How to issue use-case bound certificates with AWS Private CA

In this post, we’ll show how you can use AWS Private Certificate Authority (AWS Private CA) to issue a wide range of X.509 certificates that are tailored for specific use cases. These use-case bound certificates have their intended purpose defined within the certificate components, such as the Key Usage and Extended Key usage extensions. We will guide you on how you can define your usage by applying your required Key Usage and Extended Key usage values with the IssueCertificate API operation.

Background

With the AWS Private CA service, you can build your own public key infrastructure (PKI) in the AWS Cloud and issue certificates to use within your organization. Certificates issued by AWS Private CA support both the Key Usage and Extended Key Usage extensions. By using these extensions with specific values, you can bind the usage of a given certificate to a particular use case during creation. Binding certificates to their intended use case, such as SSL/TLS server authentication or code signing, provides distinct security benefits such as accountability and least privilege.

When you define certificate usage with specific Key Usage and Extended Key Usage values, this helps your organization understand what purpose a given certificate serves and the use case for which it is bound. During audits, organizations can inspect their certificate’s Key Usage and Extended Key Usage values to determine the certificate’s purpose and scope. This not only provides accountability regarding a certificate’s usage, but also a level of transparency for auditors and stakeholders. Furthermore, by using these extensions with specific values, you will follow the principle of least privilege. You can grant least privilege by defining only the required Key Usage and Extended Key Usage values for your use case. For example, if a given certificate is going to be used only for email protection (S/MIME), you can assign only that extended key usage value to the certificate.

Certificate templates and use cases

In AWS Private CA, the Key Usage and Extended Key Usage extensions and values are specified by using a configuration template, which is passed with the IssueCertificate API operation. The base template provided by AWS handles the most common certificate use cases, such as SSL/TLS server authentication or code signing. However, there are additional use cases for certificates that are not defined in base templates. To issue certificates for these use cases, you can pass blank certificate templates in your IssueCertificate requests, along with your required Key Usage and Extended Key usage values.

Such use cases include, but are not limited to the following:

Certificates for SSL/TLS
- Issue certificates with an Extended Key Usage value of Server Authentication, Client Authentication, or both.
Certificates for email protection (S/MIME)
- Issue certificates with an Extended Key Usage value of E-mail Protection
Certificates for smart card authentication (Microsoft Smart Card Login)
- Issue certificates with an Extended Key Usage value of Smart Card Logon
Certificates for document signing
- Issue certificates with an Extended Key Usage value of Document Signing
Certificates for code signing
- Issue certificates with an Extended Key Usage value of Code Signing
Certificates that conform to the Matter connectivity standard
- Matter-compliant device attestation certificates
- Matter-compliant subordinate CA certificates
- Matter-compliant node operational certificate
- More Matter-compliant certificates

If your certificates require less-common extended key usage values not defined in the AWS documentation, you can also pass object identifiers (OIDs) to define values in Extended Key Usage. OIDs are dotted-string identifiers that are mapped to objects and attributes. OIDs can be defined and passed with custom extensions using API passthrough. You can also define OIDs in a CSR (certificate signing request) with a CSR passthrough template. Such uses include:

Certificates that require IPSec or virtual private network (VPN) related extensions
- Issue certificates with Extended Key Usage values:
  - OID: 1.3.6.1.5.5.7.3.5 (IPSEC_END_SYSTEM)
  - OID: 1.3.6.1.5.5.7.3.6 (IPSEC_TUNNEL)
  - OID: 1.3.6.1.5.5.7.3.7 (IPSEC_USER)
Certificates that conform to the ISO/IEC standard for mobile driving license (mDL)
- Pass the ISO/IEC 18013-5 OID reserved for mDL DS: 1.0.18013.5.1.2 by using custom extensions.

It’s important to note that blank certificate templates aren’t limited to just end-entity certificates. For example, the BlankSubordinateCACertificate_PathLen0_APICSRPassthrough template sets the Basic constraints parameter to CA:TRUE, allowing you to issue a subordinate CA certificate with your own Key Usage and Extended Key Usage values.

Using blank certificate templates

When you browse through the AWS Private CA certificate templates, you may see that base templates don’t allow you to define your own Key Usage or Extended Key Usage extensions and values. They are preset to the extensions and values used for the most common certificate types in order to simplify issuing those types of certificates. For example, when using EndEntityCertificate/V1, you will always get a Key Usage value of Critical, digital signature, key encipherment and an Extended Key Usage value of TLS web server authentication, TLS web client authentication. The following table shows all of the values for this base template.

