Receiver Software Protection via Asymmetric Key Segmentation
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Solution Overview
Problem
Current conditional access/digital rights management (CA/DRM) systems for digital video broadcasting (DVB) face challenges in protecting the confidentiality and authenticity of control words, particularly in horizontal market receivers where the secret master key is known to the CA/DRM supplier, and there is a need for a solution that does not require sharing secrets between CA/DRM systems and does not require a trusted party to manage secrets after receiver personalization.
Innovation Solution
A method and apparatus that utilize a provider verification key to access a software image secured with a provider signature key, using a cryptographic function to derive descrambling information, where the function is infeasible to determine the key pair associated with the signature key and verification key, ensuring the authenticity and confidentiality of the control word without sharing secrets or managing keys post-personalization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a secret master key is shared between CA/DRM systems for key management, then system interoperability is improved, but security is worsened because the secret key may be compromised
Solution Approach 1:
The secret master key is segmented into two separate key pairs: a first key pair for the chip set manufacturer and a second key pair for the CA/DRM system operator. This segmentation eliminates the need for shared secrets while maintaining system interoperability through the cryptographic function that combines outputs from both key pairs.
Solution Approach 2:
The system transitions from symmetric key sharing to asymmetric cryptography with two independent key pairs. Each party has their own private key that never leaves their system, while public keys and cryptographic outputs enable interoperability without exposing secret materials.
2Reliability
If a trusted party manages receiver secrets after personalization, then security is improved through centralized control, but device complexity and liability are worsened
Solution Approach 1:
The trusted party's role in managing secrets after personalization is extracted and eliminated. The chip set manufacturer's secret key and the CA/DRM system operator's secret key are completely separated, with each system independently managing its own secrets without requiring ongoing trusted party involvement.
Solution Approach 2:
Each system (chip set and CA/DRM client) serves itself by independently managing its own secret keys. The chip set uses its own private key to verify authenticity, and the CA/DRM system uses its own private key to generate control words, eliminating dependency on external trusted party management.
3Reliability
If cryptographic functions are designed to be infeasible to reverse-engineer, then security is improved, but computational complexity is worsened
Solution Approach 1:
The cryptographic function is designed with sufficient complexity to be computationally infeasible to reverse-engineer, providing the necessary security margin. The function takes multiple inputs (first cryptographic output, second cryptographic output, and virtual descrambling information) and produces a control word through a complex transformation that balances security requirements with practical implementation.
Data Source
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AI summary
A method of obtaining descrambling information at a receiver, the descrambling information for enabling descrambling of scrambled content, the method comprising: using a provider verification key to access a software image, said software image having been secured by a provider using a provider signature key corresponding to the provider verification key; receiving a secured version of the descrambling information; using the accessed software image to obtain virtual descrambling information from the secured version of the descrambling information; providing the virtual descrambling information and said provider verification key as inputs to a cryptographic function to produce a given output comprising said descrambling information, wherein the cryptographic function has the property that it is infeasible to determine a key pair including a signature key and a verification key associated with the signature key, and another virtual descrambling information, such that the determined verification key and the another virtual descrambling information map to the given output of the cryptographic function.