Attestation

• 1: Key Broker Service (KBS)
• 1.1: KBS backed by AKV
• 2: Attestation Service (AS)
• 3: Reference Value Provider Service (RVPS)
• 4: KBS Client Tool

flowchart LR
    AA -- attests guest ----> KBS
    CDH -- requests resource --> KBS
    subgraph Guest
        CDH <.-> AA
    end
    subgraph Trustee
        KBS -- validates evidence --> AS
        RVPS -- provides reference values--> AS
    end
    client-tool -- configures --> KBS

1 - Key Broker Service (KBS)

The Confidential Containers Key Broker Service (KBS) facilitates remote attestation and secret delivery. The KBS is an implementation of a Relying Party from the Remote ATtestation ProcedureS (RATS) Architecture. The KBS itself does not validate attestation evidence. Instead, it relies on the Attestation-Service (AS) to verify TEE evidence.

In conjunction with the AS or Intel Trust Authority (ITA), the KBS supports the following TEEs:

• AMD SEV-SNP
• AMD SEV-SNP on Azure with vTPM
• Intel TDX
• Intel TDX on Azure with vTPM
• Intel SGX
• ARM CCA
• Hygon CSV

Deployment Configurations

The KBS can be deployed in several different environments, including as part of a docker compose cluster, part of a Kubernetes cluster or without any containerization. Additionally, the KBS can interact with other attestation components in different ways. This section focuses on the different ways the KBS can interact with other components.

Background Check Mode

Background check mode is a more straightforward and simple way to configure the Key Broker Service (KBS) and Attestation-Service (AS). The term “Background Check” is from the RATS architecture. In background check mode, the KBS directly forwards the hardware evidence of a confidential guest to the AS to validate. Once the validation passes, the KBS will release secrets to the confidential guest.

flowchart LR
    AA -- attests guest --> KBS
    CDH -- requests resource ----> KBS
    subgraph Guest
        AA <.-> CDH
    end
    subgraph Trustee
        KBS -- validates evidence --> AS
    end

In background check mode, the KBS is the relying party and the AS is the verifier.

Passport Mode

Passport mode decouples the provisioning of resources from the validation of evidence. In background check mode these tasks are already handled by separate components, but in passport mode they are decoupled even more. The term “Passport” is from the RATS architecture.

In passport mode, there are two Key Broker Services (KBSes), one that uses a KBS to verify the evidence and a second to provision resources.

flowchart LR
    CDH -- requests resource ----> KBS2
    AA -- attests guest --> KBS1
    subgraph Guest
        CDH <.-> AA
    end
    subgraph Trustee 1
        KBS1 -- validates evidence --> AS
    end
    subgraph Trustee 2
        KBS2
    end

In the RATS passport model the client typically connects directly to the verifier to get an attestation token (a passport). In CoCo we do not support direct connections to the AS, so KBS1 serves as an intermediary. Together KBS1 and the AS represent the verifier. KBS2 is the relying party.

Passport mode is good for use cases when resource provisioning and attestation are handled by separate entities.

1.1 - KBS backed by AKV

Premise

AKS

We assume an AKS cluster configured with Workload Identity and Key Vault Secrets Provider. The former provides a KBS pod with the privileges to access an Azure Key Vault (AKV) instance. The latter is an implementation of Kubernetes’ Secret Store CSI Driver, mapping secrets from external key vaults into pods. The guides below provide instructions on how to configure a cluster accordingly:

• Use the Azure Key Vault provider for Secrets Store CSI Driver in an Azure Kubernetes Service (AKS) cluster
• Use Microsoft Entra Workload ID with Azure Kubernetes Service (AKS)

AKV

There should be an AKV instance that has been configured with role based access control (RBAC), containing two secrets named coco_one coco_two for the purpose of the example. Find out how to configure your instance for RBAC in the guide below.

Provide access to Key Vault keys, certificates, and secrets with an Azure role-based access control

Note: You might have to toggle between Access Policy and RBAC modes to create your secrets on the CLI or via the Portal if your user doesn’t have the necessary role assignments.

CoCo

While the steps describe a deployment of KBS, the configuration of a Confidential Containers environment is out of scope for this document. CoCo should be configured with KBS as a Key Broker Client (KBC) and the resulting KBS deployment should be available and configured for confidential pods.