EndEntityCertificate/V1
X509v3 parameter	Value
Subject alternative name	[Passthrough from certificate signing request (CSR)]
Subject	[Passthrough from CSR]
Basic constraints	CA:`FALSE`
Authority key identifier	[Subject key identifier from CA certificate]
Subject key identifier	[Derived from CSR]
Key usage	Critical, digital signature, key encipherment
Extended key usage	TLS web server authentication, TLS web client authentication
CRL distribution points	[Passthrough from CA configuration]

When you look at blank certificate templates, you will see that there is more flexibility. For one example of a blank certificate template, BlankEndEntityCertificate_APICSRPassthrough/V1, you can see that there are fewer predefined values compared to EndEntityCertificate/V1. You can pass your own values for Extended Key Usage and Key Usage.

BlankEndEntityCertificate_APICSRPassthrough/V1
X509v3 parameter	Value
Subject alternative name	[Passthrough from API or CSR]
Subject	[Passthrough from API or CSR]
Basic constraints	CA:FALSE
Authority key identifier	[Subject key identifier from CA certificate]
Subject key identifier	[Derived from CSR]
CRL distribution points Note: CRL distribution points are included in the template only if the CA is configured with CRL generation enabled.	[Passthrough from CA configuration or CSR]

To specify your desired extension and value, you must pass them in the IssueCertificate API call. There are two ways of doing so: the API Passthrough and CSR Passthrough templates.

API Passthrough – Extensions and their values defined in the IssueCertificate parameter APIPassthrough are copied over to the issued certificate.
CSR Passthrough – Extensions and their values defined in the CSR are copied over to the issued certificate.

To accommodate the different ways of passing these values, there are three varieties of blank certificate templates. If you would like to pass extensions defined only in your CSR file to the issued certificate, you can use the BlankEndEntityCertificate_CSRPassthrough/V1 template. Similarly, if you would like to pass extensions defined only in the APIPassthrough parameter, you can use the BlankEndEntityCertificate_APIPassthrough/V1 template. Finally, if you would like to use a combination of extensions defined in both the CSR and APIPassthrough, you can use the BlankEndEntityCertificate_APICSRPassthrough/V1 template. It’s important to remember these points when choosing your template:

The template definition will always have the higher priority over the values specified in the CSR, regardless of what template variety you use. For example, if the template contains a Key Usage value of digital signature and your CSR file contains key encipherment, the certificate will choose the template definition digital signature.
API passthrough values are only respected when you use an API passthrough or APICSR passthrough template. CSR passthrough is only respected when you use a CSR passthrough or APICSR passthrough template. When these sources of information are in conflict (the CSR contains the same extension or value as what’s passed in API passthrough), a general rule usually applies: For each extension value, the template definition has highest priority, followed by API passthrough values, followed by CSR passthrough extensions. Read more about the template order of operations in the AWS documentation.

How to issue use-case bound certificates in the AWS CLI

To get started issuing certificates, you must have appropriate AWS Identity and Access Management (IAM) permissions as well as an AWS Private CA in an “Active” status. You can verify if your private CA is active by running the aws acm-pca list-certificate-authorities command from the AWS Command Line Interface (CLI). You should see the following:

"Status": "ACTIVE"

After verifying the status, make note of your private CA Amazon Resource Name (ARN).

To issue use-case bound certificates, you must use the Private CA API operation IssueCertificate.

In the AWS CLI, you can call this API by using the command issue-certificate. There are several parameters you must pass with this command:

(--certificate-authority-arn) – The ARN of your private CA.
(--csr) – The CSR in PEM format. It must be passed as a blob , like fileb://.
(--validity) – Sets the “Not After” date (expiration date) for the certificate.
(--signing-algorithm) – The signing algorithm to be used to sign the certificate. The value you choose must match the algorithm family of the private CA’s algorithm (RSA or ECDSA). For example, if the private CA uses RSA_2048, the signing algorithm must be an RSA variant, like SHA256WITHRSA.
You can check your private CA’s algorithm family by referring to its key algorithm. The command aws acm-pca describe-certificate-authority will show the corresponding KeyAlgorithm value.
(--template-arn) – This is where the blank certificate template is defined. The template should be an AWS Private CA template ARN. The full list of AWS Private CA template ARNs are shown in the AWS documentation.

