AWS Key Management Service (KMS) Customer Managed Keys (CMKs) are encryption keys that you create, own, and manage within your AWS account. Unlike AWS managed keys, which are automatically created and managed by AWS services, customer managed keys provide you with full control over the key lifecycl…AWS Key Management Service (KMS) Customer Managed Keys (CMKs) are encryption keys that you create, own, and manage within your AWS account. Unlike AWS managed keys, which are automatically created and managed by AWS services, customer managed keys provide you with full control over the key lifecycle and access policies.
Key Features:
1. **Full Control**: You define who can use and manage the keys through IAM policies and key policies. This granular access control allows you to specify which users, roles, and AWS services can perform cryptographic operations.
2. **Key Rotation**: Customer managed keys support automatic annual rotation. When enabled, AWS generates new cryptographic material yearly while retaining old material for decrypting previously encrypted data.
3. **Audit Capabilities**: All API calls to KMS are logged in AWS CloudTrail, providing complete visibility into key usage. This helps meet compliance requirements and security auditing needs.
4. **Cross-Account Access**: You can share customer managed keys across AWS accounts by configuring appropriate key policies, enabling centralized key management.
5. **Key Types**: You can create symmetric keys (single key for encryption and decryption) or asymmetric keys (public-private key pairs) depending on your use case.
6. **Integration**: CMKs integrate seamlessly with numerous AWS services including S3, EBS, RDS, Lambda, and Secrets Manager for encrypting data at rest.
7. **Cost**: Customer managed keys incur a monthly fee plus charges per API request, unlike AWS managed keys which have no monthly fee.
8. **Deletion**: Keys can be scheduled for deletion with a configurable waiting period (7-30 days), allowing time to cancel if needed.
Best Practices:
- Enable key rotation for enhanced security
- Apply least privilege access principles
- Use aliases for easier key management
- Monitor key usage through CloudTrail
Customer managed keys are essential for organizations requiring complete control over their encryption strategy and compliance with regulatory requirements.
KMS Customer Managed Keys (CMKs) - Complete Guide
Why KMS Customer Managed Keys Are Important
Customer Managed Keys (CMKs) in AWS Key Management Service (KMS) provide organizations with granular control over their encryption keys. They are essential for meeting compliance requirements, implementing least-privilege access, and maintaining full visibility into key usage through audit logs. Understanding CMKs is crucial for the AWS Developer Associate exam as they form the foundation of encryption strategies across AWS services.
What Are Customer Managed Keys?
Customer Managed Keys are symmetric or asymmetric encryption keys that you create, own, and manage within AWS KMS. Unlike AWS managed keys or AWS owned keys, CMKs give you complete control over:
• Key policies and IAM policies • Key rotation settings • Key deletion and disabling • Cross-account access • Alias management • Tagging for cost allocation
How Customer Managed Keys Work
Key Creation: You create CMKs through the AWS Console, CLI, or SDK. Each CMK is assigned a unique key ID and Amazon Resource Name (ARN).
Key Material: CMKs can use AWS-generated key material or you can import your own key material (BYOK - Bring Your Own Key).
Encryption Process: 1. Your application calls the KMS Encrypt API with plaintext data 2. KMS uses the CMK to encrypt the data 3. KMS returns ciphertext that can only be decrypted using the same CMK
Key Policies: Every CMK has a key policy that defines who can use and manage the key. This is the primary access control mechanism for CMKs.
Automatic Key Rotation: You can enable automatic annual rotation for CMKs with AWS-generated key material. The old key material is preserved for decrypting previously encrypted data.
Key Features of CMKs
• Regional: CMKs are region-specific and cannot be transferred between regions • Multi-Region Keys: Special CMKs that can be replicated to other regions • Symmetric vs Asymmetric: Symmetric keys (AES-256) are most common; asymmetric keys support RSA and ECC • Key States: Enabled, Disabled, Pending Deletion, Pending Import • Deletion: Minimum 7-day waiting period, maximum 30 days before permanent deletion
Common Use Cases
• Encrypting S3 objects with SSE-KMS • Encrypting EBS volumes • Encrypting RDS databases • Encrypting Lambda environment variables • Signing and verifying data with asymmetric keys
Exam Tips: Answering Questions on KMS Customer Managed Keys
Key Concepts to Remember:
1. Key Policy vs IAM Policy: Key policies are resource-based and must explicitly allow access. IAM policies alone are not sufficient unless the key policy grants permission to the account.
2. Cross-Account Access: Requires both the key policy in the source account AND IAM permissions in the target account.
3. Encryption Context: Additional authenticated data that provides integrity checking. If used during encryption, it must be provided during decryption.
4. Data Key Caching: The AWS Encryption SDK can cache data keys to reduce KMS API calls and costs.
5. Grants: Used for temporary, programmatic access to CMKs - common in exam scenarios involving cross-service encryption.
6. Key Rotation: Automatic rotation is annual; manual rotation requires creating a new key and updating aliases.
7. Cost Considerations: CMKs cost $1/month plus API call charges. This may appear in cost optimization questions.
Common Exam Scenarios:
• When asked about encrypting data for compliance, CMKs with audit trails via CloudTrail is typically the answer • For cross-region encryption needs, look for multi-region key options • If a question mentions rotating keys more frequently than annually, manual rotation with aliases is required • Questions about Lambda decryption failures often involve missing KMS permissions in the execution role • S3 cross-account access with SSE-KMS requires both bucket policy AND key policy permissions