Customizable redirect after successful external login

[byewokko]

Current behavior

Successful login via external identity provider redirects the user to "My account" page of Seacat Auth UI, whicxh is sort of a dead end for the user.
See

seacat-auth/seacatauth/external_login/handler.py

Line 147 in d2b7561

response = self._my_account_redirect_response(state=state)

Expected behavior

Successful login via external identity provider redirects the user to whatever the original requested URL was. If this fails, there should be a configurable fallback URL.

This must also extend to registering a new external login for an existing user.

Proposed implementation

Option 1: Adding state to the existing flow

• When an authorization request results in redirection to login, Seacat Auth stores an internal state - an unique opaque string mapped to the original requested URL (and possibly other necessary authorization metadata).
• The state string is propagated to the login page (as a query parameter). The login UI ensures this state is added to the external login URLs obtained from GET /public/features
• When external login succeeds, the external identity provider redirects the user to GET /public/ext-login/{ext_login_provider} with the state parameter.
• Seacat Auth decodes the state parameter and redirects the user to the corresponding URL.

The flow for registering new external login provider for an already logged-in user should be analogous, but i'm not sure how to store the state in this case yet.

PROs:

• Easier implementation on backend.

CONs:

• Reliance on the UI.
• Not sure how to solve it for the registration of external login providers for already logged-in users.

Option 2: Authorization flow with OpenID ACR values

• OpenID authorization endpoint defines a way for the client to request a specific authentication class (e.g. single-factor auth, two-factor auth, hardware key auth etc.) using the acr_values parameter. It is up to the authorization server to define its own ACR values.
• Seacat Auth specifies a set of ACR values for external identity providers, e.g. seacat:ext:google, seacat:ext:github etc.
• When the authorization request specifies acr_values=seacat:ext:google and prompt=login, Seacat redirect the user directly to the external identity provider login (with a state parameter).

Reference:

• https://openid.net/specs/openid-connect-modrna-authentication-1_0.html#acr_values
• IdentityServer4 uses ACR values for external identity providers, e.g. idp:google (https://stackoverflow.com/q/71159908)

PROs:

• Does not rely on UI.
• Solves both login and registration.
• Closer to standard OpenID approaches.
• Less endpoints - we might eventually ditch the /public/ext-login/{ext_login_provider} endpoints altogether.

CONs:

• More implementation work on backend.