Use AWS Private Certificate Authority to issue device attestation certificates for Matter

In this blog post, we show you how to use AWS Private Certificate Authority (CA) to create Matter device attestation CAs to issue device attestation certificates (DAC). By using this solution, device makers can operate their own device attestation CAs, building on the solid security foundation provided by AWS Private CA. This post assumes that you are familiar with the public key infrastructure (PKI) — if needed, you can review the topic What is public key infrastructure?.

Overview

Matter, governed by the Connectivity Standard Alliance (CSA), is a new open standard for seamless and secure cross-vendor connectivity for smart home devices. It provides a common language for devices manufactured by different vendors to communicate on a smart home network by using the Wi-Fi and Thread wireless protocols. The Matter 1.0 specification, released in October 2022, supports smart home bridges and controllers, door locks, HVAC controls, lighting and electrical devices, media devices, safety and security sensors, and window coverings and shades. Future versions of the Matter specification will support additional smart home devices such as cameras, home appliances, robot vacuums, and other market segments such as electric vehicle (EV) charging and energy management.

To help achieve security and interoperability, Matter requires device certification and authenticity checks before devices can join a smart home network (known as the Matter fabric). The authenticity of a Matter device is established by using an X.509 certificate called a DAC, which is provisioned by device makers. When a customer purchases a Matter-compliant smart home device and uses a smart home hub to add that device to their Matter fabric, the hub validates the DAC before allowing the device to operate on the smart home’s Matter fabric.

Matter requires DACs to be issued by a device attestation CA that is compliant with the Matter PKI certificate policy (CP). Device vendors can use AWS Private CA to host their device attestation CAs. AWS Private CA is a highly available service that helps organizations secure their applications and devices by using private certificates. In Q1 2023, AWS Private CA launched support for Matter certificates, published the Matter PKI Compliance Customer Guide, and released open source samples to help customers create and operate Matter-compliant device attestation CAs.

In this post, we demonstrate how to build a Matter device attestation CA hierarchy by using AWS Private CA. The examples will use AWS Cloud Development Kit (AWS CDK) and AWS CloudFormation to create and provision the CAs and to issue Matter-compliant DACs. We also show how you can configure other AWS services like Amazon CloudWatch, AWS CloudTrail, and Amazon Simple Storage Service (Amazon S3) to help perform the event logging, record keeping, and audit log retention that is required in the Matter standard.

The Matter CA hierarchy for DACs

When a smart home device vendor makes the decision to adopt the Matter standard, they first apply to the CSA to get a vendor ID for their organization, and one or more product IDs for the products that will be Matter-certified. Using the vendor ID and product ID, the vendor can then set up their Matter CA hierarchy to issue DACs that will be provisioned on to their Matter-certified devices. Matter prescribes a two-level CA hierarchy for issuing DACs. The product attestation authority (PAA) is at the top of the hierarchy and forms the root of trust for the DACs that chain up to it. The product attestation intermediate (PAI) is at the second level of the CA hierarchy and is the CA that issues DACs. Figure 1 illustrates a sample Matter CA hierarchy for a vendor that manufactures devices with two different product IDs.

Figure 1: Matter CA three-tier hierarchy and the role of PAA, PAIs, and DACs

PAA certificates must be approved by the CSA before they are made available to the Matter community through the Distributed Compliance Ledger (DCL). The DCL is a shared database based on blockchain technology that holds the data elements that are necessary to attest the validity of a Matter device. CSA provides access to a public DCL node that it maintains, and private nodes can be created for dedicated access. To indicate whether a particular device is Matter compliant, a certification declaration (CD) is used. A CD is a CSA-created cryptographic document encoded in the Cryptographic Message Syntax (CMS) format described in RFC 5652. The data elements in the DCL include the list of PAAs (the roots of trust) and the vendor IDs, product IDs, and CD for each Matter certified product. The DCL also contains a description of each Matter certified product, which includes elements such as the device name, vendor, firmware version, and localized strings to support internationalization.