Azure environment

Configure your Resource group, Subscription and AKS cluster name. Adjust accordingly:

export SUBSCRIPTION_ID="$(az account show --query id -o tsv)"
export RESOURCE_GROUP=my-group
export KEYVAULT_NAME=kbs-secrets
export CLUSTER_NAME=coco

Instructions

Create Identity

Create a User managed identity for KBS:

az identity create --name kbs -g "$RESOURCE_GROUP"
export KBS_CLIENT_ID="$(az identity show -g "$RESOURCE_GROUP" --name kbs --query clientId -o tsv)"
export KBS_TENANT_ID=$(az aks show --name "$CLUSTER_NAME" --resource-group "$RESOURCE_GROUP" --query identity.tenantId -o tsv)

Assign a role to access secrets:

export KEYVAULT_SCOPE=$(az keyvault show --name "$KEYVAULT_NAME" --query id -o tsv)
az role assignment create --role "Key Vault Administrator" --assignee "$KBS_CLIENT_ID" --scope "$KEYVAULT_SCOPE"

Namespace

By default KBS is deployed into a coco-tenant Namespace:

export NAMESPACE=coco-tenant
kubectl create namespace $NAMESPACE

KBS identity and Service Account

Workload Identity provides individual pods with IAM privileges to access Azure infrastructure resources. An azure identity is bridged to a Service Account using OIDC and Federated Credentials. Those are scoped to a Namespace, we assume we deploy the Service Account and KBS into the default Namespace, adjust accordingly if necessary.

export AKS_OIDC_ISSUER="$(az aks show --resource-group "$RESOURCE_GROUP" --name "$CLUSTER_NAME" --query "oidcIssuerProfile.issuerUrl" -o tsv)"
az identity federated-credential create \
 --name kbsfederatedidentity \
 --identity-name kbs \
 --resource-group "$RESOURCE_GROUP" \
 --issuer "$AKS_OIDC_ISSUER" \
 --subject "system:serviceaccount:${NAMESPACE}:kbs"

Create a Service Account object and annotate it with the identity’s client id.

cat <<EOF> service-account.yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  annotations:
    azure.workload.identity/client-id: ${KBS_CLIENT_ID}
  name: kbs
  namespace: ${NAMESPACE}
EOF
kubectl apply -f service-account.yaml

Secret Provider Class

A Secret Provider Class specifies a set of secrets that should be made available to k8s workloads.

cat <<EOF> secret-provider-class.yaml
apiVersion: secrets-store.csi.x-k8s.io/v1
kind: SecretProviderClass
metadata:
  name: ${KEYVAULT_NAME}
  namespace: ${NAMESPACE}
spec:
  provider: azure
  parameters:
    usePodIdentity: "false"
    clientID: ${KBS_CLIENT_ID}
    keyvaultName: ${KEYVAULT_NAME}
    objects: |
      array:
      - |
        objectName: coco_one
        objectType: secret
      - |
        objectName: coco_two
        objectType: secret
    tenantId: ${KBS_TENANT_ID}
EOF
kubectl create -f secret-provider-class.yaml

Deploy KBS

The default KBS deployment needs to be extended with label annotations and CSI volume. The secrets are mounted into the storage hierarchy default/akv.

git clone https://github.com/confidential-containers/kbs.git
cd kbs
git checkout v0.8.2
cd kbs/config/kubernetes
mkdir akv
cat <<EOF> akv/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
namespace: coco-tenant

resources:
- ../base

patches:
- path: patch.yaml
  target:
    group: apps
    kind: Deployment
    name: kbs
    version: v1
EOF
cat <<EOF> akv/patch.yaml
- op: add
  path: /spec/template/metadata/labels/azure.workload.identity~1use
  value: "true"
- op: add
  path: /spec/template/spec/serviceAccountName
  value: kbs
- op: add
  path: /spec/template/spec/containers/0/volumeMounts/-
  value:
    name: secrets
    mountPath: /opt/confidential-containers/kbs/repository/default/akv
    readOnly: true
- op: add
  path: /spec/template/spec/volumes/-
  value:
    name: secrets
    csi:
      driver: secrets-store.csi.k8s.io
      readOnly: true
      volumeAttributes:
        secretProviderClass: ${KEYVAULT_NAME}
EOF
kubectl apply -k akv/

