Today people are using the SDK directly or injecting secrets via the CLI. Multi-language support and integration with e.g. Kubernetes is high on the wish list. The approach we are looking at for multi-language support is to drop the dependency on the AWS Encryption SDK. Creating a read-only SDK in other languages is then straight forward.
That is certainly one approach we have looked at. The question then is where would you run the REST API? As a side car, as a separate service, e.g. using AWS API Gateway and Lambdas?
If you look at the work required to write a simplified read-only client using the AWS SDK for KMS and DynamoDB, while perhaps a bit more work than integrating with a REST API, it is not that far off, which is why we are considering that approach. That would of course not help if you are using a language that does not have an AWS SDK.
Any of the suggestions above would most likely work. Running an API sidecar or separate service would just be the cost of exposing the REST API.
I could definitely envision people deciding against using your key store when comparing to something like Vault based just on the fact that you limit extensibility with no API.
