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, the method comprising:
receiving an entitlement control message (ECM) and an entitlement management message (EMM), the ECM including a first subkey, the EMM including a second subkey, a membership data, and a receiver unique key;
decrypting or transforming the first subkey under control of the EMM; and either:
(a) in response to determining, based on the membership data, that a control word is invoked:
decrypting or transforming the second subkey under control of the EMM,
combining the decrypted or transformed second subkey with the decrypted or transformed first subkey to obtain the control word; or
(b) in response to determining, based on membership data, that the control word is revoked:
obtaining an invalid second subkey,
combining the invalid second subkey with the decrypted or transformed first subkey to obtain an invalid control word.
2. The method according to claim 1 , wherein the membership data indicates for a group having the receiver 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
in response to determining that the control word is to be revoked, disabling the decryption or transformation of the second subkey.
3. The method according to claim 1 , further comprising:
receiving decryption keys in at least one of the ECM and EMM; and
decrypting each of the first and second subkeys using the respective decryption keys,
wherein the receiver comprises hardware tamper resistance technology.
4. The method according to claim 3 , wherein one or more of the decryption keys is obtained by combining two or more decryption subkeys each obtained under control of at least one of the ECM and EMM.
5. The method according to claim 1 , further comprising:
receiving seeds in the at least one of the ECM and EMM; and
transforming each of the first and second subkeys using the respective seeds,
wherein the receiver comprises software tamper resistance technology.
6. The method according to claim 5 , wherein one or more of the seeds is obtained by combining two or more subseeds each obtained under control of at
least one of ECM and EMM.
7. The method according to claim 1 , comprising:
encrypting the control word with a local key.
8. A receiver comprising:
an input module configured for receiving an entitlement control message (ECM) and an entitlement management message (EMM), the ECM including a first subkey, the EMM including a second subkey, a membership data, and a receiver unique key; and
a processor configured to:
(a) decrypt or transform the first subkey under control of the EMM;
(b) in response to determining, based on the membership data, that a control word is invoked:
decrypt or transform the second subkey under control of the EMM,
combine the decrypted or transformed second subkey with the decrypted or transformed first subkey to obtain the control word; and
(c) in response to determining, based on the membership data, that the control word is revoked:
obtain an invalid second subkey,
combine the invalid second subkey with the decrypted or transformed first subkey to obtain an invalid control word.
9. The receiver according to claim 8 , wherein the membership data indicates for a group having the receiver 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
block the decryption or transformation of the second subkey.
10. The receiver according to claim 8 , comprising hardware tamper resistance technology, wherein the input module is configured to:
receive decryption keys in at least one of the ECM and EMM, and
wherein the processor is configured to:
decrypt each of the first and second subkeys using the respective decryption keys.
11. The receiver according to claim 10 , wherein the processor is further configured to obtain one or more of the decryption keys by combining two or more decryption subkeys each obtained under control of at least one of the ECM and EMM.
12. The receiver according to claim 8 , comprising software tamper resistance technology, wherein the input module is configured to:
receive seeds in at least one of the ECM and EMM, and wherein the processor is configured to:
transform each of the first and second subkeys using the respective seeds.
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 at least one of the ECM and EMM.
14. The receiver according to claim 8 , comprising a hardware decryption block configured to decrypt the encrypted control word using the receiver unique key.
15. The receiver according to claim 8 , wherein the processor is configured to:
encrypt the control word with a local key.
16. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by a computer processor, cause the computer processor to perform a method comprising:
receiving an entitlement control message (ECM) and an entitlement management message (EMM), the ECM including a first subkey, the EMM including a second subkey, a membership data, and a receiver unique key;
decrypting or transforming the first subkey under control of the EMM; and either:
(a) in response to determining, based on the membership data, that a control word is invoked:
decrypting or transforming the second subkey under control of the EMM,
combining the decrypted or transformed second subkey with the decrypted or transformed first subkey to obtain the control word; or
(b) in response to determining, based on the membership data, that the control word is revoked:
obtaining an invalid second subkey,
combining the invalid second subkey with the decrypted or transformed first subkey to obtain an invalid control word.