AWS REST Api as S3 proxy and accessing client specific bucket

Question

I have created AWS API Gateway as S3 proxy as per guidelines here . So final url looks something like

http://api.exmaple.com/v1/ {clientbucketname}/{key}

client will use their pre-assigned bucket name in the url. I understand that if API is authenticated using IAM user then API will have access to the bucket belongs to authenticated IAM user and can perform actions only on a particular client's bucket.
However my API is authenticated using API-KEY. Each client has their own API-KEY how do i tie API-KEY to S3 bucket so client A cannot access client B's bucket simply by changing the bucket name in the URL.

Answer 1

It is not possible without AWS_IAM authorization.

API Gateway uses the result of that authorization to locate the account, role and bucket.