Condition Matching For Derived Keys Using Crypto-key-type-choice

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Introduction

In the realm of attester designs, such as SPDM and DICE, dynamically generated keys pose a significant challenge when it comes to matching using crypto-key-type-choice or exact match semantics. This is because the dynamically generated keys are not known until they are generated, making it difficult to establish a match. In this article, we will explore a potential solution to this problem by leveraging certificate chains and parent keys to enable condition matching for derived keys.

The Challenge of Dynamically Generated Keys

In attester designs, such as SPDM and DICE, keys are often dynamically generated after deployment. This means that the keys are not known until they are generated, making it challenging to match them using traditional crypto-key-type-choice or exact match semantics. The current approach relies on pre-defined keys, which are not applicable in this scenario.

Certificate Chains: A Potential Solution

A certificate chain is a structure that could potentially allow matching on derived keys. If the derived keys have a portion of the cert chain in common and the cert chain that is in common is known before the alias (lower layer) keys have been generated, it could be used to match the derived keys. This approach relies on the existence of a certificate chain that is known before the keys are generated.

Parent Keys: A Trigger for Derived Key Matching

If a parent key is supplied in the matching criteria, it can be interpreted as an intention to match on a derived key. For example, if the crypto-key-type-choice options for cert chain (tagged-pkix-base64-cert-path-type and tagged-cert-path-thumbprint-type) are used, it could trigger verifier behavior that applies path construction to the intermediate/alias certs to the specified criteria. This would result in a full cert path that could be used as the condition to match the derived keys.

Path Construction: A Key Enabler

Path construction is a critical component in enabling condition matching for derived keys. By applying path construction to the intermediate/alias certs, a full cert path can be constructed. This full cert path can then be used as the condition to match the derived keys. The concern, however, is whether the intent was to match a specific key vs. any of the keys in the cert path.

Evidence and Singleton Keys

If Evidence contains a singleton key, path construction could be applied to result in a full cert path. Matching any part of the cert path in a condition would match the ECT. This approach raises the concern of whether the intent was to match a specific key vs. any of the keys in the cert path.

Alternative Approaches

While the approach of using certificate chains and parent keys is a potential solution, there could be other approaches to enable condition matching for derived keys. For example, if the Evidence contains a singleton key, path construction could be applied to result in a full cert path. Matching any part of the cert path in a condition would match the ECT.

Conclusion

In conclusion, condition matching for derived keys using crypto-key-type-choice is a challenging problem in attester designs. However, by leveraging certificate chains and parent keys, it is possible to enable condition matching for derived keys. The approach relies on the existence of a certificate chain that is known before the keys are generated and the use of parent keys as a trigger for derived key matching. While there could be other approaches, this solution provides a potential solution to the problem.

Recommendations

Based on the analysis, the following recommendations are made:

  • Use certificate chains to enable condition matching for derived keys.
  • Apply path construction to the intermediate/alias certs to result in a full cert path.
  • Use parent keys as a trigger for derived key matching.
  • Consider alternative approaches, such as using singleton keys in Evidence.

Future Work

Future work should focus on implementing and testing the proposed approach. This would involve:

  • Developing a prototype to demonstrate the feasibility of the approach.
  • Conducting experiments to evaluate the performance and effectiveness of the approach.
  • Refining the approach based on the results of the experiments.

Introduction

In our previous article, we explored the challenge of condition matching for derived keys using crypto-key-type-choice in attester designs, such as SPDM and DICE. We proposed a potential solution by leveraging certificate chains and parent keys to enable condition matching for derived keys. In this article, we will address some of the frequently asked questions (FAQs) related to this topic.

Q: What is the main challenge in condition matching for derived keys?

A: The main challenge is that dynamically generated keys are not known until they are generated, making it difficult to establish a match using traditional crypto-key-type-choice or exact match semantics.

Q: How does the proposed approach address this challenge?

A: The proposed approach leverages certificate chains and parent keys to enable condition matching for derived keys. If the derived keys have a portion of the cert chain in common and the cert chain that is in common is known before the alias (lower layer) keys have been generated, it can be used to match the derived keys.

Q: What is the role of parent keys in the proposed approach?

A: Parent keys are used as a trigger for derived key matching. If a parent key is supplied in the matching criteria, it can be interpreted as an intention to match on a derived key.

Q: How does path construction enable condition matching for derived keys?

A: Path construction is a critical component in enabling condition matching for derived keys. By applying path construction to the intermediate/alias certs, a full cert path can be constructed. This full cert path can then be used as the condition to match the derived keys.

Q: What are the concerns related to the proposed approach?

A: The concern is whether the intent was to match a specific key vs. any of the keys in the cert path. Additionally, there could be other approaches to enable condition matching for derived keys.

Q: What are some alternative approaches to enable condition matching for derived keys?

A: Some alternative approaches include using singleton keys in Evidence, applying path construction to result in a full cert path, and matching any part of the cert path in a condition to match the ECT.

Q: What are the benefits of the proposed approach?

A: The proposed approach enables condition matching for derived keys, which is a significant advancement in attester designs. It also provides a potential solution to the challenge of dynamically generated keys.

Q: What are the limitations of the proposed approach?

A: The proposed approach relies on the existence of a certificate chain that is known before the keys are generated. It also requires the use of parent keys as a trigger for derived key matching.

Q: What are the future directions for this research?

A: Future work should focus on implementing and testing the proposed approach. This would involve developing a prototype to demonstrate the feasibility of the approach, conducting experiments to evaluate the performance and effectiveness of the approach, and refining the approach based on the results of the experiments.

Conclusion

In conclusion, matching for derived keys using crypto-key-type-choice is a challenging problem in attester designs. However, by leveraging certificate chains and parent keys, it is possible to enable condition matching for derived keys. The proposed approach provides a potential solution to this challenge and has several benefits. However, it also has some limitations and concerns that need to be addressed. Future work should focus on implementing and testing the proposed approach to further advance this research.

Recommendations

Based on the analysis, the following recommendations are made:

  • Use certificate chains to enable condition matching for derived keys.
  • Apply path construction to the intermediate/alias certs to result in a full cert path.
  • Use parent keys as a trigger for derived key matching.
  • Consider alternative approaches, such as using singleton keys in Evidence.
  • Implement and test the proposed approach to further advance this research.

Glossary

  • Certificate chain: A structure that contains a series of certificates that are linked together.
  • Parent key: A key that is used as a trigger for derived key matching.
  • Path construction: A process that constructs a full cert path by applying path construction to the intermediate/alias certs.
  • Singleton key: A key that is used in Evidence to enable condition matching for derived keys.
  • SPDM: A protocol that is used for secure protocol discovery and management.
  • DICE: A protocol that is used for secure attestation and key exchange.