why is the Google OAuth Client Secret required for Cognito with Google as Identity Provider

Question

Hello,

why is the Client Secret required for Cognito, if I want to use Google as IdP with my hosted ui?

In the OpenId-workflow the Authorization grant code is passed to the redirect_uri by Google through Cognito.
Then my server would need to exchange the Authorization grant code for a Access token & Identity Token at google by sending the client secret.

But Cognito should not need the client secret from google here, because it doesnt request a code exchange for me.

Thank you, if s.o. knows

Accepted Answer

When using federation it is often confusing which entity is playing which role at what point.

When you integrate Cognito with Google as an ODIC identity provider, Cognito is the corresponding relying party ("app") in the relationship with Google, not your "Server"(App). Cognito is using the client id/secret to do the code exchange. Cognito does that behind the scene and it is not visible to you. After that Cognito gives your application the tokens Cognito itself generates.

Please refer to the diagram and description here for how the signals flow:
    https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pools-oidc-flow.html

Cognito acts as a relying party which provides token handling and management for authenticated users from all identity providers, so your backend systems can standardize on one set of user pool tokens.

why is the Google OAuth Client Secret required for Cognito with Google as Identity Provider

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