic data and the delta-contracting function defines a corresponding output value for each range of values for the cryptographic data so that the helper data can be made to adapt the cryptographic data for a particular medium in tolerating small deviations in the obtained cryptographic data.
2. The method of claim 1 , comprising an act of deriving a decryption key from the reliable bits.
3. The method of claim 2 , wherein the act of deriving the decryption key comprises an act of deriving the decryption key from data supplied by a playback or recording apparatus.
4. The method of claim 1 , further comprising an act of authorizing the medium if the output value of the delta-contracting function corresponds to a control value.
5. The method of claim 4 , comprising acts of applying a cryptographic function to the output of the delta-contracting function and comparing the output of the cryptographic function to the control value.
6. The method of claim 5 , in which the cryptographic function is a one-way hash function.
7. The method of claim 4 , wherein the control value is stored on a smart card.
8. The method of claim 1 , in which the delta-contracting function involves a combination of one or more of a matrix multiplication on the cryptographic data, a linear addition of at least a portion of the helper data, a quantization in which the quantization areas are defined by a portion of the helper data, and error correction decoding.
9. The method of claim 1 , wherein the variations in a physical property of a medium are naturally occurring variations in the physical parameter.
10. The method of claim 1 , comprising an act of decrypting content using the decryption key.
11. A device arranged for calculating reliable bits from cryptographic data, comprising first reading means for obtaining the cryptographic data from variations in a physical property of a medium, second reading means for reading helper data from a storage medium, and calculating means for calculating the reliable bits based on an application of a delta-contracting function to the cryptographic data and the helper data, wherein the helper data defines ranges of values for the cryptographic data and the delta-contracting function defines a corresponding output value for each range of values for the cryptographic data so that the helper data can be made to adapt the cryptographic data for a particular medium in tolerating small deviations in the obtained cryptographic data.
12. The device of claim 11 , comprising means for deriving a decryption key from the reliable bits.
13. The device of claim 11 , wherein the device is a smart card.
14. The device of claim 11 , further comprising means for authorizing the medium if the output value of the delta-contracting function corresponds to a control value.
15. The device of claim 14 , comprising means for applying a cryptographic function to the output of the delta-contracting function and comparing the output of the cryptographic function to the control value.
16. The device of claim 15 , wherein the cryptographic function is a one-way hash function.
17. The device of claim 14 , wherein the control value is stored on a smart card.
18. A non-transitory computer-readable medium having a computer program product recorded thereon, said computer program product being arranged to cause a processor to perform acts of:
obtaining cryptographic data from a property of a medium,
reading helper data from a storage medium, and
calculating reliable bits based on an application of a delta-contracting function to the cryptographic data and the helper data, wherein the helper data defines ranges of values for the cryptographic data and the delta-contracting function defines a corresponding output value for each range of values for the cryptographic data so that the helper data can be made to adapt the cryptographic data for a particular medium in tolerating small deviations in the obtained cryptographic data.