AWS KMS key rotation is a built-in feature that automatically changes the backing cryptographic material of a Customer Master Key (CMK) on a yearly basis, without altering the CMK’s identifier. This is a crucial security practice because it limits the amount of data encrypted under any single version of the cryptographic material, thereby reducing the potential impact of a key compromise.
Here’s how it works and what you need to know:
How KMS Key Rotation Works
When you enable automatic key rotation for a KMS CMK, AWS KMS creates a new version of the backing cryptographic material for that CMK every 365 days. The original CMK remains active and is used for all subsequent decryption operations. AWS KMS manages the entire process, including generating new material, associating it with the existing CMK, and making older versions available for decryption.
Crucially, the CMK’s ARN (Amazon Resource Name) and Key ID do not change. This means your applications and services that use the CMK for encryption and decryption don’t need to be reconfigured. The transition is seamless.
Enabling and Disabling Key Rotation
You can enable or disable automatic key rotation when you create a CMK or for an existing CMK.
Enabling Rotation for a New CMK (AWS CLI):
aws kms create-key \
--description "My App CMK with rotation" \
--key-usage ENCRYPT_DECRYPT \
--enable-key-rotation
This command creates a new symmetric CMK with automatic rotation enabled.
Enabling Rotation for an Existing CMK (AWS CLI):
aws kms enable-key-rotation \
--key-id arn:aws:kms:us-east-1:123456789012:key/abcdefgh-ijkl-mnop-qrst-uvwxyz012345
Replace the --key-id with the actual ARN or Key ID of your CMK.
Disabling Rotation for an Existing CMK (AWS CLI):
aws kms disable-key-rotation \
--key-id arn:aws:kms:us-east-1:123456789012:key/abcdefgh-ijkl-mnop-qrst-uvwxyz012345
Again, replace the --key-id with your CMK’s identifier.
Important Note: You can only disable rotation for customer-managed CMKs. AWS-managed CMKs have rotation enabled by default and cannot be disabled.
Verifying Key Rotation Status
You can check if key rotation is enabled for a CMK using the AWS CLI:
aws kms describe-key \
--key-id arn:aws:kms:us-east-1:123456789012:key/abcdefgh-ijkl-mnop-qrst-uvwxyz012345 \
--query 'KeyMetadata.KeyRotationStatus'
This command will return true if rotation is enabled and false if it is disabled.
How KMS Key Rotation Enhances Security
The primary security benefit of key rotation is limiting the exposure of cryptographic material. If a specific version of your CMK’s backing key were ever compromised, only the data encrypted by that specific version would be at risk. By rotating the key annually, you significantly reduce the window of opportunity for an attacker to exploit a compromised key version.
Consider this scenario: If you never rotated a key, and it was compromised today, all data encrypted with that key since its creation would be vulnerable. With annual rotation, a compromise would only expose data encrypted within the last year, assuming the compromise happened today.
Understanding Key Usage and Rotation
When you enable key rotation, AWS KMS automatically manages the cryptographic material versions.
- Encryption: When you encrypt data, KMS uses the latest version of the CMK’s cryptographic material.
- Decryption: When you decrypt data, KMS uses the correct version of the cryptographic material that was used for encryption. You don’t need to specify which version to use; KMS handles this internally.
This is why the CMK’s ID and ARN remain constant and why your applications don’t need to change.
What Happens to Older Key Material?
AWS KMS retains older versions of the cryptographic material for a CMK even after rotation. This is essential for decrypting data that was encrypted using those older versions. You can view the different versions of a CMK’s cryptographic material using the list-key-versions command in the AWS CLI, though you typically don’t need to interact with these versions directly.
Key Considerations and Best Practices
- Customer-Managed vs. AWS-Managed Keys: Automatic key rotation is enabled by default for AWS-managed CMKs and cannot be disabled. For customer-managed CMKs, you have control over enabling or disabling rotation. It’s generally recommended to enable automatic rotation for customer-managed CMKs unless you have a specific compliance or operational reason not to.
- Key Policies: Ensure your KMS key policies grant the necessary permissions for services to use the CMK for encryption and decryption. Key rotation does not affect the key policy.
- Auditing: Use AWS CloudTrail to audit all KMS API calls, including key creation, enabling/disabling rotation, and cryptographic operations. This provides a clear audit trail of key usage and management.
- Manual Key Rotation: While automatic rotation is convenient, KMS also supports manual key rotation. This allows you to schedule key rotations on a more frequent basis or at specific times, which might be required by certain compliance standards.
- Disabling Rotation: If you disable rotation, the cryptographic material for that CMK will never change. This means that if the key is ever compromised, all data encrypted by that CMK could be at risk.
- AWS Services Integration: Many AWS services integrate with KMS for encryption (e.g., S3, EBS, RDS). When you use a customer-managed CMK with these services and enable rotation, the service will automatically use the new key material for subsequent encryption operations without requiring any changes on your part.
The Counterintuitive Aspect of Key Material Versions
It’s not immediately obvious that when you encrypt data, KMS uses the current cryptographic material, but when you decrypt, it can and will use older material if that’s what the data was encrypted with. This versioning is entirely transparent to the user and the application, and it’s the core mechanism that makes key rotation seamless while ensuring all previously encrypted data remains accessible.
Next Steps
After ensuring your KMS keys are rotating, the next logical step in securing your data is to implement robust access control for your KMS keys using IAM policies and key policies.