AWS Certificate Manager (ACM) is a service that lets you efficiently provision, manage, and deploy public and private SSL/TLS certificates for use with AWS services and your internal connected resources. The certificates issued by ACM can then be used to secure network communications and establish the identity of websites on the internet or resources on private networks.
So why might you want to import a certificate into ACM, rather than using a certificate issued by ACM? According to the AWS Certificate Manager User Guide topic Importing certificates into AWS Certificate Manager, “you might do this because you already have a certificate from a third-party certificate authority (CA), or because you have application-specific requirements that are not met by ACM issued certificates.”
In this blog post, I’ll list 10 reasons why you might want to import a certificate into ACM, including what specific requirements you might have, and why you might want to use a certificate signed by a third-party CA in the first place.
1. To use an ECDSA certificate for faster TLS connections
Imported Elliptic Curve Digital Signature Algorithm (ECDSA) certificates use smaller keys than ACM issued public RSA certificates, allowing for TLS connections to be established faster. For this reason, ECDSA certificates are particularly useful for systems with limited processing resources, such as Internet of Things (IoT) devices. ACM supports imported certificates with ECDSA in 256, 384, and 521 bit variations. If you want to use an ECDSA certificate for your public-facing web application, you need to get a third-party certificate and then import it into ACM. For more information about supported cryptographic algorithms for imported certificates, see Prerequisites for importing certificates in the AWS Certificate Manager User Guide.
2. To control your certificate’s renewal cycle
When you import a certificate into ACM, you have greater control over its renewal cycle simply because you can re-import it as frequently as you want. You also have control over how often your imported certificate’s private key can be rotated. As a best practice, you should rotate your certificate’s private key based on your certificate’s usage frequency.
Note: When you re-import your certificate, to maintain the existing associations during renewal, ensure that you specify the existing certificate’s Amazon Resource Name (ARN). For more information and step-by-step instructions, see Reimporting a certificate in the AWS Certificate Manager User Guide.
3. To use certificate pinning
You might have an application that requires certificate pinning, which is the practice of bypassing the typical hierarchical model of trust that is governed by certificate authorities. With certificate pinning, a host’s identity is trusted based on a specific certificate or public key. As a certificate pinning best practice, AWS recommends that public certificates issued by ACM should not be pinned because ACM will generate a new public/private key pair at the next renewal phase, which essentially replaces the pinned certificate with a new one, causing service disruption along the process. If you want to use certificate pinning, you can pin an imported certificate because imported certificates are not subject to managed renewal, thereby reducing the risk of production impact.
4. To use a higher-assurance certificate
You might want to use a higher-assurance certificate, such as an organization validation (OV) or extended validation (EV) certificate. Certificates issued by ACM currently only support domain validation (DV). If the domain you want to protect is an application that requires OV or EV, you can import OV or EV certificates into ACM by using a third-party certificate of either type. You can use the ACM API action ImportCertificate to import OV or EV certificates into ACM.
5. To use a self-signed certificate
For internal testing environments where your developers want speed and flexibility, self-signed certificates are issued faster and effortlessly. However, it’s important to know that self-signed certificates are not trusted by default, which means that self-signed certificates need to be installed inside the trust stores of the intended clients, to avoid the risk of your users getting into the habit of ignoring browser warnings. For more information, see the additional requirements for self-signed certificates in Prerequisites for importing certificates in the AWS Certificate Manager User Guide.
6. To use an IP address for the certificate’s subject
By design, the subject field of an ACM certificate can only identify a fully qualified domain name (FQDN). If you want to use an IP address for the certificate’s subject, then you can create the certificate and import it to ACM.
7. To exceed the number of domains allowed by the ACM quotas
Certificates issued by ACM are subject to the ACM service quotas. The default quota for ACM is 10 domain names for each ACM certificate, and you can request an increase to the quota up to a maximum of 100 domain names for each certificate. However, if you import certificates, they are not subject to the quotas, and you can use a public certificate with more than 100 FQDNs in its domain scope without having to go through the process of requesting any limit increases.
8. To use a private certificate issued by ACM Private CA with the IssueCertificate API action
Certificates provisioned with the IssueCertificate API action have a private status and cannot be associated directly with an AWS integrated service, such as an internal Application Load Balancer. Instead, a private certificate issued by AWS Certificate Manager Private Certificate Authority (ACM Private CA) with the IssueCertificate API action needs to be exported and then imported into ACM before the association can be made. The same is true for certificate templates as well, which are configuration templates that can be passed as parameters to the IssueCertificate API action as a means to have greater control over the private certificate’s extensions.
9. To use a private certificate issued by your on-premises CA
You might want to use a private certificate issued by your on-premises CA instead of using ACM Private CA. To administer your internal public key infrastructure (PKI), AWS generally recommends that you use ACM Private CA. However, you might still come across scenarios where a certificate signed by your on-premises CA is better suited for your specific needs. For example, you might want to have a common root of trust, for consistency and interoperability purposes across a hybrid PKI solution. Furthermore, using an external parent CA with ACM Private CA also allows you to enforce CA name constraints. For more information, see Signing private CA certificates with an external CA in the AWS Certificate Manager Private Certificate Authority User Guide.
10. To use a certificate for something other than securing a public website
In addition to securing a public website, you can use certificates for other purposes. For example, you can import client and server certificates as part of an OpenVPN setup. For more information about this example, see How can I generate server and client certificates and their respective keys on a Windows server and upload them to AWS Certificate Manager (ACM)? In addition, you can import a code-signing certificate for use with AWS IoT Device Management. For more information about how to import a code-signing certificate, see (For IoT only) Obtain and import a code-signing certificate in the AWS Signer Developer Guide.
In this blog post, you learned about some of the reasons you might want to import a certificate into AWS Certificate Manager (ACM). For more information about importing certificates into ACM and step-by-step instructions, see Importing certificates into AWS Certificate Manager in the AWS Certificate Manager User Guide. For the latest pricing information, see the AWS Certificate Manager Pricing page on the AWS website. You can also use the AWS pricing calculator to estimate costs.
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