After the Matter CA hierarchy is set up, the CA operator must complete a certification practice statement (CPS) that describes how the CA operator will comply with the physical, operational, and logical controls specified in the Matter PKI CP. These controls address a large list of scenarios. These include the creation of the CA keys, storage of these key pairs, physical access controls for the HSMs, and separation of roles that perform administrative and operational tasks. The CA operators must submit their CPS, along with the PAA certificate, to the CSA for final approval. After the CPS is approved by the CSA, the PAA certificate is added to the list of approved Matter root certificates. This will now allow a Matter-compliant smart home network hub to verify the validity of the DAC that is provisioned on the Matter-certified device.

Build Matter CA hierarchies by using AWS Private CA

AWS Private CA has open sourced samples on GitHub that help you to create Matter-ready CA hierarchies and can issue DACs for your devices. These samples use the AWS CDK and CloudFormation templates to set up and configure services like Amazon CloudWatch, AWS CloudTrail and Amazon S3, to help you meet the Matter PKI compliance requirements. In the following sections, we walk you through an introduction to these samples, and discuss how you can use them to build your Matter CA hierarchies in AWS Private CA.

Solution architecture and building blocks

Figure 2 shows the Matter device attestation PKI architecture that is created when you run the GitHub samples. The architecture works as follows:

A PAA is created by a PKI administrator, using the generatePaa key.
You create PAIs by using the generatePaiCnt key in a single AWS Region, or you can choose to house the PAIs in different Regions.
AWS CloudTrail captures the AWS API actions that are performed.
Amazon CloudWatch receives a stream of filtered events specific to AWS Private CA and stores it in a dedicated MatterAudit log group.
The logs are stored in Amazon S3 buckets.
Logs can only be read by using the auditor-specific AWS Identity and Access Management (IAM) role that is configured for read-only access.
The sample script configures the S3 buckets to automatically move log data to Amazon S3 Glacier storage by using S3 Lifecycle rules (Figure 2 shows an example of monthly backup plan for S3), in order to meet the long-term log retention requirements of the Matter standard.
To issue DACs by using the PAIs created by the samples, you upload your certificate signing requests (CSRs) to the input/output S3 bucket.
New CSRs are picked up by an AWS Lambda function through the use of Amazon Simple Queue Service (Amazon SQS).
The DAC Signing Lambda function uses a PAI to generate a certificate.
The Lambda function writes the generated certificates (which are the DACs) in the input/output S3 bucket in the Privacy Enhanced Mail (PEM) format.

Figure 2: Matter device attestation PKI architecture topology that uses AWS Private CA

Prerequisites

For a typical deployment, make sure you have a working host with the following software dependencies installed:

Using the following Git command, download sample scripts from the Matter PKI CDK samples on GitHub.

$ git clone https://github.com/aws-samples/aws-private-ca-matter-infrastructure.git
$ cd aws-private-ca-matter-infrastructure

Finally, set up the AWS CDK Toolkit with your AWS account’s credentials (see the AWS Cloud Development Kit documentation for details).

Deploy the PAA

The first step is to create and deploy a Matter PAA by running the following command:

$ cdk deploy ‐‐context generatePaa=1 ‐‐parameters vendorId=FFF1 ‐‐parameters validityInDays=3600

After the command completes, you should see the following output:

MatterStackPAA.CloudTrailArn = arn:aws:cloudtrail:us-east-1:111111111111:trail/MatterStackPAA-MatterAuditTrail586D7D12-akSFjLJyEumz

MatterStackPAA.LogGroupName = MatterAudit

MatterStackPAA.PAA = VID=FFF1 CN=PAA arn:aws:acm-pca:us-east-1:111111111111:certificate-authority/99999999-9999-9999-9999-999999999999

MatterStackPAA.PAACertArn = arn:aws:acm-pca:us-east-1:111111111111:certificate-authority/99999999-9999-9999-9999-999999999999/certificate/99999999999999999999999999999999

MatterStackPAA.PAACertLink = https://console.aws.amazon.com/acm-pca/home?region=us-east-1#/details?arn=arn:aws:acm-pca:us-east-1:111111111111:certificate-authority/99999999-9999-9999-9999-999999999999&tab=certificate

Stack ARN:
    arn:aws:cloudformation:us-east-1:111111111111:stack/MatterStackPAA/11111111-1111-1111-1111-111111111111

Note the following fields in the output:

CloudTrailArn — The Amazon Resource Name (ARN) of the CloudTrail trail that is collecting logs of performed operations.
LogGroupName — The ARN of the CloudWatch log group where you can find logs of Matter-relevant operations.
PAA — The vendor ID, common name, and ARN of the AWS Private CA root certificate authority created as the PAA.
PAACertArn — The ARN of the certificate of the PAA in AWS Private CA.
PAACertLink — The link to the PAA certificate in the AWS Private CA console.

