AWS API Gateway with cognito authorization - aws-api-gateway

Question

Currently I'm developing serverless architecture where there are set of resources and methods in AWS API gateway. I plan to add Cognito authentication(user pool) and authorization as secure layer to AWS API gateway.
There are 3 authorizer in AWS API Gateway which are IAM, Cognito User Pool and custom lambda.
For my use case, the sign-in and sign-up(authentication) are using cognito user pool via API gateway. It's perfect works. My user will given app client id and client secret to enable both processes. Once after sign-in, my intention is get user able to use the access token(returned by user pool) to access resource via api gateway.
However, my user can has different role such admin, owner or guest. User only can access the authorized api. My approach is to put user into different group in user pool, assign IAM policy to group and enable identity pool. This force me to change the authorization type in api gateway to IAM. and IAM require every request to be signed by Signature V4.
It means every requests have to sign up by session token, access key, secret (returned after exchange id token with federated pool) instead of using access token based approach. So in my use case, after my user sign-in via api gateway, my client app(web/mobile/postman tool) has to generate signature and put in Authorization header. Is there alternative ways to control authorisation in user pool group but using access token in api gateway? My understanding is access token (in Authorization header) is much easier to use than complex signed signature process.
Correct me if I'm wrong. Thanks.

Answer 1

Will this help instead?
Create groups in user pool and assign IAM role to the group.
And then add users to the group.
More documentation here: https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pools-user-groups.html

Answer 2

You are on the right track but you have chosen one of the paths of Authorizing the user requests on AWS. You are using IAM and Identity Pool and because of that, you are forced to sign every request using Signature V4 provided by AWS.
Instead of going for I would suggest as per my experience to choose custom Lambda Authorizer over Identity Pool. In this case, your authentication will remain as it is that you have already built. But instead of applying IAM as Authorizer in API gateway, you can create a lambda function that will receive the ARN of the API gateway that the user wanted to authorize in the request and user ID_TOKEN that you received in during authentication from User Pool.
{
...
"cognito:roles": [
"arn:aws:iam::**********:role/addBookSellerRole"
],
"exp": 1565758916,
"iat": 1565755316,
...
}
You can see you will get the roles array from the ID_TOKEN in JWT received from authentication. You can use this role to fetch the attached policies to this role. Follow this document to fetch the policies from the role.
Once you get the policy JSON you can compare it with the ARN of the method received in request with the list of policies. And this way you have to generate a Policy Document that will either Allow or Deny the request.
To read more about it visit my medium blog Authorization using Cognito + API Gateway + IAM.