By reading Cognito Identity Provider document, I understand that it looks like it provides out-of-box integration with Facebook / Google / Twitter as Identity Providers.
My application is a developer focused application so I would like enable users sign-up/sign-in with their Github account besides the above Identity Provider's accounts. Is that possible?
If possible, how much extra work (comparing the out-of-box Facebook/Google social sign-up feature) I need to do?
Since first writing this answer, I implemented and published a project that provides a wrapper for Cognito to talk to GitHub. It comes with a SAM/cloudformation deploy script, so you can create a CloudFormation stack that provides the wrapper very easily.
So, OpenID Connect is built on top of OAuth2.0. It's an extension - in OpenID Connect, the OAuth endpoints are there (with one or two extensions or changes), plus some new endpoints.
My understanding from reading the Cognito documentation and the relevant bits of the OpenID Connect and OAuth2.0 specs is that Cognito only uses four of the OpenID endpoints - Authorization, token, userinfo and jwks. You can specify each endpoint separately when configuring an OpenID Connect provider in Cognito. This means it is possible to provide OpenID Connect for github by implementing these endpoints.
Here's a rough plan for implementation:
Authorization: In the spec, this looks to be the same as the OAuth2.0 endpoint (plus some additional parameters that I don't think are relevant to using github as an identity provider). I think you could:
Use the github Auth URL: https://github.com/login/oauth/authorize
Set your GitHub OAuth app to redirect to https://<your_cognito_domain>/oauth2/idpresponse
For the other endpoints, you'll have to roll them yourselves:
Token: This is used to get the access and ID tokens - using a code returned by the authorization callback. It looks the same as the OAuth2.0 endpoint, but also returns an idToken. It looks possible to make an implementation that passes through the code to github's token endpoint (https://github.com/login/oauth/access_token) to get the accessToken, and then generates an idToken, signed with your own private key.
UserInfo: This doesn't exist at all in OAuth2.0, but I think that much of the contents could be filled in with requests to the /user github endpoints (since at this point the request contains the authenticated access_token). Incidentally, this is the reason that there's no open source shim to wrap OAuth2.0 with OpenID connect - OpenID connect's primary contribution is a standardised way of communicating user data - and since OAuth doesn't have a standardised way to do this, we have to write a custom one specific to GitHub (or any other OAuth-only provider we wanted to use for federation).
JWKS: This is the JSON Web Key Set document containing the public key(s) that can be used to verify the tokens generated by the token endpoint. It could be a flat file.
I have implemented the above, and the approach works. I open-sourced the project here.
Unfortunately it's not possible. Cognito Federated Identities can support any OIDC Identity Provider but OAuth2.0 spec does not give that flexibility, so there's no easy way to achieve this unless we add special support for Github.
