Conditional entitlement processing for obtaining a control word
Embodiments of the invention provide an improved method and an improved receiver for obtaining a control word. Two or more subkeys are obtained in a receiver. Each subkey was encrypted under control of a key received in an entitlement message or transformed under control of a seed received in an entitlement message. After decryption or transformation, the subkeys are combined to obtain the control word. Typically at least one of the entitlement messages is a positive entitlement message and at least one of the entitlement messages is a negative entitlement message. Embodiments of the invention can be used in a conditional access system such as a Pay-TV system.
1. A method in a receiver of enabling descrambling of scrambled content, the method comprising:
receiving a positive entitlement message and a negative entitlement message from a head-end system, wherein the negative entitlement message instructs the receiver to revoke an entitlement to use a particular key;
obtaining a first subkey using first entitlement data received in the positive entitlement message, the first subkey being under control of the first entitlement data;
obtaining a second subkey using second entitlement data received in the negative entitlement message, the second subkey being under control of the second entitlement data; and
deriving a control word operable to descramble the scrambled content by forming a combination, using a processor of the receiver, of the subkeys.
2. The method according to claim 1 , wherein the negative entitlement message comprises group membership data indicating for one or more receivers whether or not the control word is to be revoked, the method further comprising:
processing the group membership data to determine for the receiver whether or not the control word is to be revoked; and
if the control word is to be revoked, for the second subkey that is under control of the negative entitlement message disabling the obtaining of the second subkey or obtaining an invalid second subkey.
3. The method according to claim 1 , wherein the receiver comprises hardware tamper resistance technology and wherein the first entitlement data comprises a first decryption key and the second entitlement data comprises a second decryption key, the method further comprising receiving a first encrypted subkey encrypted under the first decryption key and a second encrypted subkey encrypted under the second decryption key, and wherein obtaining the first subkey comprises decrypting the first encrypted subkey using the first decryption key and obtaining the second subkey comprises decrypting the second encrypted subkey using the second decryption key.
4. The method according to claim 3 , wherein one or more of the decryption keys is derived by combining two or more decryption subkeys each obtained under control of further entitlement data received in a decryption subkey specific entitlement message.
5. The method according to claim 1 , wherein the receiver comprises software tamper resistance technology and wherein the first entitlement data comprises a first seed and the second entitlement data comprises a second seed, the method further comprising receiving a first transformed subkey and a second transformed subkey, and wherein obtaining the first subkey comprises transforming the first transformed subkey using the first seed and obtaining the second subkey comprises transforming the second transformed subkey using the second seed.
6. The method according to claim 5 , wherein one or more of the seeds is derived by combining two or more subseeds each obtained under control of further entitlement data received in a subseed specific entitlement message.
7. The method according to claim 1 , wherein the derived control word is encrypted under a receiver unique key and the method further comprises obtaining the receiver unique key and decrypting the derived control word using the receiver unique key.
8. A receiver for enabling descrambling of scrambled content, the receiver comprising:
at least one processor; and
at least one memory operatively coupled to the at least one processor and storing instructions which, when executed by the at least one processor, cause the at least one processor to:
receive a positive entitlement message and a negative entitlement message from a head-end system;
obtain a first subkey using first entitlement data received in the positive entitlement message, the first subkey being under control of the first entitlement data;
obtain a second subkey using second entitlement data received in the negative entitlement message, the second subkey being under control of the second entitlement data, wherein the negative entitlement message instructs the receiver to revoke an entitlement to use a particular key; and
derive a control word operable to descramble the scrambled content by forming a combination of the subkeys.
9. The receiver according to claim 8 , wherein the negative entitlement message comprises group membership data indicating for one or more receivers whether or not the control word is to be revoked, wherein the processor is further configured to process the group membership data to determine for the receiver whether or not the control word is to be revoked, and wherein the processor is further configured to block obtainment of the second subkey or to obtain an invalid second subkey.
10. The receiver according to claim 8 , comprising hardware tamper resistance technology, wherein the first entitlement data comprises a first decryption key and the second entitlement data comprises a second decryption key, wherein the input module is further configured to receive a first encrypted subkey encrypted under the first decryption key, and a second encrypted subkey encrypted under the second decryption key, and wherein the processor is configured to:
derive the first subkey by decrypting the first encrypted subkey using the first decryption key, and
derive the second subkey by decrypting the second encrypted subkey using the second decryption key.
11. The receiver according to claim 10 , wherein the processor is further configured to derive one or more of the decryption keys by combining two or more decryption subkeys each obtained under control of further entitlement data received in a decryption subkey specific entitlement message.
12. The receiver according to claim 8 , comprising software tamper resistance technology, wherein the first entitlement data comprises a first seed and the second entitlement data comprises a second seed, wherein the input module is further configured to receive a first transformed subkey and a second transformed subkey,
and wherein the processor is configured to:
obtain the first subkey by transforming the first transformed subkey using the first seed, and
obtain the second subkey by transforming the second transformed subkey using the second seed.
13. The receiver according to claim 12 , wherein the receiver is further configured to obtain one or more of the seeds by combining two or more subseeds each obtained under control of further entitlement data received in a subseed specific entitlement message.
14. The receiver according to claim 8 , wherein the processor is configured to derive the control word as an encrypted control word encrypted under a receiver unique key, the receiver further comprising a hardware decryption block configured to obtain the receiver unique key and to decrypt the encrypted control word using the receiver unique key.