Deploy one or more PAIs

To deploy two PAIs that chain up to the PAA that you created in the previous step, run the following command:

$ PAA_ARN=$(awk '/MatterStackPAA.PAA = /{ print substr($0, match($0, "arn")); }' DeployPAA.log)
$ cdk deploy --context generatePaiCnt=2 ‐‐parameters vendorId=FFF1 ‐‐parameters productIds=0100,0102 ‐‐parameters validityInDays=365 ‐‐parameters paaArn=$PAA_ARN

If you want to create more or fewer PAIs, you can modify the generatePaiCnt and productIds parameters to reflect the correct number of PAIs that you want.

After the command completes, you should see the following output:

MatterStackPAI.CertArnPAI0 = arn:aws:acm-pca:us-east-1:111111111111:certificate-authority/99999999-9999-9999-9999-999999999999/certificate/11111111111111111111111111111111

MatterStackPAI.CertArnPAI1 = arn:aws:acm-pca:us-east-1:111111111111:certificate-authority/99999999-9999-9999-9999-999999999999/certificate/22222222222222222222222222222222

MatterStackPAI.CertLinkPAI0 = https://console.aws.amazon.com/acm-pca/home?region=us-west-1#/details?arn=arn:aws:acm-pca:us-west-1:111111111111:certificate-authority/11111111-1111-1111-1111-111111111111&tab=certificate

MatterStackPAI.CertLinkPAI1 = https://console.aws.amazon.com/acm-pca/home?region=us-west-1#/details?arn=arn:aws:acm-pca:us-west-1:111111111111:certificate-authority/22222222-2222-2222-2222-222222222222&tab=certificate

MatterStackPAI.PAI0 = VID=FFF1 PID=0100 CN=PAI0 arn:aws:acm-pca:us-west-1:111111111111:certificate-authority/11111111-1111-1111-1111-111111111111

MatterStackPAI.PAI1 = VID=FFF1 PID=0102 CN=PAI1 arn:aws:acm-pca:us-west-1:111111111111:certificate-authority/22222222-2222-2222-2222-222222222222

Note the following fields in the output:

CertArnPAI0 — The ARN of the certificate of the first PAI in AWS Private CA.
CertArnPAI1 — The ARN of the certificate of the second PAI in AWS Private CA.
CertLinkPAI0 — The link to the first PAI certificate in the AWS Private CA console.
CertLinkPAI1 — The link to the second PAI certificate in the AWS Private CA console.
PAI0 — The vendor ID, product ID, common name, and ARN of the AWS Private CA subordinate certificate authority created as the first PAI.
PAI1 — The vendor ID, product ID, common name, and ARN of the AWS Private CA subordinate certificate authority created as the second PAI.

Deploy multi-Region PAIs

The following command is an example of how you can generate a PAI in a different Region. To specify a Region, either use the CDK_DEFAULT_REGION environment variable or specify a different AWS profile by using the –profile parameter (see AWS CDK documentation for more details).

$ CDK_DEFAULT_REGION=us-east-1
$ cdk deploy ‐‐context generatePaiCnt=1 ‐‐parameters vendorId=FFF1 ‐‐parameters productIds=0100 ‐‐parameters validityInDays=365 ‐‐parameters paaArn=<PAA-ARN-FROM-PAA-OUTPUT>

Issue DACs

After the Matter CA hierarchy is set up, you can issue DACs by writing your CSRs directly to the DacInputS3ToSQSS3Bucket S3 bucket. The SqsToDacIssuingLambda function signs the CSRs by using the specified PAI to create a DAC, and then the function writes that DAC back to the S3 bucket.

