Understanding AWS KMS Key Rotation

Introduction

Among the various strategies to enhance data security within AWS KMS, key rotation is vital, especially when dealing with customer-managed keys in AWS KMS. In this blog post, we'll demystify what key rotation is, why it's important, and how it works in AWS KMS, making this concept accessible even to beginners.

What is Key Rotation?

Key rotation refers to the practice of regularly changing encryption keys. This process involves creating new keys for data encryption and retiring the old ones. The idea is to limit the amount of data encrypted with a single key, thereby reducing the risk if a key is compromised. Regularly rotating keys enhances security by ensuring that even if an attacker gets hold of a key, it will be useful for a limited time only.  AWS supports automatic and manual key rotation for customer-managed keys. 

Why is Key Rotation Important?

1. Enhanced Security: Regular key rotation, now annually for AWS managed keys, makes unauthorized access even more challenging.
2. Compliance: Staying compliant with regulations that require frequent key rotations is now simpler with AWS's automatic rotation.
3. Reduced Impact of a Breach: Annual rotation means that compromised keys put a smaller window of data at risk.

How Does AWS KMS Key Rotation Work?

AWS KMS supports two main types of keys within a KMS account: AWS-managed keys, and customer-managed keys. Additionally, there are AWS-owned keys, which are used by AWS services and are not visible to users nor stored in customer accounts.

Automatic Key Rotation - AWS and Customer managed keys

AWS-managed keys are automatically rotated every year to enhance security.

For customer-managed keys with AWS key material, users can enable yearly (365 days) automatic key rotation from the Key rotation tab of the key.

Manual Key Rotation - Customer-managed keys

Customer-managed keys in AWS Key Management Service (KMS) also support manual rotation, which is particularly useful in scenarios where automatic key rotation is not suitable or available. For instance:

1. Automatic key rotation is not available for AWS KMS Asymmetric keys, AWS KMS Keys with external key material, and AWS KMS Keys with External key stores.
2. Manual rotation is also necessary when a specific rotation frequency, different from the default one-year period, is required. This may be the case for certain compliance requirements, where a more frequent rotation schedule, such as every 90 or 180 days, is mandated.

Steps to follow for manual key rotation

Manual key rotation for customer-managed AWS KMS keys, including symmetric keys, AWS KMS Asymmetric keys, AWS KMS Keys with external key material, and AWS KMS Keys configured with External Key Stores, can be carried out as follows:

1. Create a New KMS Key: Initiate the process by creating a new customer-managed KMS key within AWS KMS. This newly created key will serve as the replacement for the old key.

2. Update Key Aliases and Policies: Review and update any aliases or policies that reference the old key to point to the new key. Aliases are invaluable in this context as they facilitate the transition to the new key without necessitating changes to application code.

3. Re-Encrypt Data: Proceed to re-encrypt data using the new KMS key. For infrequently accessed data, you may choose to re-encrypt it upon next access. However, for data that's accessed regularly, it's advisable to proactively re-encrypt it to ensure security continuity.

4. Retire the Old KMS Key: After re-encrypting all necessary data with the new key and updating all relevant references from the old key, you should change the status of the old KMS key to "disabled." Disabling a key blocks its use for any new cryptographic operations but retains it for decrypting data previously encrypted under it, should you need to access this data again. It's important to note that, for keys with external material or those in external key stores, managing the key material or external store accordingly is also part of the rotation process.

5. Monitor and Audit: It's crucial to continually monitor the usage of the old KMS key to verify that it's not being used for new encryption tasks. Additionally, conducting regular audits of your AWS environment ensures that all data and resources have been transitioned to use the new key.

Conditions and Best Practices

• Rotation Period: AWS recommends rotating our CMKs at least once every 12 months. However, specific regulatory requirements or company policies may necessitate more frequent rotations.
• Backward Compatibility: Ensure that our system is designed to handle key rotation smoothly, especially if we're implementing manual rotation. This includes maintaining access to older keys for decryption purposes.
• Key Material: When rotating keys, be mindful of whether we're using AWS-generated key material or importing our own. The rotation process might differ slightly based on this.
• Record Keeping: Maintain a record of key usage and rotation schedules. This is crucial for audit purposes and for troubleshooting any issues that might arise.

Conclusion

Key rotation is a fundamental aspect of maintaining a robust encryption strategy on AWS. By regularly rotating our Customer Managed Keys, we enhance the security of our encrypted data, reducing the risk of compromise. Whether we opt for automatic rotation for its simplicity or manual rotation for greater control, the key is to establish a consistent rotation process that aligns with our security policies and compliance requirements. Remember, the goal of key rotation is not just to comply with best practices but to actively protect our data in the ever-evolving landscape of cyber threats.