The core requirement is to encrypt data in one AWS Region and decrypt it in another using the same logical key. AWS KMS multi-Region keys are designed for this exact purpose. A multi-Region primary key is created in one Region, and replica keys are created in other Regions. These keys are interoperable because they share the same underlying key material and key ID. An application in any of these Regions can use its local replica key (via the regional KMS endpoint) to encrypt or decrypt data, ensuring low latency and high availability while maintaining a consistent encryption standard across Regions for the replicated S3 data.

B: Creating new, independent customer-managed KMS keys in each Region would result in keys with different key material, making it impossible to decrypt data in one Region that was encrypted in another.

C: AWS Private CA is used for creating and managing private X.509 certificates, primarily for securing network communications (TLS) or authenticating entities, not for encrypting application data at rest.

D: Exporting the key material from an AWS-generated KMS key is not a supported feature. This is a fundamental security design of AWS KMS to ensure the key material never leaves the FIPS-validated hardware security modules (HSMs).

1. AWS KMS Developer Guide

"Multi-Region keys in AWS KMS": "Multi-Region keys are AWS KMS keys in different AWS Regions that can be used interchangeably to encrypt and decrypt data in any of these Regions... Each set of related multi-Region keys has the same key ID and key material." This directly supports the functionality described in option A.

2. AWS KMS Developer Guide

"How multi-Region keys work": "Because related multi-Region keys have the same key material

data encrypted with a multi-Region key in one AWS Region can be decrypted with a related multi-Region key in a different AWS Region." This confirms the interoperability required by the question.

3. AWS KMS Developer Guide

"Importing key material in AWS KMS": This section details the process for using your own key material (BYOK). It implicitly confirms that you cannot export AWS-generated key material. The documentation states

"When you use a KMS key with imported key material

you remain responsible for the key material

" which is not the case for standard KMS keys where AWS manages the material securely.

4. AWS Private Certificate Authority User Guide

"What is AWS Private Certificate Authority?": "AWS Private Certificate Authority (AWS Private CA) is a managed private CA service that helps you easily and securely manage the lifecycle of your private certificates." This defines the service's purpose

which is distinct from encrypting data at rest.