How to evaluate and use ECDSA certificates in AWS Certificate Manager

AWS Certificate Manager (ACM) is a managed service that enables you to provision, manage, and deploy public and private SSL/TLS certificates that you can use to securely encrypt network traffic. You can now use ACM to request Elliptic Curve Digital Signature Algorithm (ECDSA) certificates and associate the certificates with AWS services like Application Load Balancer (ALB) or Amazon CloudFront. As a result, you get the benefit of managed renewal, where ACM can automatically renew ECDSA certificates before they expire. Previously, you could only request certificates with an RSA 2048 key algorithm from ACM. ECDSA certificates could be imported to ACM, but imported certificates cannot use managed renewal.

You can request both ECDSA P-256 and P-384 certificates from ACM. If you do not request an ECDSA certificate, ACM will issue an RSA 2048 certificate by default.

In this blog post, we will briefly examine the differences between RSA and ECDSA certificates, discuss some important considerations when evaluating which certificate type to use, and walk through how you can request an ECDSA certificate and associate it with an application load balancer in AWS.

Cryptographic certificates overview

TLS certificates are used to secure network communications and establish the identity of websites over the internet, as well as the identity of resources on private networks. Public certificates that you request through ACM are obtained from Amazon Trust Services, which is an Amazon managed public certificate authority (CA).

Private certificates are issued through certificate authorities, which you can create and manage by using AWS Private Certificate Authority (AWS Private CA).

Both public and private certificates can help customers identify resources on networks and secure communication between these resources. Public certificates identify resources on the public internet, whereas private certificates do the same for private networks. One key difference is that applications and browsers trust public certificates by default, but an administrator must explicitly configure applications and devices to trust private certificates.

RSA and ECDSA primer

RSA and ECDSA are two widely used public-key cryptographic algorithms—algorithms that use two different keys to encrypt and decrypt data. In the case of TLS, a public key is used to encrypt data, and a private key is used to decrypt data. Public key (or asymmetric key) algorithms are not as computationally efficient as symmetric key algorithms like AES. For this reason, public key algorithms like RSA and ECDSA are primarily used to exchange secrets between two parties initiating a TLS connection. These secrets are then used by both parties to decipher the same symmetric key that actually encrypts the data in transit.

RSA stands for Rivest, Shamir, and Adleman: the researchers who first publicly described this algorithm in 1977. The basic functionality of RSA relies on the idea that large prime numbers are very difficult to efficiently factor. ECDSA, or Elliptic Curve Digital Signature Algorithm, is based on certain unique mathematical properties of elliptic curves that make them very useful for cryptographic operations. The cryptographic utility of ECDSA comes from a concept called the discrete logarithm problem.

Considerations when choosing between RSA and ECDSA

What are the important differences between RSA and ECDSA certificates? When should you choose ECDSA certificates to encrypt network traffic? In this section, we’ll examine the security and performance considerations that help to determine whether ECDSA or RSA certificates are the best choice for your workload.

Security

In cryptography, security is measured as the computational work it takes to exhaust all possible values of a symmetric key in an ideal cipher. An ideal cipher is a theoretical algorithm that has no weaknesses, so you must try every possible key to discover which is the correct key. This is similar to the idea of “brute forcing” a password: trying every possible character combination to find the correct password.

Let’s imagine you have a 112-bit key ideal cipher, which means it would take 2¹¹² tries to exhaust the key space—we would say this cipher has a 112-bit security strength. However, it is important to realize that security strength and key length are not always equal—meaning that an encryption key with a length of 112 bits will not always have a 112-bit security strength.

ECDSA provides higher security strength for lower computational cost. ECDSA P-256, for example, provides 128-bit security strength and is equivalent to an RSA 3072 key. Meanwhile, ECDSA P-384 provides 192-bit security strength, equivalent to the key associated with an RSA 7680 certificate. In other words, an ECDSA P-384 key would require 2¹⁹² tries to exhaust the key space.

The following table provides an in-depth comparison of the different security strengths for RSA key lengths and ECDSA curve types. Note that only RSA 2048 and ECDSA P-256 and P-384 are currently issued by ACM. However, ACM does support the import and usage of the other certificate types listed in the table. For more information, see Importing certificates into AWS Certificate Manager.

Performance

ECDSA provides a higher security strength (for a given key length) than RSA but does not add performance overhead. For example, ECDSA P-256 is as performant as RSA 2048 while providing security strength that is comparable to RSA 3072.

ECDSA certificates also have up to a 50% smaller certificate size when compared to RSA certificates, and are therefore more suitable to protect data-in-transit over low bandwidth or for applications with limited memory and storage, such as Internet of Things (IoT) devices.

Take a look at the following certificate examples; you can see the size difference between RSA and ECDSA certificates.

