Explanation
Creating an AUTH token and storing it in AWS Secrets Manager and configuring the existing cluster to use the AUTH token and configure encryption in transit, and updating the application to retrieve the AUTH token from Secrets Manager when necessary and to use the AUTH token for authentication, would meet the requirements for user authentication and end-to-end encryption.
AWS Secrets Manager is a service that enables you to easily rotate, manage, and retrieve database credentials, API keys, and other secrets throughout their lifecycle. Secrets Manager also enables you to encrypt the data and ensure that only authorized users and applications can access it.
By configuring the existing cluster to use the AUTH token and encryption in transit, all data will be encrypted as it is sent over the network, providing additional security for the data stored in ElastiCache.
Additionally, by updating the application to retrieve the AUTH token from Secrets Manager when necessary and to use the AUTH token for authentication, it ensures that only authorized users and applications can access the cache.

Explanation
This solution will provide the highest availability for the database, as the read replicas will allow the database to be available in multiple Regions, thus reducing the chances of disruption. Additionally, the promotion of the cross-Region read replica to become a standalone DB instance will ensure that the database is still available even if one of the Regions experiences disruptions.

Explanation
https://d1.awsstatic.com/architecture-diagrams/ArchitectureDiagrams/NAT-gateway-centralized-egress-ra.pdf?d

Explanation
https://docs.aws.amazon.com/AmazonS3/latest/userguide/ServerSideEncryptionCustomerKeys.html Clearly says we need following header for SSE-C x-amz-server-side-encryption-customer-algorithm Use this header to specify the encryption algorithm. The header value must be AES256.