Test

The KBS pod should be running, the pod events should give indication of possible errors. From a confidential pod the AKV secrets should be retrievable via Confidential Data Hub:

$ kubectl exec -it deploy/nginx-coco -- curl http://127.0.0.1:8006/cdh/resource/default/akv/coco_one
a secret

2 - Attestation Service (AS)

The Attestation Service (AS or CoCo-AS) verifies hardware evidence. The AS was designed to be used with the Key Broker Service (KBS) for Confidential Containers, but it can be used in a wide variety of situations. The AS can be used anytime TEE evidence needs to be validated.

Today, the AS can validate evidence from the following TEEs:

• Intel TDX
• Intel SGX
• AMD SEV-SNP
• ARM CCA
• Hygon CSV
• Intel TDX with vTPM on Azure
• AMD SEV-SNP with vTPM on Azure

Overview

                                      ┌───────────────────────┐
┌───────────────────────┐ Evidence    │  Attestation Service  │
│                       ├────────────►│                       │
│ Verification Demander │             │ ┌────────┐ ┌──────────┴───────┐
│    (Such as KBS)      │             │ │ Policy │ │ Reference Value  │◄───Reference Value
│                       │◄────────────┤ │ Engine │ │ Provider Service │
└───────────────────────┘ Attestation │ └────────┘ └──────────┬───────┘
                        Results Token │                       │
                                      │ ┌───────────────────┐ │
                                      │ │  Verifier Drivers │ │
                                      │ └───────────────────┘ │
                                      │                       │
                                      └───────────────────────┘

The Attestation Service (AS) has a simple API. It receives attestation evidence and returns an attestation token containing the results of a two-step verification process. The AS can be consumed directly as a Rust crate (library) or built as a standalone service, exposing a REST or gRPC API. In Confidential Containers, the client of the AS is the Key Broker Service (KBS), but the evidence originates from the Attestation Agent inside the guest.

The AS has a two-step verification process.

1. Verify the format and provenance of evidence itself (e.g. check the signature of the evidence).
2. Appraise the claims presented in the evidence (e.g. check that measurements match reference values).

The first step is accomplished by one of the platform-specific Verifier Drivers. The second step is driven by the Policy Engine with help from the Reference Value Provider Service (RVPS).

3 - Reference Value Provider Service (RVPS)

Reference Value Provider Service (RVPS) is a component to receive software supply chain provenances / metadata, verify them and extract the reference values. All the reference values are stored inside RVPS. When Attestation Service (AS) queries specific software claims, RVPS will response with related reference values.

Architecture

RVPS contains the following components:

• Pre-Processor: Pre-Processor contains a set of *wares (like middleware). These wares can process the input Message and then deliver it to the Extractors.

• Extractors: Extractors has sub-modules to process different type of provenance. Each sub-module will consume the input Message, and then generate an output Reference Value.

• Store: Store is a trait object, which can provide key-value like API. All verified reference values will be stored in the Store. When requested by Attestation Service (AS), related reference value will be provided.

Message Flow

The message flow of RVPS is like the following figure:

Message

A protocol helps to distribute provenance of binaries. It will be received and processed by RVPS, then RVPS will generate Reference Value if working correctly.

{
    "version": <VERSION-NUMBER-STRING>,
    "type": <TYPE-OF-THE-PROVENANCE-STRING>,
    "provenance": #provenance,
}

• "version": This field is the version of this message, making extensibility possible.
• "type": This field specifies the concrete type of the provenance the message carries.
• "provenance": This field is the main content passed to RVPS. This field contains the payload to be decrypted by RVPS. The meaning of the provenance depends on the type and concrete Extractor which process this.

Trust Digests

It is the reference values really requested and used by Attestation Service to compare with the gathered evidence generated from HW TEE. They are usually digests. To avoid ambiguity, they are named trust digests rather than reference values.

4 - KBS Client Tool

This is a simple client for the Key Broker Client (KBS) that facilitates testing of the KBS and other basic attestation flows.

You can run this tool inside of a TEE to make a request with real attestation evidence. You can also provide pre-existing evidence or use the sample attester as a fallback.

The client tool can also be used to provision the KBS/AS with resources and policies.