Entitlement management message epoch as an external trusted time source
An external trusted time source is implemented over a network for conditional access system (CAS)/digital rights management (DRM) client devices. A client device includes untrusted software and a trusted execution environment (TEE) for processing an entitlement management message (EMM) that includes an epoch sequence number (ESN) transmitted from an EMM server using a first network connection. A remaining client key set (CKS) lifetime value is stored and updated in the TEE based on the ESN processed.
1. A method for implementing an external trusted time source over a network to a conditional access system (CAS) client device, the method comprising:
transmitting an entitlement management message (EMM) to a trusted execution environment (TEE) of the CAS client device from an EMM server using a first network connection, the EMM including an epoch sequence number (ESN);
generating an updated remaining client key set (CKS) lifetime value in the TEE based on an initial lifetime duration value for the CKS, and the ESN value in the EMM;
sealing the CKS, updated ESN value in the EMM, and updated remaining CKS lifetime value as unit information; and
delivering the updated remaining CKS lifetime value and updated ESN value in the EMM from the TEE to untrusted software in the CAS client device, and storing the updated remaining CKS lifetime value and updated ESN value.
2. The method according to claim 1 , wherein the EMM is transmitted to the untrusted software before sending the EMM to the TEE.
3. The method according to claim 1 , wherein the initial lifetime duration value for the CKS is delivered to the CAS client device as part of the CKS.
4. The method according to claim 1 , wherein the initial lifetime duration value for the CKS is a pre-configured value that is part of a CAS client implementation.
5. The method according to claim 1 , further comprising the step of initializing the remaining CKS lifetime value and ESN value when the first EMM is transmitted to the TEE after installing a CKS.
6. The method according to claim 1 , wherein the TEE does not accept sealed unit information after a reboot unless it also contains an EMM related remaining CKS lifetime value and ESN value.
7. The method according to claim 1 , wherein the EMM server moves to a next epoch transition over the course of a pre-defined epoch period, wherein the ESN in the EMM is incremented by M in value.
8. The method according to claim 7 , further comprises updating the sealed unit information with the new incremented value for the ESN by M in value and decrementing the remaining CKS lifetime value by N in value.
9. The method according to claim 8 , wherein when the remaining CKS lifetime value is at a lower bound when EMM is received by the TEE, the updated remaining CKS lifetime value in the TEE, and the ESN value in the EMM is not generated and the EMM is deleted or ignored.
10. The method according to claim 8 , wherein the remaining CKS lifetime value is at a lower bound after the decrementing, the EMM is deleted or ignored and sealed unit information is sent with a remaining CKS lifetime value at the lower bound.
11. The method according to claim 7 , wherein there is a time lapse between the last ESN received and the current ESN received from the EMM server so that the ESN is incremented X in value, wherein X is a multiple of M, further comprising decrementing the remaining CKS lifetime by Y, wherein the same value of the multiple of M for X, is the multiple of N for Y.
12. The method according to claim 11 , wherein the remaining CKS lifetime value is at a lower bound after the decrementing, the EMM is deleted or ignored and sealed unit information is sent with a remaining CKS lifetime value at the lower bound.
13. The method according to claim 1 , wherein the TEE is a first instance TEE, the method further comprising one or more instances of TEE including at least a second instance TEE, each instance of TEE comprising a different installed CKS, wherein the EMM server transmits a separate EMM with a separate ESN to each of the one or more instances of TEE, each of the one or more instances of TEE generates an updated remaining CKS lifetime value based on the initial lifetime duration value for the CKS of each of the one or more instances of TEE, and the ESN value in the EMM for each of the one or more instances of TEE; sealing the CKS, updated ESN value in the EMM, and updated remaining CKS lifetime value as unit information in each of the one or more instances of TEE; and delivering the updated remaining CKS lifetime value and updated ESN value in the EMM from each of the one or more instances of TEE to untrusted software in the CAS client device, and storing the updated remaining CKS lifetime value and updated ESN value for each of the one or more instances of TEE.
14. The method according to claim 1 , further comprising forming an attestation check message, and transmitting the attestation check message from the CAS client device to an attestation proxy server (APS) using a second network connection; and transmitting the attestation check response from the APS to the untrusted software using the second network connection, and sending the attestation check response from the untrusted software to the TEE.
15. The method according to claim 14 , further comprising transmitting the attestation check message to an attestation server using a third network connection to verify authenticity, and transmitting from the attestation server a result as the verification result to the APS.
16. The method according to claim 15 , wherein the attestation check message is transmitted each time there is a change in the ESN value in the sealed unit information in the TEE.
17. The method according to claim 16 , wherein the verification result is negative, the failure is logged by the client device and an alarm is initiated.
18. The method according to claim 16 , wherein the ESN value during the last successful verification result is persistently stored, and with each EMM that is transmitted, the EMM ESN value is compared to the sealed value of the ESN from the last successful verification result.
19. A non-transitory computer-readable recording medium in each of a client device and an entitlement management message (EMM) server for implementing an external trusted time source over a network to a conditional access system (CAS) client device, each non-transitory computer-readable recording medium storing one or more programs which when executed by a respective processor performs a method comprising:
transmitting an EMM to a trusted execution environment (TEE) of the CAS client device from the EMM server using a first network connection, the EMM including an epoch sequence number (ESN);
generating an updated remaining client key set (CKS) lifetime value in the TEE based on an initial lifetime duration value for the CKS, and the ESN value in the EMM;
sealing the CKS, updated ESN value in the EMM, and updated remaining CKS lifetime value as unit information; and
delivering the updated remaining CKS lifetime value and updated ESN value in the EMM from the TEE to untrusted software in the CAS client device, and storing the updated remaining CKS lifetime value and updated ESN value.
20. An apparatus for implementing an external trusted time source over a network to a conditional access system (CAS) client device, the apparatus comprising:
a non-transitory memory configured to store one or more programs, untrusted software, and a trusted execution environment (TEE);
a hardware processor configured to execute the one or more programs to perform operations of the untrusted software and the TEE; and
a network interface configured to establish communication with a network using a network connection,
wherein when the one or more programs are executed by the hardware processor:
the TEE of the CAS client device receives an entitlement management message (EMM) from an EMM server using the network connection, wherein the EMM includes an epoch sequence number (ESN),
the TEE generates an updated remaining client key set (CKS) lifetime value based on an initial lifetime duration value for the CKS, and the ESN value in the EMM,
the TEE seals the CKS, updated ESN value in the EMM, and updated remaining CKS lifetime value as unit information, and
the TEE delivers the updated remaining CKS lifetime value and ESN to untrusted software in the CAS client server, and stores the updated remaining CKS lifetime value.