We’ll now demonstrate how to issue use-case bound end-entity certificates by using blank end-entity certificate templates. We will issue two different certificates. One will be bound for email protection, and one will be bound for smart card authentication. Email protection and smart card authentication certificates have specific Extended Key Usage values which are not defined by any base template. We’ll use CSR passthrough to issue the smart card authentication certificate and use API passthrough to issue the email protection certificate.

The certificate templates that we will use are:

For CSR passthrough: BlankEndEntityCertificate_CSRPassthrough/V1
For API Passthrough: BlankEndEntityCertificate_APIPassthrough/V1

Important notes about this demo:

These commands are for demo purposes only. Depending on your specific use case, email protection certificates and smart card authentication certificates may require different extensions than what’s shown in this demo.
You will be generating RSA 2048 private keys. Private keys need to be protected and stored properly and securely. For example, encrypting private keys or storing private keys in a hardware security module (HSM) are some methods of protection that you can use.
We will be using the OpenSSL command line tool, which is installed by default on many operating systems such as Amazon Linux 2023. If you don’t have this tool installed, you can obtain it by using the software distribution facilities of your organization or your operating system, as appropriate.

Use API passthrough

We will now demonstrate how to issue a certificate that is bound for email protection. We’ll specify Key Usage and Extended Key Usage values, and also a subject alternative name through API passthrough. The goal is to have these extensions and values in the email protection certificate.

Extensions:

	X509v3 Key Usage: critical
	Digital Signature, Key Encipherment
	X509v3 Extended Key Usage:
	E-mail Protection
	X509v3 Subject Alternative Name:
	email:john_doe@example.com

To issue a certificate bound for email protection

Use the following command to create your keypair and CSR with OpenSSL. Define your distinguished name in the OpenSSL prompt.
```
openssl req -out csr-demo-1.csr -new -newkey rsa:2048 -nodes -keyout private-key-demo-1.pem
```

Use the following command to issue an end-entity certificate specifying the EMAIL_PROTECTION extended key usage value, the Digital Signature and Key Encipherment Key Usage values, and the subject alternative name john_doe@example.com. We will use the Rfc822Name subject alternative name type, because the value is an email address.

Make sure to replace the data in arn:aws:acm-pca:<region>:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 with your private CA ARN, and adjust the signing algorithm according to your private CA’s algorithm. Assuming my PCA is type RSA, I am using SHA256WITHRSA.

aws acm-pca issue-certificate --certificate-authority-arn arn:aws:acm-pca:<region>:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 --csr fileb://csr-demo-1.csr --template-arn arn:aws:acm-pca:::template/BlankEndEntityCertificate_APIPassthrough/V1 --signing-algorithm "SHA256WITHRSA" --validity Value=365,Type="DAYS" --api-passthrough "Extensions={ExtendedKeyUsage=[{ExtendedKeyUsageType="EMAIL_PROTECTION"}],KeyUsage={"DigitalSignature"=true,"KeyEncipherment"=true},SubjectAlternativeNames=[{Rfc822Name="john_doe@example.com"}]}"

If the command is successful, then the ARN of the issued certificate is shown as the result:

{
    "CertificateArn": "arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111/certificate/123465789123456789"
}

Proceed to the Retrieve the Certificate section of this post to retrieve the certificate and certificate chain PEM from the CertificateArn.

Use CSR passthrough

We’ll now demonstrate how to issue a certificate that is bound for smart card authentication. We will specify Key Usage, Extended Key Usage, and subject alternative name extensions and values through CSR passthrough. The goal is to have these values in the smart card authentication certificate.

Extensions:

	X509v3 Key Usage: critical
	Digital Signature
	X509v3 Extended Key Usage:
	TLS Web Client Authentication, Microsoft Smartcard Login
	X509v3 Subject Alternative Name:
	othername: UPN::john_doe@example.com

We’ll generate our CSR by requesting these specific extensions and values with OpenSSL. When we call IssueCertificate, the CSR passthrough template will acknowledge the requested extensions and copy them over to the issued certificate.

To issue a certificate bound for smart card authentication

Use the following command to create the private key.

openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out private-key-demo-2.pem

Create a file called openssl_csr.conf to define the distinguished name and the requested CSR extensions.

