Key Differences Between Customer Managed Keys (CMKs) and AWS Managed Keys for Server-Side Encryption

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ustomer Managed Keys (CMKs) and AWS Managed Keys are both used for server-side encryption in AWS services, but there are key differences between them:

  1. Ownership and Control:
    • Customer Managed Key (CMK): You, as the AWS account holder, have full control over CMKs. You create, manage, and control the policies and permissions for CMKs. This gives you more flexibility to customize your encryption approach and access controls.
    • AWS Managed Key: AWS Managed Keys are handled by AWS services, and you don’t have direct control over them. AWS services like S3, EBS, RDS, etc., automatically generate and manage these keys on your behalf. They are more straightforward to use but offer less customization and control.
  2. Use Cases:
    • Customer Managed Key (CMK): Typically used when you have specific compliance requirements or need fine-grained control over key access and rotation. CMKs are suitable for scenarios where you need to manage the encryption keys for your data.
    • AWS Managed Key: AWS Managed Keys are convenient for general encryption needs and scenarios where you want encryption without the complexity of key management. They are often used for ease of use and speed of setup.
  3. Key Rotation:
    • Customer Managed Key (CMK): You are responsible for managing key rotation for CMKs. AWS KMS provides tools for key rotation and tracking, but you need to initiate the rotation process.
    • AWS Managed Key: AWS manages key rotation automatically for AWS Managed Keys. This ensures that keys are rotated at predefined intervals without your direct intervention.
  4. Cross-Account Access:
    • Customer Managed Key (CMK): You can share access to CMKs across AWS accounts using key policies and resource-based policies. This is useful for scenarios where multiple AWS accounts need to access the same keys.
    • AWS Managed Key: AWS Managed Keys are typically not shared across AWS accounts because AWS services use them within the same account.
  5. Customization:
    • Customer Managed Key (CMK): You can set custom key policies, control who has access to the key, and manage key usage as per your requirements.
    • AWS Managed Key: Limited customization options because AWS services use them according to their predefined configurations.

In summary, the choice between Customer Managed Keys (CMKs) and AWS Managed Keys depends on your specific requirements. If you need more control, compliance, and customization over key management, CMKs are a better choice. If you want the simplicity and automation of key management by AWS services, AWS Managed Keys are a suitable option.

Understanding the Inaccessibility of Customer Managed Keys (CMKs) in AWS KMS

Downloading the Customer Managed Key (CMK) directly from the AWS Key Management Service (KMS) is not possible. CMKs are integral to your security infrastructure, and AWS KMS does not offer an option to export or retrieve the key material for a CMK.

AWS KMS is purposefully designed to provide a secure and meticulously managed environment for key handling, adhering to a fundamental security principle of maintaining strict control over the keys. The key material, generated and stored securely by AWS KMS, is safeguarded and inaccessible to users. While you can utilize the CMK for encryption and decryption tasks, you cannot extract or access the key material itself.

Should you ever need to transfer a CMK to a different AWS account or region, the recommended approach is to create a new CMK within the desired account or region. Subsequently, you can re-encrypt your data using this new CMK. This approach ensures that the key material remains securely managed within the AWS KMS service, preserving its integrity.

It’s essential to exercise care and diligence in the management of CMKs and to establish a robust backup and recovery strategy. Losing access to a CMK or encountering a security breach could potentially result in data loss or unauthorized access, underlining the significance of prudent key management practices

Demo : Setting Up Amazon S3 Bucket Encryption with Customer Managed Key (CMK) Using the AWS CLI

To set up Customer Managed Key (CMK) encryption for an Amazon S3 bucket using the AWS CLI, you can follow these steps:

  1. Create a Customer Managed Key (CMK):Use the AWS Key Management Service (KMS) to create a CMK:
    aws kms create-key
    This command will return the ARN of the newly created CMK. Note this ARN, as you will need it later.
  2. Configure S3 Bucket Encryption with CMK:Configure your S3 bucket to use the CMK for server-side encryption:
    aws s3api put-bucket-encryption --bucket your-bucket-name --server-side-encryption-configuration '{"Rules": [{"ApplyServerSideEncryptionByDefault": {"SSEAlgorithm": "aws:kms", "KMSMasterKeyID": "your-cmk-arn"}}]}'
    Replace "your-bucket-name"
with your S3 bucket name and "your-cmk-arn"
with the ARN of the CMK you created in step 1.
  • Set Up IAM Policies:Ensure that the appropriate IAM policies are in place to grant users, roles, or services the necessary permissions for accessing the CMK and S3 bucket. These policies should include permissions for KMS and S3 actions.
  • Testing and Validation:Test your setup to confirm that objects stored in the S3 bucket are automatically encrypted using the specified CMK. Also, validate that users and services have the correct permissions for accessing the bucket and its encrypted contents.

    • By following these steps with the AWS CLI, you can set up Customer Managed Key (CMK) encryption for your Amazon S3 bucket, giving you control over your encryption keys and enhancing security and compliance for your data stored in S3.

    When you encrypt an S3 bucket with a Customer Managed Key (CMK), IAM users like “Peter” can access the S3 bucket and read its content with decrypted content if they have the necessary permissions. Here’s how you can set this up:


    When you encrypt an S3 bucket with a Customer Managed Key (CMK), IAM users like “Peter” can access the S3 bucket and read its content with decrypted content if they have the necessary permissions. Here’s how you can set this up:

    1. IAM Policy for S3 Access:
      • First, ensure that Peter has an IAM policy attached that grants him access to the S3 bucket and its objects. The policy should include permissions like s3:ListBucket for listing the bucket contents and s3:GetObject for retrieving objects from the bucket.
    2. IAM Policy for KMS Access:
      • Peter also needs permissions to use the CMK for decryption. This means he should have permissions like kms:Decrypt for the specified CMK.
    3. Bucket and Key Configuration:
      • Ensure that the S3 bucket is configured to use the specified CMK for server-side encryption, and that the objects stored in the bucket are encrypted with this CMK.
    4. Testing:
      • After setting up the policies and ensuring that the CMK is used for encryption, you can test Peter’s access. He should be able to use the S3 API to retrieve objects from the bucket. AWS S3 will automatically handle the decryption process when Peter accesses the objects.

    With the appropriate IAM policies, S3 bucket encryption, and CMK configuration, Peter can access the S3 bucket and read its contents with the data automatically decrypted using the specified CMK. This ensures that only authorized users with the necessary permissions can access and decrypt the data stored in the S3 bucket.

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