Consider a small IoT sensor device that tracks temperature in an office building. This device typically has very low storage capacity and compute power, so the smaller ECDSA certificate will be easier to process and store. In the case of an IoT device, you might not be able to store the entire RSA certificate chain on the device due to memory limitations and the larger size of RSA certificates. This can make it more difficult to validate the chain of trust for that certificate.

Using ECDSA, customers can take advantage of the smaller size of the certificates (and the certificate trust chain) and store the entire chain of trust on the IoT device itself, enabling the IoT device to more easily validate the certificate.

When should I use ECDSA certificates from ACM?

In general, you should consider using ECDSA certificates wherever possible, because they provide stronger security (for a given key length) compared to RSA, without impacting performance. You can also choose to issue ECDSA certificates from ACM to implement 128-bit or 192-bit TLS security, where previously you could request up to 112-bit security from ACM by using RSA 2048 certificates.

ECDSA certificates are strongly recommended for applications that need to securely send data over low-bandwidth connections, or when you are using IoT devices that might not have much memory or computational power to store and process the larger certificate sizes that RSA offers.

If your application is not ECDSA compatible, you will need to continue using RSA certificates. RSA 2048 remains the default certificate type issued by ACM, in order to prevent compatibility issues with legacy applications or with applications that do not support ECDSA certificate types. We will provide links to check if your application is compatible with ECDSA certificate types in the next section of this blog.

Getting started with ECDSA certificates

Modern browsers and operating systems are ECDSA compatible. That said, some custom applications might not be ECDSA compatible. You can check whether your calling application is ECDSA compatible by accessing the following links from your application:

ECDSA P-256

ECDSA P-384

When you access one of these links, you should see a message stating “Expected Status: good”. This indicates that the application is ECDSA compatible. See Figure 1 for an example of a successful result.

Figure 1: ECDSA application compatibility example

When you terminate your TLS traffic with ALB, you can work around compatibility concerns by binding both ECDSA and RSA certificates for a given domain. ALB will prioritize and present the ECDSA certificate when the calling application is ECDSA compatible and will use the RSA certificate if the calling application is not ECDSA compatible. We’ll walk through this configuration in the demonstration portion of this post.

How to request an ECDSA certificate from ACM

You can use the ACM console, APIs, or AWS Command Line Interface (AWS CLI) to issue public or private ECDSA P-256 and P-384 TLS certificates. When you request certificates by using the API or AWS CLI, you can use the request-certificate API action with either EC_prime256v1 or EC_secp384r1 as the key-algorithm parameter to request a P-256 or P-384 ECDSA certificate, respectively.

Certificates have a defined validity period, and ACM will attempt to renew certificates that were issued by ACM and that are in use before they expire. ACM will also attempt to automatically bind the renewed certificates with an integrated service. ACM issued private ECDSA certificates can also be exported and used on other workloads to terminate TLS traffic.

Associate an ECDSA certificate with an Application Load Balancer for TLS

To demonstrate how to request and use ECDSA certificates from ACM, let’s examine a common use case: requesting a public certificate from ACM and associating it with an ALB. This walkthrough will also include requesting an RSA 2048 certificate and associating it with the same ALB, to facilitate TLS connections for applications that do not support ECDSA. ALB will prioritize and present the ECDSA certificate when the calling application is ECDSA compatible, and will use the RSA certificate if the calling application is not ECDSA compatible.

This procedure has the following prerequisites:

• An AWS Identity and Access Management (IAM) user or role that has the appropriate permissions to request certificates from ACM and create an ALB
• A public domain that you own
• A public subnet, or IAM permissions to create one

To request an ECDSA certificate from ACM

1. Navigate to the ACM console and choose Request a certificate.
2. Choose Request a public certificate, and then choose Next.
3. For Fully qualified domain name, enter your domain name.
4. Choose DNS validation. DNS validation is recommended wherever possible, because it enables automatic renewal of ACM issued certificates with no action required by the domain owner. If you use Amazon Route 53, you can use ACM to directly update your DNS records. DNS-validated certificates will be renewed by ACM as long as the certificate is in use and the DNS record is in place.
   

   Figure 2: Requesting a public ECDSA certificate

5. In the Key algorithm options section, select your preferred algorithm based on your security requirements:
   • ECDSA P-256 — Equivalent in security strength to RSA 3072
   • ECDSA P-384 — Equivalent in security strength to RSA 7680
   

   Figure 3: Key algorithms

6. (Optional) Add tags to help you identify and manage your certificate. You can find more information on using tags in Tagging AWS resources in the AWS General Reference.
7. Choose Request to request the public certificate.

   The certificate will now be in the Pending Validation state until the domain can be validated, either through DNS or email validation, depending on your selection in the previous steps. For information on how to validate ownership of the domain name or names, see Validating domain ownership in the AWS Certificate Manager User Guide.

