The service in question is Amazon Simple Storage Service (S3), which fulfills all the specified criteria by providing default encryption for stored data, and securing each customer's data with unique keys managed by the service itself, without any additional configuration needed for data access. Amazon Elastic Block Store (EBS) and Amazon Relational Database Service (RDS) provide encryption capabilities, but they may require initial setup and configuration, and do not always use a unique key per customer unless specified. AWS Key Management Service (KMS) is a key management service and not a data storage service, and thus does not directly encrypt data at rest.