Cloud Trust Models Explained

1. Defining Trust in a Cloud Context

In everyday language, trust is a feeling. In computing, and especially for your exams, we need a more formal, quantifiable definition. Trust in the cloud is a relationship between two parties: the one who trusts (the trustor) and the one who is trusted (the trustee).

To grasp these foundational concepts, please read the following excerpt from a research paper on trust in cloud computing.

From the reading, here are the key definitions to memorize:

• Trustor: The party that places trust in another. In a cloud context, this can be the Cloud Service Consumer (CSC) or the Cloud Service Provider (CSP).
• Trustee: The party that is being trusted by the trustor.
• Trust: A quantifiable and direct relationship, representing the subjective willingness or expectation of the trustor that the trustee will perform a specific action, even with potential risks.
• Trustworthiness: An internal, objective attribute of the trustee that reflects its capability and reliability, forming the basis for the trustor's subjective assessment.

In simple terms: A provider's certifications and security infrastructure contribute to its trustworthiness (objective fact). A customer's decision to use that provider is an act of trust (subjective assessment).

2. The Core Trust Models: User, Provider, and Mutual

Trust in the cloud is not a one-way street. Both the customer and the provider have assets to protect, leading to different perspectives or "models" of trust. Your syllabus identifies three: user trust, provider trust, and mutual trust.

The following reading explains the two fundamental orientations of trust evaluation.

Based on the reading, we can define the three models as follows:

A. User Trust Model (CSC-Oriented)

This is the most common perspective. It focuses on the customer's need to trust the provider.

• Definition: The User Trust Model (also known as the Cloud Service Consumer-oriented model) is the process by which a customer evaluates the trustworthiness of a cloud provider.
• Trustor: Cloud Service Consumer (CSC)
• Trustee: Cloud Service Provider (CSP)
• Goal: To help the CSC select a reliable and secure provider that will protect its data and meet service level agreements (SLAs). The CSC worries about data breaches, service downtime, and vendor lock-in.

B. Provider Trust Model (CSP-Oriented)

This model flips the perspective. The provider also needs to trust its customers.

• Definition: The Provider Trust Model (also known as the Cloud Service Provider-oriented model) is the process by which a provider evaluates the trustworthiness of its customers (or tenants).
• Trustor: Cloud Service Provider (CSP)
• Trustee: Cloud Service Consumer (CSC)
• Goal: To protect the cloud platform and other tenants from malicious user behavior. The CSP worries about users launching denial-of-service attacks, hosting illegal content, or attempting to compromise the underlying infrastructure (a "VM escape" attack, as we discussed previously).

C. Mutual Trust Model

This is not a separate mechanism but rather the ideal state achieved when both user and provider trust models are successfully established.

• Definition: The Mutual Trust Model is a bidirectional relationship where the CSC trusts the CSP to provide secure and reliable services, and the CSP trusts the CSC to use those services in a lawful and non-malicious manner.
• Mechanism: It is achieved through the combined application of CSC-oriented and CSP-oriented trust mechanisms. For example, a customer signs a detailed SLA (establishing user trust), while also agreeing to an Acceptable Use Policy (AUP) and undergoing identity verification (establishing provider trust).

For your exam revision, here is a summary table:

3. How is Trust Practically Established?

Defining the models is the first step. The second is understanding the practical mechanisms used to calculate and establish trust. These mechanisms are what we refer to as "trust models" in a more technical sense.

Here is a summary of the four primary types of trust models, which you can use to answer questions about how trust is established:

1. Agreement-Based Trust Models:

   • Trust is established through formal contracts negotiated between the CSC and CSP.
   • The cornerstone of this model is the Service Level Agreement (SLA), which defines specific, measurable metrics for performance, availability, and security, along with penalties for failure.
   • This is a primary mechanism for establishing User Trust.

2. Certificate/Key-Based Trust Models:

   • Trust is established through cryptographic proof and hardware attestation.
   • This includes using digital certificates to verify the integrity of software components and leveraging technologies like a Trusted Platform Module (TPM) to ensure that a server boots up in a known, secure state.
   • This model provides strong technical assurance and is fundamental to establishing User Trust in the provider's infrastructure.

3. Feedback-Based Trust Models (Reputation-Based):

   • Trust is calculated based on collective feedback, ratings, and historical performance. This can be from other users or from independent third parties.
   • Examples include:
     • User reviews and public ratings of a service.
     • Third-party audit reports (e.g., SOC 2, ISO 27001), which serve as expert "feedback."
     • The concept of Trust as a Service (TaaS), where a third-party entity continuously monitors and rates the trustworthiness of various CSPs.
   • This is a key model for establishing User Trust.

4. Domain-Based Trust Models:

   • This model, originating from grid computing, divides the cloud into different "domains." Trust is calculated differently for interactions within a domain versus between domains.
   • Each domain manager maintains trust tables for entities based on their interaction history. If no direct history exists, it seeks recommendations from other domains.
   • This can be used to establish both User Trust (a user's domain trusting a provider's domain) and Provider Trust (a provider's domain tracking the reputation of user domains).

4. Trust and the Shared Responsibility Model

Finally, it's essential to connect these trust models to the Shared Responsibility Model we've discussed throughout this course. Trust is what enables each party to have confidence that the other is managing their part of the technology stack correctly.

Consider an IaaS model:

• User Trust: The user trusts the provider to secure the physical infrastructure (servers, storage, networking) and the virtualization layer. They might verify this using a feedback-based model (reviewing the provider's ISO 27001 certification) and an agreement-based model (signing an SLA that guarantees network uptime).
• Provider Trust: The provider trusts the user to secure their own virtual machines, operating systems, and applications. They might enforce this using a provider-oriented model that monitors network traffic from the user's VMs to detect malicious behavior.

The trust model provides the framework for verifying that responsibilities are being met.

Conclusion

In this lesson, we formalized the concept of trust in cloud computing and explored the different models that govern the relationship between customers and providers.

Key Takeaways:

• Trust is a subjective assessment by a trustor (e.g., a customer) of a trustee's (e.g., a provider's) ability to perform as expected.
• The three main perspectives are:
  • User Trust (CSC-Oriented): The customer needs to trust the provider.
  • Provider Trust (CSP-Oriented): The provider needs to trust the customer.
  • Mutual Trust: A bidirectional state where both parties trust each other.
• Trust is practically established through several models: agreement-based (SLAs), certificate/key-based (TPM, crypto), feedback-based (reputation, audits), and domain-based.
• These trust models are the mechanisms used to verify that each party is fulfilling its duties under the Shared Responsibility Model.

Preview of the Next Lesson:

This concludes our module on Cloud Security and Governance. Our next module, "Cloud Monitoring and Financial Management," dives into the operational aspects of managing a cloud environment. We will begin by looking at a key component of agreement-based trust models in great detail. Your next lesson will address the learning outcome: Define a Service Level Agreement (SLA) and explain its key components (e.g., uptime, performance metrics, penalties).