8. Take note of the certificate ARN; you will need this later to identify the certificate.

To request an RSA 2048 certificate from ACM

1. To request a public RSA 2048 certificate, use the same steps noted in the preceding section, but select RSA 2048 in the Key algorithm options section.
2. Make sure that both certificates you request have the same fully qualified domain name.

   For more information on requesting public certificates from ACM, see Requesting a public certificate.

To create a new Application Load Balancer and associate a default certificate

1. Navigate to the Amazon Elastic Compute Cloud (EC2) console. In the left navigation pane, under Load Balancing, choose Load Balancers.
2. Choose Create Load Balancer.

   For this post, we will use an Application Load Balancer. You can view more details on each type of Load Balancer, and see a feature-to-feature breakdown, on the Elastic Load Balancing features page.

3. For the Application Load Balancer type, choose Create.
4. Enter a name for your load balancer.
5. Select the scheme and IP address type of the application load balancer. For this post, we will choose Internet-facing for the scheme and use the IPv4 address type.
   

   Figure 4: Create an application load balancer

6. In the Network mapping section of this page, you will need to select a VPC and at least two Availability Zones and one public subnet per zone. If you do not already have a public subnet in two Availability Zones, see these instructions for creating a public subnet.
   

   Figure 5: Network mapping for ALB

7. Next, you need to create a secure listener. Under Listeners and routing, choose the HTTPS protocol (Port 443) in the drop-down list.
8. Under Default action, choose Forward. For Target Group, select a target group for the ALB to send traffic to.
   • Target groups can consist of EC2 instances, AWS Lambda functions, or IP addresses.
   • If you don’t have an existing target group, use these instructions to create one.
9. Under Secure listener settings, you will associate the RSA 2048 certificate with the new Application Load Balancer.

   Choose the appropriate security policy for your organization—you can compare policies on this page.

10. Under Default SSL/TLS certificate, verify that From ACM is selected, and then in the drop-down list, select the RSA certificate you requested earlier.
    

    Note: We are using the RSA certificate as the default so that the ALB will use this certificate if the connecting client does not support ECDSA or the Server Name Indication (SNI) protocol. This is to maximize availability and compatibility with legacy applications.

    

    Figure 6: Secure listener settings

11. (Optional) Add tags to the Application Load Balancer.
12. Review your selections, and then choose Create load balancer.
    

    Figure 7: Review and create load balancer

To associate the ECDSA certificate with the Application Load Balancer

1. In the EC2 console, select the new ALB you just created, and choose the Listeners tab.
2. In the SSL Certificate column, you should see the default certificate you added when you created the ALB. Choose View/edit certificates to see the full list of certificates associated with this ALB.
   

   Figure 8: ALB listeners

3. Under Listener certificates for SNI, choose Add certificate.
   

   Figure 9: Listener certificates for SNI

4. Under ACM and IAM certificates, select the ECDSA certificate you requested earlier.
   

   Note: You can use the certificate ARN to identify the appropriate certificate.

5. Choose Include as pending below to add the ECDSA certificate to the listener.
   

   Figure 10: Adding the ECDSA certificate to the load balancer listener

6. Under Listener certificates for SNI, confirm that the ECDSA certificate is listed as pending, and choose Add pending certificates.
   

   Figure 11: Confirm addition of pending certificates

Great! We’ve used ACM to request a public ECDSA certificate and a public RSA 2048 certificate. Next, we associated both of these certificates with an Application Load Balancer to facilitate TLS communications between the load balancer and client devices.

If clients support the SNI protocol, the ALB uses a smart certificate selection algorithm. The load balancer will select the best certificate that the client can support from the certificate list. Certificate selection is based on the following criteria, in the following order:

• Public key algorithm (prefer ECDSA over RSA)
• Hashing algorithm (prefer SHA over MD5)
• Key length (prefer the longest key)
• Validity period

In the earlier example, this means if clients support SNI and ECDSA, the ECDSA certificate will be prioritized and presented to the client. If the client does not support SNI or ECDSA, the RSA certificate will be used to maximize compatibility with legacy applications.

Conclusion

In this blog post, we discussed the basic differences between RSA and ECDSA certificates, when you might choose ECDSA over RSA, and how you can use AWS Certificate Manager to request public or private ECDSA certificates. We also covered how to request a public ECDSA certificate from ACM and associate it with an Application Load Balancer. Finally, we showed you how to request an RSA 2048 certificate and associate it with the same load balancer to facilitate TLS for applications that do not support ECDSA certificates.

To learn more about using ACM to issue ECDSA certificates, see our YouTube video: AWS Certificate Manager (ACM) – How to evaluate and use ECDSA certificates. You can also refer to the AWS Certificate Manager documentation for more details, and then get started issuing ECDSA certificates with AWS Certificate Manager.

 
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