Are KMS custom key stores right for you?

October 29, 2021: AWS KMS is replacing the term customer master key (CMK) with AWS KMS key and KMS key. The concept has not changed. To prevent breaking changes, AWS KMS is keeping some variations of this term. More info.

You can use the AWS Key Management Service (KMS) custom key store feature to gain more control over your KMS keys. The KMS custom key store integrates KMS with AWS CloudHSM to help satisfy compliance obligations that would otherwise require the use of on-premises hardware security modules (HSMs) while providing the AWS service integrations of KMS. However, the additional control comes with increased cost and potential impact on performance and availability. This post will help you decide if this feature is the best approach for you.

KMS is a fully managed service that generates encryption keys and helps you manage their use across more than 45 AWS services. It also supports the AWS Encryption SDK and other client-side encryption tools, and you can integrate it into your own applications. KMS is designed to meet the requirements of the vast majority of AWS customers. However, there are situations where customers need to manage their keys in single-tenant HSMs that they exclusively control. Previously, KMS did not meet these requirements since it offered only the ability to store keys in shared HSMs that are managed by KMS.

AWS CloudHSM is a service that’s primarily intended to support customer-managed applications that are specifically designed to use HSMs. It provides direct control over HSM resources, but the service isn’t, by itself, widely integrated with other AWS managed services. Before custom key store, this meant that if you required direct control of your HSMs but still wanted to use and store regulated data in AWS managed services, you had to choose between changing those requirements, not using a given AWS service, or building your own solution. KMS custom key store gives you another option.

How does a custom key store work?

With custom key store, you can configure your own CloudHSM cluster and authorize KMS to use it as a dedicated key store for your keys rather than the default KMS key store. Then, when you create keys in KMS, you can choose to generate the key material in your CloudHSM cluster. Your AWS KMS keys (KMS keys) never leave the CloudHSM instances, and all KMS operations that use those keys are only performed in your HSMs. In all other respects, the KMS keys stored in your custom key store are used in a way that is consistent with other KMS keys.

This diagram illustrates the primary components of the service and shows how a cluster of two CloudHSM instances is connected to KMS to create a customer controlled key store.
 

Figure 1: A cluster of two CloudHSM instances is connected to KMS to create a customer controlled key store

Because you control your CloudHSM cluster, you can take direct action to manage certain aspects of the lifecycle of your keys, independently of KMS. Specifically, you can verify that KMS correctly created keys in your HSMs and you can delete key material and restore keys from backup at any time. You can also choose to connect and disconnect the CloudHSM cluster from KMS, effectively isolating your keys from KMS. However, with more control comes more responsibility. It’s important that you understand the availability and durability impact of using this feature, and I discuss the issues in the next section.

Decision criteria

KMS customers who plan to use a custom key store tell us they expect to use the feature selectively, deciding on a key-by-key basis where to store them. To help you decide if and how you might use the new feature, here are some important issues to consider.

Here are some reasons you might want to store a key in a custom key store:

• You have keys that are required to be protected in a single-tenant HSM or in an HSM over which you have direct control.
• You have keys that are explicitly required to be stored in an HSM validated at FIPS 140-2 level 3 overall (the HSMs used in the default KMS key store are validated to level 2 overall, with level 3 in several categories, including physical security).
• You have keys that are required to be auditable independently of KMS.

And here are some considerations that might influence your decision to use a custom key store:

• Cost — Each custom key store requires that your CloudHSM cluster contains at least two HSMs. CloudHSM charges vary by region, but you should expect costs of at least $1,000 per month, per HSM, if each device is permanently provisioned. This cost occurs regardless of whether you make any requests of the KMS API directly or indirectly through an AWS service.
• Performance — The number of HSMs determines the rate at which keys can be used. It’s important that you understand the intended usage patterns for your keys and ensure that you have provisioned your HSM resources appropriately.
• Availability — The number of HSMs and the use of availability zones (AZs) impacts the availability of your cluster and, therefore, your keys. The risk of your configuration errors that result in a custom key store being disconnected, or key material being deleted and unrecoverable, must be understood and assessed.
• Operations — By using the custom key store feature, you will perform certain tasks that are normally handled by KMS. You will need to set up HSM clusters, configure HSM users, and potentially restore HSMs from backup. These are security-sensitive tasks for which you should have the appropriate resources and organizational controls in place to perform.

Getting Started

Here’s a basic rundown of the steps that you’ll take to create your first key in a custom key store within a given region.

1. Create your CloudHSM cluster, initialize it, and add HSMs to the cluster. If you already have a CloudHSM cluster, you can use it as a custom key store in addition to your existing applications.
2. Create a CloudHSM user so that KMS can access your cluster to create and use keys.
3. Create a custom key store entry in KMS, give it a name, define which CloudHSM cluster you want it to use, and give KMS the credentials to access your cluster.
4. Instruct KMS to make a connection to your cluster and log in.
5. Create an AWS KMS key (KMS key) in KMS in the usual way except now select CloudHSM as the source of your key material. You’ll define administrators, users, and policies for the key as you would for any other KMS key.
6. Use the key via the existing KMS APIs, AWS CLI, or the AWS Encryption SDK. Requests to use the key don’t need to be context-aware of whether the key is stored in a custom key store or the default KMS key store.

Summary

Some customers need specific controls in place before they can use KMS to manage encryption keys in AWS. The new KMS custom key store feature is intended to satisfy that requirement. You can now apply the controls provided by CloudHSM to keys managed in KMS, without changing access control policies or service integration.

However, by using the new feature, you take responsibility for certain operational aspects that would otherwise be handled by KMS. It’s important that you have the appropriate controls in place and understand the performance and availability requirements of each key that you create in a custom key store.

If you’ve been prevented from migrating sensitive data to AWS because of specific key management requirements that are currently not met by KMS, consider using the new KMS custom key store feature.

If you have feedback about this blog post, submit comments in the Comments section below. If you have questions about this blog post, start a new thread on the AWS Key Management Service discussion forum.

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