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Confidential Kubernetes
We use the term Confidential Kubernetes to refer to the concept of using confidential-computing technology to shield entire Kubernetes clusters from the infrastructure. The three defining properties of this concept are:
- Workload shielding: the confidentiality and integrity of all workload-related data and code are enforced.
- Control plane shielding: the confidentiality and integrity of the cluster's control plane, state, and workload configuration are enforced.
- Attestation and verifiability: the two properties above can be verified remotely based on hardware-rooted cryptographic certificates.
Each of the above properties is equally important. Only with all three in conjunction, an entire cluster can be shielded without gaps. This is what Constellation is about.
Constellation's approach is to run all nodes of the Kubernetes cluster inside Confidential VMs (CVMs). This gives runtime encryption for the entire cluster. Constellation augments this with transparent encryption of the network and storage. Thus, workloads and control plane are truly end-to-end encrypted: at rest, in transit, and at runtime. Constellation manages the corresponding cryptographic keys inside CVMs. Constellation verifies the integrity of each new CVM-based node using remote attestation. Only "good" nodes receive the cryptographic keys required to access the network and storage of a cluster. (A node is "good" if it's running a signed Constellation image inside a CVM and is in the expected state.) Towards the DevOps engineer, Constellation provides a single hardware-rooted certificate from which all of the above can be verified. As a result, Constellation wraps an entire cluster into one coherent confidential context. The concept is depicted in the following.
In contrast, managed Kubernetes with CVMs, as it's for example offered in AKS and GKE, only provides runtime encryption for certain worker nodes. Here, each worker node is a separate (and typically unverified) confidential context. This only provides limited security benefits as it only prevents direct access to a worker node's memory. The large majority of potential attacks through the infrastructure remain unaffected. This includes attacks through the control plane, access to external key management, and the corruption of worker node images. This leaves many problems unsolved. For instance, Node A has no means to verify if Node B is "good" and if it's OK to share data with it. Consequently, this approach leaves a large attack surface, as is depicted in the following.
The following table highlights the key differences in terms of features:
| Managed Kubernetes with CVMs | Confidential Kubernetes (Constellation✨) | |
|---|---|---|
| Runtime encryption | Partial (data plane only) | Yes |
| Node image verification | No | Yes |
| Full cluster attestation | No | Yes |
| Transparent network encryption | No | Yes |
| Transparent storage encryption | No | Yes |
| Confidential key management | No | Yes |
| Cloud agnostic / multi-cloud | No | Yes |