SNS Topic encryption

A target is a subscription

If you don't know the source of messages, you can check CloudTrail (if enabled) Here is an example of encrypted SNS + SQS https://www.youtube.com/watch?v=I7f2c5Qgz9w

The main idea is that you need to use a Customer Managed KMS key (KMS CMK), because this type of keys allows managing Key Policy and configuring cross-service communication