I created a AWS API Gateway set with authentication = AWS_IAM to call a Lambda function. Now, to call this API I understand that I need to sign the request and as stated in the AWS documentation the correct way is to add the Authorization header calculated using AWS Signature V4 which need an access_key and a secret_key. On my client side the user authenticate with AWS Cognito first and receive the JWT tokens (id token access token and refresh token) but I cannot find the access_key/secret _key in them. How can I calculate the AWS Signature V4 from the tokens received from AWS Cognito?
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1 Answers
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I believe you can't (with 99,99999 of certainty)!
Please confirm that you are authenticating your users with AWS Cognito User Pool. You probably are because Cognito User Pool is the service that provides JWT. In this case, the token will assure the service that receives it (API Gateway) that your user is registered in a specific identity directory (User Pool). Your service should evaluate if it will provide access or not to its resources for users registered in this specific directory with the provided claims (groups, roles, etc).
When you secure your API Gateway endpoints with AWS_IAM you are saying that only identities that AWS can recognize inside its own identity directory (Users or Roles) are allowed to perform actions on the resource. In general, users registered in Cognito User Pools are not considered by AWS as valid identities.
For a Cognito User Pool user to be considered a valid AWS identity, you have two options:
1 - Configure your AWS account to use external Identity Providers and Federation. Not a simple thing and a solution to a different use case. In summary, don't choose this one.
2 - Use another AWS product (with a name that creates a lot of confusion) called Cognito Identity Pool. This service evaluates if the JWT token is allowed in that context (you configure it inside the Identity Pool). If is a valid token from a registered identity directory, Cognito Identity Pool will exchange your JWT token for a AWS Access Key, AWS Secret Key and AWS Session Token associated with a specific IAM Role. You can then use these keys to sign your request. But keep in mind that with this change you will lose your capacity to identity the specific user in API Gateway and in the downstream services called by API Gateway. If you need to have the JWT token in your downstream services, you can do it with a little bit of additional effort. You can't find a way here: https://stackoverflow.com/a/57961207/6471284
