The correct answer is Create a new S3 bucket in eu-central-1 with SSE-KMS using a multi-Region KMS key, configure replication to a bucket in eu-west-3 that uses the related replica key, and copy current data into the new source bucket. This option uses AWS KMS multi-Region keys so the same key material and key ID are available in both Regions which meets the security policy requirement for identical key material.

Multi-Region KMS keys provide a primary key and a related replica key that share key material and a consistent key identity across Regions. Creating a new source bucket encrypted with the multi-Region key and copying existing objects into that bucket ensures historical objects are re-encrypted under the multi-Region key so Amazon S3 replication can replicate encrypted objects to the eu-west-3 bucket that uses the related replica key. This gives a native, automated replication path and preserves the requirement that encryption and decryption use the same key material and key ID.

Enable replication on the existing bucket in eu-central-1 to a bucket in eu-west-3 and share the current AWS KMS key from eu-central-1 with eu-west-3 is not possible because AWS KMS keys are Regional and you cannot share a single-Region key across Regions. You must use a separate regional key or a multi-Region key pair for cross-Region decryption.

Use an Amazon EventBridge schedule to trigger an AWS Lambda function daily that copies objects from eu-central-1 to eu-west-3 with permissions to encrypt and decrypt using the KMS key is operationally heavier and has a worse recovery point objective than native S3 replication. This pattern also does not automatically satisfy the requirement for identical key material and key ID across Regions without creating and managing multi-Region keys and re-encrypting objects, which defeats the simplicity of the solution.

Convert the existing single-Region AWS KMS key to a multi-Region key and use Amazon S3 Batch Replication to backfill historical objects to eu-west-3 is invalid because you cannot convert an existing single-Region KMS key into a multi-Region key. The supported approach is to create a new multi-Region key and re-encrypt objects so replication can use the related replica key.

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