To test this procedure by using OpenSSL

Use the following command to create a key pair for the DAC:

$ openssl ecparam -name prime256v1 -out certificates/ecparam

Create the DAC CSR with DAC as the common name; the following is an example:

$ openssl req -config certificates/config.ssl -newkey param:certificates/ecparam -keyout key-DAC.pem -out cert-DAC.csr -sha256 -subj "/CN=DAC"

Request CSR signing by using a PAI identified by its UUID:

$ aws s3 cp cert-DAC.csr s3://Matterstackpai-dacinputs3tosqss3bucket<remainder of your bucket name>/arn:aws:acm-pca:<region>:<account>:certificate-authority/<PAI UUID>/<PID>/cert-DAC.csr

The PAI will respond with the certificate as soon as it finishes processing the CSR, and the certificate will be made available in the same S3 bucket:

$ aws s3 cp s3://Matterstackpai-dacinputs3tosqss3bucket<remainder of your bucket name>/arn:aws:acm-pca:<region>:<account>:certificate-authority/<PAI UUID>/<PID>/cert-DAC.pem

Audit trails, record keeping, and retention

The Matter PKI CP mandates controls and processes that are required to operate Matter device attestation CAs. The Matter PKI CDK samples on GitHub configure IAM roles, AWS CloudTrail, Amazon CloudWatch, and Amazon S3 to help you meet these requirements. Specifically, the samples configure the following:

Permissions
- The /MatterPKI/MatterIssueDACRole role, for issuing and revoking DACs from PAIs in AWS Private CA.
- The /MatterPKI/MatterIssuePAIRole role, for issuing and revoking PAIs from the PAA in AWS Private CA.
- The /MatterPKI/MatterManagePAARole role, for monitoring and managing the PAA.
- The /MatterPKI/MatterAuditorRole role, for viewing, collecting, and managing the logs that pertain to Matter PKI resources.
Logging
- Audit logs should be maintained for operations performed in AWS that are related to the Matter PKI. This is accomplished by using AWS CloudTrail to store logs of API calls in S3.
- In order to preserve the integrity of the data in the audit logging S3 bucket, the sample scripts configure S3 Object Lock to make objects write-once, read-many.
- A resource policy is applied to the S3 bucket that grants IAM role access only to the auditor.
- Data in this S3 bucket is backed up to a vault by using AWS Backup. From there, the data can be restored in case of an emergency.
- The logs are retained in S3 for two months after creation and then are automatically moved into S3 Glacier for the rest of their five-year lifecycle, where they can be restored if necessary.
- In order to make sure that the logs can be quickly queried by an auditor, the sample script sets up CloudTrail to continuously send events that are filtered to include only events relevant to Matter PKI to the AWS CloudWatch log group MatterAudit. You should restrict access to the stored logs by enabling log integrity validation to assure that the log remains computationally infeasible to modify, delete or forge.

Verify and validate certificates for Matter compliance

The Matter standards working group provides the CHIP Certificate Tool (chip-cert), a CLI utility that you can use to verify the conformance of the certificates that form a DAC chain. If you use the sample script we provided earlier to generate DACs, this tool is run as part of the certificate issuance process to verify that the certificate and chain are Matter compliant and no misconfiguration has taken place.

Use the following command to validate a DAC chain by using chip-cert manually.

$./chip-cert validate-att-cert ‐‐dac <dac-certificate-file-in-PEM> ‐‐pai <pai-certificate-file-in-PEM> ‐‐paa <paa-certificate-file-in-PEM>

Best practices for assuring security, scalability, and resiliency

For a list of best practices and recommendations for using AWS Private CA effectively, see AWS Private CA best practices.

Cost considerations

When you install the Matter PKI CDK, you will incur charges for the deployment and use of AWS Private CA and the associated services. To delete a private CA permanently, see Deleting your private CA.

Conclusion

In this blog post, we discussed in depth the use of AWS Private CA to create and operate Matter device attestation CAs in order to issue DACs. We walked you through how you can use our open source samples on GitHub to create Matter PAAs and PAIs, and how to configure other AWS services like Amazon CloudWatch, AWS CloudTrail and Amazon S3 to help you meet the compliance requirements that are stipulated by the Matter PKI CP. We also showed you how you can use the Matter PAIs to issue DACs for your smart home devices and verify that the issued DACs meet the Matter requirements. To learn more about using the GitHub samples, see the README file located in the GitHub repository. We welcome your feedback to help us improve the samples.

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