Following is an example of OpenSSL configuration file content. You can copy this configuration to the openssl_csr.conf file and adjust the values to your requirements. You can find further reference on the configuration in the OpenSSL documentation.

[ req ]
default_bits = 2048
prompt = no
default_md = sha256
req_extensions = my_req_ext
distinguished_name = dn

#Specify the Distinguished Name
[ dn ]
countryName                     = US
stateOrProvinceName             = VA 
localityName                    = Test City
organizationName                = Test Organization Inc
organizationalUnitName          = Test Organization Unit
commonName                      = john_doe


#Specify the Extensions
[ my_req_ext ]
keyUsage = critical, digitalSignature
extendedKeyUsage = clientAuth, msSmartcardLogin 

#UPN OtherName OID: "1.3.6.1.4.1.311.20.2.3". Value is ASN1-encoded UTF8 string
subjectAltName = otherName:msUPN;UTF8:john_doe@example.com

In this example, you can specify your Key Usage and Extended Key Usage values in the [ my_req_ext ] section of the configuration. In the extendedKeyUsage line, you may also define extended key usage OIDs, like 1.3.6.1.4.1.311.20.2.2. Possible values are defined in the OpenSSL documentation.

Create the CSR, defining the configuration file.

openssl req -new -key private-key-demo-2.pem -out csr-demo-2.csr -config openssl_csr.conf

(Optional) You can use the following command to decode the CSR to make sure it contains the information you require.

openssl req -in csr-demo-2.csr -noout  -text

The output should show the requested extensions and their values, as follows.

	X509v3 Key Usage: critical
	Digital Signature
	X509v3 Extended Key Usage:
	TLS Web Client Authentication, Microsoft Smartcard Login
	X509v3 Subject Alternative Name:
	othername: UPN:: <your_user_here>

Issue the certificate by using the issue-certificate command. We will use a CSR passthrough template so that the requested extensions and values in the CSR file are copied over to the issued certificate.
Make sure to replace the data in arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 with your private CA ARN and adjust the signing algorithm and validity to for your use case. Assuming my PCA is type RSA, I am using SHA256WITHRSA.
```
aws acm-pca issue-certificate --certificate-authority-arn arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 --csr fileb://csr-demo-2.csr --template-arn arn:aws:acm-pca:::template/BlankEndEntityCertificate_CSRPassthrough/V1 --signing-algorithm "SHA256WITHRSA" --validity Value=365,Type="DAYS"
```
If the command is successful, then the ARN of the issued certificate is shown as the result:
```
{
    "CertificateArn": "arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111/certificate/123465789123456789"
}
```

Retrieve the certificate

After using issue-certificate with API passthrough or CSR passthrough, you can retrieve the certificate material in PEM format. Use the get-certificate command and specify the ARN of the private CA that issued the certificate, as well as the ARN of the certificate that was issued:

aws acm-pca get-certificate --certificate-arn arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111/certificate/123465789123456789 --certificate-authority-arn arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 --output text

You can use the --query command with the AWS CLI to get the certificate and certificate chain in separate files.

Certificate

aws acm-pca get-certificate --certificate-authority-arn  arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 --certificate-arn arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111/certificate/123465789123456789 --output text --query Certificate > certfile.pem

Certificate chain

aws acm-pca get-certificate --certificate-authority-arn  arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111 --certificate-arn arn:aws:acm-pca:us-east-1:<accountID>:certificate-authority/11111111-1111-1111-1111-111111111111/certificate/123465789123456789 --output text --query CertificateChain > certchain.pem

After you retrieve the certificate, you can decode it with the openssl x509 command. This will allow you to view the details of the certificate, including the extensions and values that you defined.

openssl x509 -in certfile.pem -noout -text

Conclusion

In AWS Private CA, you can implement the security benefits of accountability and least privilege by defining the usage of your certificates. The Key Usage and Extended Key Usage values define the usage of your certificates. Many certificate use cases require a combination of Key Usage and Extended Key Usage values, which cannot be defined with base certificate templates. Some examples include document signing, smart card authentication, and mobile driving license (mDL) certificates. To issue certificates for these specific use cases, you can use blank certificate templates with the IssueCertificate API call. In addition to the blank certificate template, you must also define the specific combination of Key Usage and Extended Key Usage values through CSR passthrough, API passthrough, or both.

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